Review of “Natural Product Synthesis” on Nature Chemistry (2011~2013) Speaker: Dr

Review of “Natural Product Synthesis” on Nature Chemistry (2011~2013) Speaker: Dr. Tao Xu Dong group seminar January 29th, 2014 Contents

• 5 publicaons in 2011 • 4 publicaons in 2012 • 5 publicaons in 2013 Selecve in-depth discussion • Conclusion and summary NATURE CHEMISTRY DOI: 10.1038/NCHEM.1044Solanoeclepin A-combang hunger ARTICLES

a b c

Potato cyst nematodes d HO Cyst nematodes Coupling Potatoes affected by CO2H reaction on potato’s root HCyst nematodes CO2H HO G Stereoselective O O Simmons–Smith A D * Cyst nematodes are parasites that live on and destroy plants such as soybeans, potatoes O CO H E H reaction 2 Intramolecular O C B F Glycinoeclepin A (1) Diels–Alder O Base-induced * Damage caused by cyst nematodes are worldwide. (> 50 countries reported) reaction O OH intramolecular OMe cyclization Solanoeclepin A (2) * Mechanisms have been figured out and Chemists were needed. Figure 1 | Cyst nematodes and their effect on crops. a,PotatocystnematodeGlobodera rostochiensis. b, Cysts on potato roots, which protect the nematodes until hatch stimulants produced by the host plant are present. c, Potato field affected by potato cyst nematodes. d,StructuresofglycinoeclepinA(1)and solanoeclepin A (2)—key hatch-stimulating substancesTanino for the K, Miyashita M etc. soybean cyst and potatoNat. Chem cyst nematodes,. 3:484 respectively. The key disconnections in a planned total synthesis effort for solanoeclepin A (2)areshown.

CN CN CN CN TBHP CN CN

Me3Al 1) DBU, CH2Cl2 Ti(Oi-Pr)4 TMSOTf mCPBA Al(OTf)3 67% for 3 steps MS4Å 2,6-lutidine (CH Cl) CH2Cl2 (CH2Cl)2 2) CH2=CHMgBr OH CH2Cl2 OH 2 2 O H 92% O then HF•Py O AcO AcO AcO O CeCl3, THF HO 96% 97% 3 4 5 6 7 8

1) DIBAL, THF CO2Et CN TBSO 2) TBSOTf, 2,6-lutidine CN TBSO CH2Cl2 LDA then 1) DIBAL, THF 93% for 2 steps TBSCl, HMPA 1) DIBAL, CH2Cl2 100% 3) mCPBA, (CH Cl) THF 2) (EtO) P(O)CH CO Et 2) CONMe 2 2 2 2 2 O 2 (S)-epoxide 9 74% OTBS 99% OTBS NaH, THF TBSO TBSO BuB O 96% for 2 steps TBSO OTBS (R)-epoxide 9' 14% O 9 10 11 CONMe2 Et2Zn, CH2I2, CH2Cl2 H CH2OH H CH2OBn 100% (dr = 94:6) TBSO HO 1) HF•Py, THF 1) BOMCl, DIPEA H H 1) NaH, BnBr 1) o-NO2C6H4SeCN 2) TBSCl, imidazole TBAI, CH2Cl2 TBAI, DMF Bu3P, THF DMF 99%

2) TBAF, THF 2) H2O2, THF 3) DMP, CH2Cl2 2) t-BuOCH(NMe2)2 OTBS TBSO OTBS 75% for 2 steps TBSO OTBS 88% for 2 steps TBSO 4) HF•Py, THF O OH DMF 12 13 14 86% for 4 steps 15

H CH2OBn G Tf2O H 2,6-(t-Bu)2Py Me2N D E CH2Cl2 F OHC O OBOM 92% for 2 steps OTf OBOM 16 17

Figure 2 | Stereoselective synthesis of right-hand segment 17. The DEFG ring system was constructed via 1,2-rearrangement of the vinyl group and the key intramolecular cyclization reaction of epoxy nitrile 9. mCPBA, m-chloroperbenzoic acid; Al(OTf)3, aluminium triﬂuoromethanesulfonate; DBU,

1,8-diazabicyclo[5.4.0]undec-7-ene; TBHP, tert-butyl hydroperoxide; MS4Å, molecular sieves 4Å; TMSOTf, trimethylsilyl trifluoromethanesulfonate; HF†Py, hydrogen fluoride pyridine complex; DIBAL, diisobutylaluminum hydride; TBSOTf, tert-butyldimethylsilyl trifluoromethanesulfonate; LDA, lithium diisopropylamide; TBSCl, tert-butyldimethylsilyl chloride; HMPA, hexamethylphosphoramide; BnBr, benzyl bromide; TBAI, tetrabutylammonium iodide; DMF, N,N-dimethylformamide; TBAF, tetrabutylammonium fluoride; THF, tetrahydrofuran; DMP, Dess–Martin periodinane; BOMCl, benzyl chloromethyl ether;

DIPEA, N,N-diisopropylethylamine; Tf2O, triﬂuoromethanesulfonic anhydride; 2,6-(t-Bu)2Py, 2,6-di-(tert-butyl)pyridine. including a furan (diene component) and a,b-unsaturated ketone tricyclo[5.2.1.01,6]decane skeleton (DEF ring system). The targeted (dienophile) moieties in the molecule. molecule 9 was a trans-fused indane derivative bearing two quatern- Our ﬁrst objective focused on the stereoselective synthesis of the ary asymmetric carbon atoms at the bridge heads; its synthesis crucial precursor epoxy nitrile 9 for construction of the therefore appeared very challenging. Figure 2 presents the successful

NATURE CHEMISTRY | VOL 3 | JUNE 2011 | www.nature.com/naturechemistry 485 NATURE CHEMISTRY DOI: 10.1038/NCHEM.1044 ARTICLES

a b c

Total Synthesis of Solanoeclepin A

d HO Coupling CO2H reaction H CO2H HO G Stereoselective O O Simmons–Smith A D O CO H E H reaction 2 Intramolecular O C B F Glycinoeclepin A (1) Diels–Alder O Base-induced reaction O OH intramolecular OMe cyclization Isolaon: 1) Masamune, T. Solanoeclepin A (2) Nature 1982, 297, 495. Figure 1 | Cyst nematodes2) andMasamune their effect on, T. J. Chem. crops. a,PotatocystnematodeIsolaon: 1) Mulder, J. G., CT Int. Appl. WO 93 Globodera rostochiensis. b, Cysts on potato roots, which protect the nematodes until hatch stimulants producedSoc. Chem. by the hostCommun plant are. 1985 present., c, Potato ﬁeld02 083 ( affected1992 by potato) (Chem. cyst nematodes.Abstr. 118, 185844z). d,StructuresofglycinoeclepinA(1)and solanoeclepin A (2)—key hatch-stimulating222. substances for the soybean cyst2) Schenk, H. et al. Croat. Chem. and potato cyst nematodes, respectively.Acta The 72, 593– key disconnections in a planned total synthesis effort for solanoeclepin A (2)areshown. 50 ug out of 113 kg 606 (1999). dry roots of kidney bean CN CN CN CN TBHP CN CN

Me3Al 1) DBU, CH2Cl2 Ti(Oi-Pr)4 TMSOTf mCPBA Al(OTf)3 67% for 3 steps MS4Å 2,6-lutidine • Agricultural chemicals and nemacides kill plants as well. (CH Cl) CH2Cl2 (CH2Cl)2 2) CH2=CHMgBr OH CH2Cl2 OH 2 2 • Hatching smulaon could be an alternave. O H 92% O then HF•Py O AcO AcO AcO O CeCl3, THF HO 96% 97% 3 • Four hatching stages, stage II are root infecous and detrimental. 4 5 6 7 8 • Spreading smulus over potato ﬁles aer harvest. 1) DIBAL, THF CO2Et CN TBSO 2) TBSOTf, 2,6-lutidine CN TBSO CH2Cl2 LDA then 1) DIBAL, THF 93% for 2 steps TBSCl, HMPA 1) DIBAL, CH2Cl2 100% 3) mCPBA, (CH Cl) THF 2) (EtO) P(O)CH CO Et 2) CONMe 2 2 2 2 2 O 2 (S)-epoxide 9 74% OTBS 99% OTBS NaH, THF TBSO TBSO BuB O 96% for 2 steps TBSO OTBS (R)-epoxide 9' 14% O 9 10 11 CONMe2 Et2Zn, CH2I2, CH2Cl2 H CH2OH H CH2OBn 100% (dr = 94:6) TBSO HO 1) HF•Py, THF 1) BOMCl, DIPEA H H 1) NaH, BnBr 1) o-NO2C6H4SeCN 2) TBSCl, imidazole TBAI, CH2Cl2 TBAI, DMF Bu3P, THF DMF 99%

2) TBAF, THF 2) H2O2, THF 3) DMP, CH2Cl2 2) t-BuOCH(NMe2)2 OTBS TBSO OTBS 75% for 2 steps TBSO OTBS 88% for 2 steps TBSO 4) HF•Py, THF O OH DMF 12 13 14 86% for 4 steps 15

H CH2OBn G Tf2O H 2,6-(t-Bu)2Py Me2N D E CH2Cl2 F OHC O OBOM 92% for 2 steps OTf OBOM 16 17

NATURE CHEMISTRY | VOL 3 | JUNE 2011 | www.nature.com/naturechemistry 485 NATURE CHEMISTRY DOI: 10.1038/NCHEM.1044 ARTICLES

a b c

Figure 1 | Cyst nematodes and their effect on crops. a,PotatocystnematodeGlobodera rostochiensis. b, Cysts on potato roots, which protect the nematodes until hatch stimulants produced by the host plant are present. c, Potato ﬁeld affected by potato cyst nematodes. d,StructuresofglycinoeclepinA(1)and solanoeclepin A (2)—key hatch-stimulating substances for the soybean cyst and potato cyst nematodes, respectively. The key disconnections in a planned total synthesis effort for solanoeclepinTotal Synthesis of A (2)areshown. Solanoeclepin A

CN CN CN CN TBHP CN CN

2) TBAF, THF 2) H2O2, THF 3) DMP, CH2Cl2 2) t-BuOCH(NMe2)2 OTBS TBSO OTBS 75% for 2 steps TBSO OTBS 88% for 2 steps TBSO 4) HF•Py, THF O OH DMF 12 13 14 86% for 4 steps 15

H CH2OBn G Tf2O H 2,6-(t-Bu)2Py Me2N D E CH2Cl2 F OHC O OBOM 92% for 2 steps OTf OBOM 16 17

NATURE CHEMISTRY | VOL 3 | JUNE 2011 | www.nature.com/naturechemistry 485 ARTICLES Total Synthesis of SolanoeclepinNATURE CHEMISTRY A DOI: 10.1038/NCHEM.1044

H CH OBn MeO 2

OTMS TMSO TMS H O HO 1) PPTS TMSO Bu3SnF t-BuLi DMF, H O PdCl2[P(o-tol)3]2 MeO 17 2 THF MeO 2) TMSCl MeO DMF O OBOM OBOM O OTf imidazole, DMF O OTf 44% O TMS 94% for 3 steps 18 19 20

TMSO O O O 1) CH3CO2H, H2O 1) SeO2 Me2AlCl 62% for 2 steps H H 1,4-dioxane, H2O MeI, Ag2O diethyl ether 2) DMP, CH Cl O O O 2 2 O 2) Cu(OAc) O DMF O MeO 90% O 2 O O O O MeOH OH 97% OMe 74% for 2 steps 21 22 23 24

H CH2OBn

H DIBAL HO HO IBX HO + toluene CH2Cl2 DMSO OBOM O OH O HO O HO O 43% for 2 steps O O OMe OMe OMe

25 26 27

Figure 3 | Stereoselective synthesis of the heptacyclic compound 27. The ABC ring system was constructed based on the Lewis acid-induced intramolecular Diels–Alder reaction of 20 and subsequent functionalization of the C ring. PPTS, pyridinium p-toluenesulfonate; TMSCl, trimethylsilyl chloride; IBX, 2-iodoxybenzoic acid. synthesis of the requisite precursor 9, starting from bicyclic acetoxy (TBSCl) and hexamethylphosphoramide (HMPA) in a one-pot nitrile 3, available on a large scale in an optically active form using a operation to produce the tricyclo[5.2.1.01,6]decane derivative 10 in protocol recently reported by us25. Epoxidation of 3 with m-chlor- nearly quantitative yield. The stereochemistry of the product was operoxy- benzoic acid (mCPBA) in dichloromethane (CH2Cl2)pro- unambiguously confirmed by X-ray crystallographic analysis of vided b-epoxide 4 stereoselectively, which, on treatment with the corresponding p-bromobenzoate. (An ORTEP drawing and trimethylaluminium (Me3Al) in the presence of aluminium tri- the CIF file of the crystalline compounds are provided in the fluoro- methanesulfonate (Al(OTf)3) in 1,2-dichloroethane Supplementary Information.) ((CH2Cl)2), underwent a Meinwald rearrangement to give keto With intermediate 10 in hand, we next focused on the synthesis acetate 5 as a single product. After treatment of the keto acetate of the cyclopropane ring of the side chain. For this purpose, 10 was with 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) in CH2Cl2, the transformed into unsaturated ester 11 (96%, two steps). After resulting enone (67% yield, three steps) was subjected to the reduction of the ester with DIBAL, the resulting allylic alcohol Grignard reaction with vinylmagnesium bromide in the presence was subjected to the Simmons–Smith reaction in the presence of 26 of cerium(III) chloride (CeCl3) in tetrahydrofuran (THF) to afford 1,3,2-dioxaborolane ligand to give cyclopropane 12 with the allylic alcohol 6 in 96% yield. As expected, addition of the desired stereochemistry in quantitative yield. We had thus estab- Grignard reagent occurred exclusively from the opposite side of lished the carbocyclic framework of the DEFG ring system. the angular methyl group. Oxidation of the allylic alcohol with Product 12 was further converted to hydroxy ketone 15 in several tert-butyl hydroperoxide (TBHP) in the presence of titanium(IV) steps, from which enol triflate 17, the key fragment, was successfully i isopropoxide (Ti(O Pr)4) and molecular sieves 4Å (MS4Å) in synthesized by way of keto enamine 16 (91%, three steps). This pro- CH2Cl2 afforded a-epoxy alcohol 7 in 92% yield. vided the fully functionalized DEFG ring system bearing three con- The next 1,2-rearrangement of the vinyl group was efficiently secutive stereocentres in a highly stereoselective manner. performed with trimethylsilyl trifluoromethanesulfonate (TMSOTf) and 2,6-lutidine in (CH2Cl)2 to furnish the desired hydroxy ketone Construction of the left-hand segment and completion of the 8 in 97% yield. Thus, the stereoselective synthesis of the trans- total synthesis. We then focused on the construction of the ABC fused indane derivative bearing two quaternary asymmetric carbon ring system including the 7-oxabicyclo[2.2.1]heptan-2-one moiety. atoms was established. The requisite epoxy nitrile 9 was synthesized To construct this particular ring system effectively, we designed in 69% yield by a three-step reaction sequence comprising: the synthetic route shown in Fig. 3, which involved an (i) stereoselective reduction of the ketone moiety with diisobutylalu- intramolecular Diels–Alder reaction between the furan and an minium hydride (DIBAL) in THF, (ii) protection of the hydroxyl unsaturated ketone moiety in precursor 20. groups with a tert-butyldimethylsilyl (TBS) group, (iii) epoxidation Synthesis of 20 began with the addition reaction of 17 with of the vinyl group with mCPBA in CH2Cl2 giving rise to a separable 4-methoxy-5-(trimethylsilyl)fur-2-yl lithium in THF at –78 8C, mixture of the (S)-epoxide 9 (74%) and (R)-epoxide 9′ (14%). which afforded a 2 alcohol 18 quantitatively. Removal of the The next step was the key intramolecular cyclization reaction to con- TMS group in the furan with pyridinium p-toluenesulfonate struct the four-membered carbocycle.Thereactionproceededwithhigh (PPTS) in wet DMF and protection of the hydroxyl group with efficiency by treatment of 9 with lithium diisopropylamide (LDA) in chlorotrimethylsilane (TMSCl) and imidazole in DMF furnished THF at 0 8Candsubsequentadditionoftert-butyldimethylsilyl chloride 19 (94% yield, three steps). The crucial palladium-catalysed

486 NATURE CHEMISTRY | VOL 3 | JUNE 2011 | www.nature.com/naturechemistry NATURE CHEMISTRYTotal Synthesis of DOI: 10.1038/NCHEM.1044 Solanoeclepin A ARTICLES

H CH2OBn H CH2OBn

1) OsO4, pyridine H t-BuOH H TMSCl TMSO 45% (68% based HO TMSCl TMSO imidazole on recovered 28) O imidazole O 27 OBOM 2) NaIO , CH CN OBOM DMF OBOM DMF O 4 3 O O 90% O O O O 70% for 2 steps O O OMe OMe OMe

28 29 30

H CH OH H CH OH H H 2 2 CH2OH CHO

TMSCl H H H H imidazole H2 CH CO H, H O DMF DMP Pd(OH)2 O 3 2 2 O O O then aq. THF CH Cl THF OH 2 2 O OH 88% for 3 steps OTMS OTMS 31 32 33 34

H H CO2H CO2H

H H TMSO HO NaClO2, NaH2PO4 O O 2-methyl-2-butene 3 M HCl OTMS OH t-BuOH, H O O CH CO H, H O O 2 O 3 2 2 O 97% for 2 steps O 63% O OMe OMe

35 Solanoeclepin A (2)

Figure 4 | Total synthesis of solanoeclepin A (2).

a 100 b 100 Solanoeclepin A Tomato root leachate (%) (%) 80 80

60 60

40 40 G. rostochiensis G. G. rostochiensis G.

20 20 Hatch of Hatch Hatch of Hatch 0 0 –6 –7 –8 –9 –10 –11 –12 –13 Water 1:2 1:10 1:100 Concentration (log M) Dilution

Figure 5 | Hatching stimulation effect of synthetic solanoeclepin A on G. rostochiensis. a,DosageresponsetosyntheticsolanoeclepinAofin vitro %hatch of G. rostochiensis after 21 days at 22 8C. Solanoeclepin A was diluted to produce a series of concentrations from 1 1026 to 1 10213 gml21 in water. % × × hatch (Nj – Nj )/Ne ; Nj ,numberofhatchedjuvenilesat21days;Nj ,numberofhatchedjuvenilesat0day;Ne , number of eggs at 0 day, 250 , ¼ 21 0 0 21 0 0 Ne , 350 eggs/each in 10 ml petri dish. Results are shown as means+s.e.m. (n 3). b,Dosageresponsetotomatorootleachate(TRL)ofin vitro %hatch 0 ¼ of G. rostochiensis.Resultsareshownasmeans+s.e.m. (n 3). TRL was collected from a commercial hydroponic tomato production system ( 500 l of ¼ " nutrient feed solution per day passed through 12,000 plants of tomato in a 4,300 m2 greenhouse). Dilutions refer to the original tomato culture media filtrate preparations. coupling reaction of 19 with 4-methyl-2-trimethylsilyloxy-1,3-pen- product 23 was further converted to 27 by way of 25 in three tadiene successfully occurred by combination of bis[(tris-o-tolyl)- steps (42%). By-products including 26 produced upon reduction phosphine]palladium dichloride and tributyltin fluoride27 in DMF of 24 were recycled to 24 by oxidation with Dess–Martin periodi- resulting in the formation of 20 as a single product in 44% yield. nane (DMP). The crucial intramolecular Diels–Alder reaction occurred in the pres- With the functionalized heptacyclic compound 27 in hand, the ence of dimethylaluminum chloride (Me2AlCl) in ether giving rise to final remaining tasks for the total synthesis of solanoeclepin A the heptacyclic compound 21 in a stereoselective manner. were: (i) oxidative cleavage of the exo-methylene group on the To introduce the oxygen functionality at the C7 position, 21 was four-membered carbon ring to the corresponding ketone; (ii) con- further transformed into triketone 22 in two steps (90%). Oxidation version of the primary alcohol protected with a benzyl group to car- of the C7 methylene group was successfully performed by initial boxylic acid as shown in Fig. 4. treatment of 22 with selenium dioxide (SeO2) in 1,4-dioxane fol- The first task was performed by treatment of 28 with osmium lowed by treatment of the resulting hydroxy ketone with tetroxide (OsO4) followed by oxidative cleavage of the resulting copper(II) acetate in MeOH, giving rise to 23 in 74% yield. The diol with sodium periodate (NaIO4) to give cyclobutanone 30.

NATURE CHEMISTRY | VOL 3 | JUNE 2011 | www.nature.com/naturechemistry 487 Total synthesis of solanoeclepin A

• First total synthesis of solanoeclepin A achieved in 0.18% yield and 52 steps from 3- Me cyclohexenone. (average yield 88.6%) • The synthec sample is only 65% as effecve indicate a cofactor maybe responsible for hatching smulaon. ARTICLES PUBLISHED ONLINE: 23 MAY 2011 | DOI: 10.1038/NCHEM.1050 ARTICLES Synthesis of conolidine, a potentPUBLISHED ONLINE: non-opioid 23 MAY 2011 | DOI: 10.1038/NCHEM.1050 analgesicSynthesis for of conolidine, tonic and persistent a potent non-opioid pain Michaelanalgesic A. Tarselli1,KirstenM.Raehal for tonic2,3 and,AlexK.Brasher persistent1, John M. Streicher pain2,3,ChadE.Groer2,3, Michael D. Cameron2,LauraM.Bohn2,3* and Glenn C. Micalizio1* Michael A. Tarselli1,KirstenM.Raehal2,3,AlexK.Brasher1, John M. Streicher2,3,ChadE.Groer2,3, ManagementMichael D. of Cameron chronic pain2,LauraM.Bohn continues to represent2,3 and an Glenn area of C. great Micalizio unmet1 biomedical need. Although opioid analgesics are typically embraced as the mainstay of pharmaceutical* interventions in this* area, they suffer from substantial liabilities that include addiction and tolerance, as well as depression of breathing, nausea and chronic constipation. Because of their suboptimalManagement therapeutic of chronic profile, pain continues the search to represent for non-opioid an area analgesics of great to unmet replace biomedical these well-established need. Although therapeutics opioid analgesics is an importantare typically pursuit. embraced Conolidine as the mainstayis a rare of C5-nor pharmaceutical stemmadenine interventions natural in product this area, recently they suffer isolated from from substantial the stem liabilities bark of Tabernaemontanathat include addiction divaricata and tolerance,(a tropical as well flowering as depression plant used of breathing, in traditional nausea andChinese, chronic Ayurvedic constipation. and Because Thai medicine). of their Althoughsuboptimal structurally therapeutic related profile, alkaloids the search have for been non-opioid described analgesics as opioid to analgesics, replace these no therapeutically well-established relevant therapeutics properties is an of conolidineimportant have pursuit. previously Conolidine been is reported. a rare C5-nor Here, we stemmadenine describe the natural first de product novo synthetic recently pathway isolated to from this the exceptionally stem bark rare of C5-norTabernaemontana stemmadenine divaricata natural product,(a tropical the flowering first asymmetric plant used synthesis in traditional of any member Chinese, of this Ayurvedic natural andproduct Thai class, medicine). and the discoveryAlthough that structurally (+++++ )-, (1 related)- and alkaloids (2)-conolidine have been are potent described and as efficacious opioid analgesics,non-opioid noanalgesics therapeutically in an relevantin vivo model properties of tonic of andconolidine persistent have pain. previously been reported. Here, we describe the first de novo synthetic pathway to this exceptionally rare C5-nor stemmadenine natural product, the first asymmetric synthesis of any member of this natural product class, and the discovery that (+++++ )-, (1)- and (2)-conolidine are potent and efficacious non-opioid analgesics in an in vivo model of tonic andhe persistent search for pain. the next generation of therapeutics for the treat- of this natural product class has been described, and no robust ment of pain continues to define an active area of scientific source of conolidine has been identified to fuel medicinal evalu- Tpursuit. Among current pharmaceutical interventions, ation. The lack of a source of 1, the scarcity of chemical approaches opioid analgesicshe search for (i.e. the Fig. next 1a) generation represent ofthe therapeutics most widely for embraced the treat-1. suitableof this natural for the synthesis product class of the has C5-nor been stemmadenine described, and skeleton no robust and Unfortunately,ment of these pain continuesagents are to clinically define an problematic active area as of a scientific result of thesource compelling of conolidine drug-like has beenmolecular identified structure to fuel of conolidinemedicinal evalu- motiv- theirT well-establishedpursuit. Among adverse current properties pharmaceutical that include addiction, interventions, tol- atedation. our The initial lack of chemical a source of investigations1, the scarcity aimed of chemical at securing approaches ample 1 erance,opioid depression analgesics (i.e. of breathing, Fig. 1a) represent nausea theand most chronic widely constipation embraced2. quantitiessuitableConolidine for of the this synthesis rare alkaloid-fighng chronic pain of the C5-norby synthetic stemmadenine means. skeleton and HenceUnfortunately, the identification these agents of are effective clinically non-opioid problematic analgesics as a result toof the compelling drug-like molecular structure of conolidine motiv- their well-established adverse properties that include addiction, tol- ated our initial chemical investigations aimed at securing ample replace these well-established therapeutics is widely held as an2 a HO MeO MeO importanterance, depression area of investigation. of breathing, Here, nausea we describeand chronic an interdisciplin- constipation . quantities of this rare alkaloid by synthetic means. aryHence study the that identification has led to the of discovery effective non-opioid that a rare plant-derived analgesics to replace these well-established therapeutics is widely held as an O N Me O N Me O N Me natural product, first isolated in 20043, possesses potent non- a HO MeO MeO important area of investigation. Here, we describe an interdisciplin- opioid analgesic properties and is effective in alleviating chemically H H OH ary study that has led to the discovery that a rare plant-derived N Me O N Me O N Me induced, inflammatory and acute tonic pain.3 Overall, efforts have HOO O O natural product, first isolated in 2004 , possesses potent non- Morphine Hydrocodone Oxycodone definedopioid analgesicthe first propertiesde novo synthetic and is effective pathway in alleviating to an exceptionally chemically H H OH rareinduced, C5-nor inflammatory stemmadenine and natural acute tonic product, pain. and Overall,Common opioid analgesics morphine, hydrocodone and oxycodone. Although extremely effecve, the efforts first asym- have HO O O metric synthesis of any natural product in this class. These advances b defined the first de novo synthetic pathway to anwell-known side effects are obvious such as addicon, depression, nausea and conspaon exceptionally MorphineN HydrocodoneN OxycodoneN have fuelled the discovery that (+)-, ( )- and (2)-conolidine are 5 5 rare C5-nor stemmadenine natural product,þ and the first asym- potent non-opioid analgesics in vivo. b H N H H metric synthesis of any natural product in this class. These advances O H R N N HN N HN haveTabernaemonta fuelled the discovery divaricata thatis a(+ flowering)-, ( )- and tropical (2)-conolidine plant that has are Me Me 5 Me HO 5 been used historically in traditional Chinese,þ Ayurvedic and Thai N potent non-opioid analgesics in vivo. H O1 H 2 H R 3 ConolidineH PericineHN StemmadenineHN medicines,Tabernaemonta with applications divaricata spanningis a flowering treatment tropical of plant fever, that pain, has Me Me Me HO 4 scabies and dysentery . The search for the medicinally relevant (R = CO2Me) been used historically in traditional Chinese, Ayurvedic and Thai 1 2 3 componentsmedicines, with of this applications plant has spanning resulted in treatment the isolation of fever, of a pain, vast Conolidine Pericine Stemmadenine 4 4 arrayscabies of indole and dysentery alkaloids. Thethat searchpossess for diverse the medicinally biological profiles relevant. Figure 1 | Opioid analgesics and stemmadenine-based alkaloids.(R = CO2Me) Conolidinecomponents (1) of is this an plant exceedingly has resulted rare component in the isolation ofConolidine a of Malayan a vast, are rare alkaloids derived from plants used in tradional Chinese medicine. a,Commonopioidanalgesicsmorphine,hydrocodoneandoxycodone. T.array divaricata of indole, isolated alkaloids in only that 0.00014% possess diverse yield from biological theConolidine stem profiles bark4 itself can be isolated in just 0.00014% yield from the stem bark of T. . AlthoughFigure 1 | extremelyOpioid analgesics effective, and well-known stemmadenine-based side effects compromise alkaloids. divaricata. ofConolidine this small flowering(1) is an plant exceedingly (Fig. 1b) rare3. Although component no of biological a Malayan or thea,Commonopioidanalgesicsmorphine,hydrocodoneandoxycodone. therapeutic profile of these agents. b,Conolidine,pericineand medicinalT. divaricata properties, isolated of in conolidine only 0.00014% have yield been from described, the stem other bark stemmadenineAlthough extremely are rare effective, alkaloids well-known derived sidefrom effects plants compromise used in traditional moreof this abundant small flowering alkaloids plant common (Fig. to 1b) this3. Although species have no biological1) been Kam impli-, T.-S., or Chinese,thePand therapeutic, H.-S., AyurvedicChoo profile and, Y.-M. & Komiyama, K of Thai these medicine. agents. b Conolidine,Conolidine,pericineandChem. itself canBiodivers be isolated. 1, 646 (2004). in catedmedicinal as opioid properties analgesics of4,5 conolidine. have been described, other juststemmadenine 0.00014% are yield rare from alkaloids the stem derived bark from of T. plants divaricata used.Thelownatural in traditional moreThe abundant molecular alkaloids structure common of 1 defines to this it as species a member have been of the impli- C5- abundance,Chinese, Ayurvedic established and Thai utility medicine. of T. divaricata Conolidinein traditional itself can medicine be isolated and in norcated stemmadenine as opioid analgesics family4,5 of. natural products, other members of syntheticjust 0.00014% challenge yield posed from the by the stem structure bark of T. of divaricata conolidine.Thelownatural served to motivate whichThe have molecular represented structure significant of 1 defines challenges it as a member to modern of the asym- C5- ourabundance, pursuit ofestablished a programme utility to of accomplishT. divaricata thein traditional first synthetic medicine pathway and to metricnor stemmadenine synthesis. In fact, family no of asymmetric natural products, synthesis other of any members member of conolidinesynthetic challenge and evaluate posed its by biological the structure properties. of conolidine served to motivate which have represented significant challenges to modern asym- our pursuit of a programme to accomplish the first synthetic pathway to metric synthesis. In fact, no asymmetric synthesis of any member conolidine and evaluate its biological properties. 1Department of Chemistry, The Scripps Research Institute, Jupiter, Florida, USA, 2Department of Molecular Therapeutics, The Scripps Research Institute, Jupiter, Florida, USA, 3Department of Neuroscience, The Scripps Research Institute, Jupiter, Florida, USA. *e-mail: [email protected]; [email protected] 1Department of Chemistry, The Scripps Research Institute, Jupiter, Florida, USA, 2Department of Molecular Therapeutics, The Scripps Research Institute, 3 NATUREJupiter, CHEMISTRY Florida, USA,| VOLDepartment 3 | JUNE 2011 of | Neuroscience,www.nature.com/naturechemistry The Scripps Research Institute, Jupiter, Florida, USA. *e-mail: [email protected]; [email protected]

NATURE CHEMISTRY | VOL 3 | JUNE 2011 | www.nature.com/naturechemistry 449 ARTICLES PUBLISHED ONLINE: 23 MAY 2011 | DOI: 10.1038/NCHEM.1050

Synthesis of conolidine, a potent non-opioid analgesic for tonic and persistent pain Michael A. Tarselli1,KirstenM.Raehal2,3,AlexK.Brasher1, John M. Streicher2,3,ChadE.Groer2,3, Michael D. Cameron2,LauraM.Bohn2,3* and Glenn C. Micalizio1*

Management of chronic pain continues to represent an area of great unmet biomedical need. Although opioid analgesics are typically embraced as the mainstay of pharmaceutical interventions in this area, they suffer from substantial liabilities that include addiction and tolerance, as well as depression of breathing, nausea and chronic constipation. Because of their suboptimal therapeutic profile, the search for non-opioid analgesics to replace these well-established therapeutics is an important pursuit. Conolidine is a rare C5-nor stemmadenine natural product recently isolated from the stem bark of Tabernaemontana divaricata (a tropical flowering plant used in traditional Chinese, Ayurvedic and Thai medicine). Although structurally related alkaloids have been described as opioid analgesics, no therapeutically relevant properties of conolidine have previously been reported. Here, we describe the first de novo synthetic pathway to this exceptionally rare C5-nor stemmadenine natural product, the first asymmetric synthesis of any member of this natural product class, and the discovery that (+++++ )-, (1)- and (2)-conolidine are potent and efficacious non-opioidSynthec design and total synthesis analgesics in an in vivo model of tonic and persistent pain. ARTICLES NATURE CHEMISTRY DOI: 10.1038/NCHEM.1050 he search for the next generation of therapeutics for the treat- of this natural product class has been described, and no robust ment of pain continues to define an active area of scientific sourcea of conolidine has been identified to fuel medicinalExcision evalu- of C5 Tpursuit. Among current pharmaceutical interventions, ation. The lack of a source of 1, the scarcity of chemical approaches opioid analgesics (i.e. Fig. 1a) represent the most widely embraced1. suitable for the synthesis of the C5-nor stemmadenine skeletonH and N 5 (1) H O N N Unfortunately, these agents are clinically problematic as a result of the compelling drug-like molecular2 2 structure of conolidine motiv- (2) (CF CO) O, –5 °C R N their well-established adverse properties that include addiction, tol- atedH ourR initial chemical investigations3 2 aimed atH securingR ample H H 2 HN then dilute NaOH N HO erance, depression of breathing, nausea and chronic constipation . quantitiesMe HO of this rare alkaloid by synthetic means. Me R2O Me Hence the identification of effective non-opioid analgesics to 3 4 5 replace these well-established therapeutics is widely held as an a HO MeO MeO important area of investigation. Here, we describe an interdisciplin- b H PMB H N + N N ary study that has led to the discovery that a rare plant-derived N MeN N Me N N20Me 3 1 O + O O 15 natural product, first isolated in 2004 , possesses potent non- 19 HO HO H H N H R Me H 19 opioid analgesic properties and is effective in alleviating chemically H O H H OH Me O N Me Me induced, inflammatory and acute tonic pain. Overall, efforts have HO Me O O H defined the first de novo synthetic pathway to an exceptionally Morphine 6 Hydrocodone Oxycodone 7 (S)-8 9 rare C5-nor stemmadenine natural product, and the first asym- Figure 2 | Development of a synthesis strategy for conolidine inspired by the biosynthetic proposal for the conversion of stemmadenine to vallesamine. metric synthesis of any natural product in this class. These advances b a,Biomimeticsemisynthesisofvallesamine(N 5N) from5 stemmadenine (3). Nb,Retrosynthesisofconolidine(5 1) that features a cyclization process supported by have fuelled the discovery that (+)-, ( )- and (2)-conolidine arethe minimization of A-1,3 strain (7 1)—astrategyinspiredbytheconformationalconstraintslikelyoperativeinthecyclizationof4 (a). þ N ! potent non-opioid analgesics in vivo. H O H H R H HN HN Tabernaemonta divaricata is a flowering tropical plant that has Me Me Me PMB HO PMB PMB been used historically in traditional Chinese, Ayurvedic and Thai 1 2 3 N N (1) KH, Bu3SnCH2I N N (1) 10, Dess-Martin medicines, with applications spanning treatment of fever, pain, ConolidinePMBCl, DCE, reflux Pericine Stemmadenine THF periodinane, DCM/H O 20 + 2 4 HO HO (R = CO Me) Me scabies and dysentery . The search for the medicinally relevant 2 19 (2) 12, THF then, NaBH4, MeOH (2) n-BuLi, THF 57% 76% components of this plant has resulted in the isolation of a vast Me Bohn L. M.; MicalizioMe G. C. etc. Nature ChemistryMe OH, 2011, 3, 449. OH 4 Li array of indole alkaloids that possess diverse biological profiles . Figure 1 | Opioid analgesics and stemmadenine-based alkaloids. N 9 810 (E:Z = 12:1) 11 12 Conolidine (1) is an exceedingly rare component of a Malayan a,Commonopioidanalgesicsmorphine,hydrocodoneandoxycodone. PhO2S T. divaricata, isolated in only 0.00014% yield from the stem bark Although extremely effective, well-known side effects compromise 3 PMB of this small flowering plant (Fig. 1b) . Although no biological or the therapeutic profile of these agents. b,Conolidine,pericineandH medicinal properties of conolidine have been described, other stemmadenineN are rare alkaloids derived fromN plants used in traditional (1) Na(Hg) amalgam N CF CO H, then N more abundant alkaloids common to this species have been impli- Chinese, Ayurvedic and(2) MnO Thai2, medicine.DCM Conolidine itself can be isolated3 2 in + 4,5 N cated as opioid analgesics . just 0.00014% yield(3) from ACE-Cl, the stem DCE, bark of T. divaricata.Thelownatural H N H Me N Me (CH2O)n, CH3CN H O H O H HO 84 °C O N The molecular structure of 1 defines it as a member of the C5- abundance, established utility of T. divaricata in traditional medicine80 and °C Me Me PhO S (43% over 5 steps) H a nor stemmadenine family of natural products, other members of synthetic2 challenge posed by the structure of conolidine served to95% motivate 1 13 7 (±)-Conolidine which have represented significant challenges to modern asym- our pursuit of a programme to accomplish the first synthetic pathway to 9-steps from 9 metric synthesis. In fact, no asymmetric synthesis of any member conolidine and evaluate its biological properties. 18% overall yield

Figure 3 | Execution of a synthesis pathway to conolidine (1). Synthesis of (+)-conolidine (1)isaccomplishedinjustninestepsand18%overallyieldfrom 1Department of Chemistry, The Scripps Research Institute, Jupiter, Florida, USA,the commercially2Department available of Molecular pyridine Therapeutics,9.AStill’sallylstannylmethylether-based[2,3]-Wittigrearrangementisusedasakeystereoselectivetransformationin The Scripps Research Institute, this pathway that culminates in the acid-mediated conversion of 7 to conolidine 1. aYield reported is for the preparation of the 1 H SO salt (the compound Jupiter, Florida, USA, 3Department of Neuroscience, The Scripps Research Institute, Jupiter, Florida, USA. *e-mail: [email protected]; [email protected] 2 2 4 used in subsequent in vivo experiments); spectral data reported in the Supplementary Information are of the free base of 1. PMB, p-methoxybenzyl; DCE, 1,2-dichloroethane; DCM, dichloromethane; ACE-Cl, a-chloroethyl chloroformate. NATURE CHEMISTRY | VOL 3 | JUNE 2011 | www.nature.com/naturechemistry 449

Results and discussion stability to the potentially unstable b,g-unsaturated ketone 7 (ref. 9). At the outset of our studies, two anticipated challenges guided the The preparation of substrate 7 was then deemed possible from conception of a chemical pathway to this class of natural products: allylic alcohol 8, and ultimately from the readily available pyridine 9. (1) formation of a strained 1-azabicyclo[4.2.2]-decane and (2) As depicted in Fig. 3a, efforts commenced with commercially stereochemical control in the generation of the exocyclic available pyridine 9. Initial N-alkylation, followed by hydride trisubstituted alkene. reduction, provided 8 in 57% yield. With precedent firmly estab- Our retrosynthetic planning began with an examination of the lished for stereoselective Claisen rearrangement of related substrates biosynthetic pathway for conversion of stemmadenine (3) to valles- providing the undesired alkene isomer11,12, an unusual stereoselective amine (5), a proposal later realized in vitro by Scott and colleagues transformation of 8 was required to establish the desired alkene (Fig. 2a)6,7. Here, excision of the C5 carbon occurs by a sequence of geometry at C19–C20. Although our initial experiments aimed at (1) N-oxidation, (2) fragmentation, (3) hydrolysis and (4) cycliza- accomplishing such a stereoselective transformation focused on the tion (through 4)8. application of reductive cross-coupling chemistry between 8 and a The ease with which cyclization takes place in this system is prob- suitably functionalized alkyne13, these studies encountered unantici- ably influenced by a conformational preference imparted by the pated failure as a result of low levels of reactivity associated with the stereodefined C19–C20 (E)-ethylidene unit, thereby providing a allylic alcohol component in this class of metallacycle-mediated con- bias to support cyclization through 4 (Fig. 2a). With this as a vergent coupling reaction. We sought to identify a stereoselective guiding principle, we speculated that conolidine (1) could derive transformation suitable for the conversion of allylic alcohol 8 to a from iminium ion 6 through a related ring closure by means of a functionalized (E)-exo-ethylidine-containing product. conformation that would be similarly stabilized by minimization With this goal in mind, we turned our attention to Still’s allyl of allylic strain (Fig. 2b)9. While providing a conformational bias stannylmethyl ether-based [2,3]-Wittig rearrangement14. Whereas to facilitate the establishment of the 1-azabicyclo[4.2.2]-core, the few reports describe the application of this reaction to the establish- stereochemistry of this alkene was expected to impart kinetic ment of stereodefined exo-alkylidene cycloalkanes15–17, the unique

450 NATURE CHEMISTRY | VOL 3 | JUNE 2011 | www.nature.com/naturechemistry ARTICLES NATURE CHEMISTRY DOI: 10.1038/NCHEM.1050

a Excision of C5

H N N 5 (1) H2O2 N (2) (CF CO) O, –5 °C R N H R 3 2 H R H H HN then dilute NaOH N HO Me HO Me R2O Me 3 4 5

b H PMB N H + N N 20 N N 1 + 15 19 HO HO N H H O H R Me H 19 H O N Me Me H Me Me 6 7 (S)-8 9

Figure 2 | Development of a synthesis strategy for conolidine inspired by the biosynthetic proposal for the conversion of stemmadenine to vallesamine. a,Biomimeticsemisynthesisofvallesamine(5) from stemmadenine (3). b,Retrosynthesisofconolidine(1) that features a cyclization process supported by the minimization of A-1,3 strain (7 1)—astrategyinspiredbytheconformationalconstraintslikelyoperativeinthecyclizationofTotal synthesis of conolidine 4 (a). !

PMB PMB PMB

N N (1) KH, Bu3SnCH2I N N (1) 10, Dess-Martin PMBCl, DCE, reflux THF 20 periodinane, DCM/H2O HO HO + Me 19 (2) 12, THF then, NaBH4, MeOH (2) n-BuLi, THF Me 57% Me 76% Me OH OH Li N 9 810 (E:Z = 12:1) 11 12 PhO2S

PMB H N N (1) Na(Hg) amalgam N CF CO H, then N (2) MnO2, DCM 3 2 + N (3) ACE-Cl, DCE, H N H Me N Me (CH2O)n, CH3CN H O H O H 84 °C HO O N 80 °C Me Me PhO S H 2 (43% over 5 steps) 95%a 1 13 7 (±)-Conolidine NATURE CHEMISTRY9 steps DOI: 10.1038/NCHEM.10509-steps from 9 ARTICLES 18% overall yield 18% overall yield NATURE CHEMISTRY DOI: 10.1038/NCHEM.1050 ARTICLES 16. Wang, X. J., Hart, S. A., Xu, B., Mason, M. D., Goodell, F. R. & Etzkorn, F. A. Figure 3 | Execution of a synthesis pathway to conolidine (1). Synthesis of (+)-conolidineTable 1 | Relative (1)isaccomplishedinjustninestepsand18%overallyieldfrom drug potencies in both phases of the Serine-cis-proline and serine-trans-proline isosteres: stereoselective synthesis of formalin test (i.p.; 95% confidence interval) from data the commercially availablePMB pyridine 9.AStill’sallylstannylmethylether-based[2,3]-WittigrearrangementisusedasakeystereoselectivetransformationinPMB (Z)- and (E)-alkene mimics by Still–Wittig and Ireland–Claisen rearrangements. (±)-8 presenteda in Fig. 5c,d. 1 J. Org. Chem. 68, 2343–2349 (2003). N this pathway that culminates in the acid-mediated conversion of 7 to conolidine 1. Yield reportedN is for the preparation of the 2 H2SO4 salt (the compound 8-stepsb N N 8-stepsb 17. Fristad, W. E. & Paquette, L. A. Oxidation chemistry of a cis-2-(3- used in subsequent in vivo experiments); spectral data reported in the Supplementary Information are of the free baseRelative of 1 potencies. PMB, p-methoxybenzyl; (ED ,mgkg21) DCE, HO HO N 50 flurylidene)ethanol. Heterocycles 31, 2219–2224 (1990). N H H H O 1,2-dichloroethane; DCM, dichloromethane;Amano lipase ACE-Cl, PS a-chloroethyl chloroformate.DrugO H Phase 1 Phase 2 18. Le Bars, D., Gozariu, M. & Cadden, S. W. Animal models of nociception. vinyl acetate/hexanes Me Me Me Me (2)-Conolidine sulfate 5.6 (4.0–7.8) 6.0 (3.7–9.7) Pharmacol. Rev. 53, 597–652 (2001). 4 Å mol. sieves, 18 h 19. Collier, H. O. J., Dinneedn L. C., Johnson, C. A. & Schneider, C. The abdominal (–)-1 40%a 39% Morphine sulfate(+)-1 4.6 (3.3–6.4) 2.4 (1.7–3.2) Results and discussion stability to the potentially unstable b,g-unsaturated ketone 7 (ref. 9). constriction response and its suppression by analgesics in the mouse. Br. J. 90% e.e. ( 92% e.e.) ( 92% e.e.) 90% e.e. ≥ At the outset≥ of our studies, two anticipated challenges≥ guided the The preparation≥ of substrate 7 was then deemed possible from Pharmac. Chemother. 32, 295–310 (1968). conception of a chemical pathway to this class of natural products: syntheticallylic alcohol pathway8, toand any ultimately C5-nor stemmadenine. from the readily With available an efficient pyridine20.9. Mogil, J. S., Kest, B., Sadowski, B. & Belknap, J. K. Differential genetic mediation of sensitivity to morphine in genetic models of opiate antinociception: influence Figure 4 | Asymmetric synthesis of ((1)1)- and formation (2)-conolidine. of aResolution strainedof 1-azabicyclo[4.2.2]-decane racemic 8,bylipase-catalysedacylation,definesarobustmeanstoaccess and (2) sourceAs of depicted this alkaloid in Fig. secured 3a, efforts by synthetic commenced means with (nine commercially steps, optically active samples of either antipode of conolidine (1). aAfter hydrolysis. bApart from the conditions for oxidation of an intermediate primary alcohol, of nociceptive assay. J. Pharmacol. Exp. Ther. 276, 532–544 (1996). stereochemical control in the generation of the exocyclic 18%available overall pyridine yield), the9. production Initial N-alkylation, of sufficient followedquantities by of the hydride21. Dubuisson, D. & Dennis, S. G. The formalin test: a quantitative study of the this eight-step sequence is identical totrisubstituted that describedalkene. in Fig. 3. naturalreduction, product provided (and related8 in 57% antipode) yield. wasWith followed precedent bythe firmly first estab- analgesic effects of morphine, meperidine, and brain stem stimulation in rats Our retrosynthetic planning began with an examination of the evaluationlished for of stereoselective this alkaloid Claisenin vivo. rearrangement These combined of studies related substrateshave and cats. Pain 4, 161–174 (1977). stereochemical course of this transformation in acyclic settings pad produces a biphasic pain responseresulted that in is the demonstrated discovery that by the conolidine11,12 (1) is a potent non- 22. McNamara, C. R. et al. TRPA1 mediates formalin-induced pain. Proc. Natl Acad. biosynthetic pathway for conversion of stemmadenine (3) to valles- providing the undesired alkene isomer , an unusual stereoselective Sci. USA 104, 13525–13530 (2007). suggested its potential utility in solvingamine ( the5), aproblem proposal at later hand. realized inanimals vitro by attending Scott and to colleaguesand licking theopioidtransformation injected analgesic paw (nocifensive that of 8 is effectivewas required beha- at alleviating to establish chemically the desired induced, alkene23. Woolf, C. J. Evidence for a central component of post-injury pain 21 As illustrated in Fig. 3, allylic alcohol(Fig. 2a)8 6,7was. Here, converted excision to its of corre- the C5 carbonviours) occurs. Responses by a sequence to the of initialacutegeometry phase and (Phase persistent at C19–C20. 1) occur tonic Althoughpain. within Further our initial studies experiments in neuropathic aimed at hypersensitivity. Nature 306, 686–688 (1983). sponding tributylstannylmethyl ether,(1) N-oxidation, and subsequently (2) fragmentation, exposed to (3)the hydrolysis first 10 min and of formalin (4) cycliza- injectionpainaccomplishing and models reflect will the such be direct undertaken a stereoselective chemical to determine transformation more widespread focused on the24. Tjølsen, A., Berge, O. G., Hunskaar, S., Rosland, J. H. & Hole, K. The formalin therapeutic promise22 for the treatment of chronic pain. Finally, test: an evaluation of the method. Pain 51, 5–17 (1992). n-BuLi. The products from the [2,3]-Wittigtion (through rearrangement4)8. were gen- action at nociceptive primary afferentapplication nerve offibres reductive. Following cross-coupling a chemistry between 8 and a although determination of the pharmacological mechanism of 25. Li, X., Kamenecka, T. M. & Cameron, M. D. Bioactivation of the epidermal erated in 76% yield (over two steps), favouring the formation of the brief intermediary phase (10–20 min after formalin injection)13 in growth factor receptor inhibitor gefitinib: implications for pulmonary and The ease with which cyclization takes place in this system is prob- actionsuitably associated functionalized with the alkyne potent, these analgesic studies properties encountered of unantici- this desired isomer 10 (10:11 12:1).ably Oxidation influenced to the by corresponding a conformationalwhich preference no pain imparted response by occurs, the apated second failure pain as response a result (Phase of low 2)levels of reactivity associated with the hepatic toxicities. Chem. Res. Toxicol. 22, 1736–1742 (2009). ¼ alkaloid remains an area of intense current investigation, the 26. Wess, J. et al. Muscarinic receptor subtypes mediating central and peripheral b,g-unsaturated aldehyde with Dess–Martinstereodefined periodinane, C19–C20 and (E nucleo-)-ethylidenebecomes unit, thereby evident providing (20–40 min a afterallylic formalin alcohol component injection) during in this class of metallacycle-mediated con- philic addition of the organolithium reagent 12, then gave a mixture of which the animal once again attendsresults toof thethese injected studies paw. mark This the establishment of a chemical antinociception studied with muscarinic receptor knockout mice: a review. Life bias to support cyclization through 4 (Fig. 2a). With this as a vergent coupling reaction. We sought to identify a stereoselective Sci. 72, 2047–2054 (2003). stereoisomeric alcohols 13 that was subsequently advanced to the second phase models a persistentfoundation pain state suitable that is believedfor investigating to be the therapeutic potential of guiding principle, we speculated that conolidine (1) could derive thistransformation unique alkaloid suitable as a potent for the non-opioid conversion analgesic. of allylic alcohol 8 to27. a Stone, L. S., Fairbanks, C. A. & Wilcox, G. L. Moxonidine, a mixed alpha(2)- cyclization substrate 7 by a simple three-step sequence. due in part to injury-provoked sensitization of central nervous adrenergic and imidazoline receptor agonist, identifies a novel adrenergic target from iminium ion 6 through a related ring closure by means of a functionalized (E)-exo-ethylidine-containing product. Conversion of amino ketone 7 to its corresponding trifluoroace- system neurons and to inflammatory factors18,23,24. The application for spinal analgesia. Ann. NY Acad. Sci. 1009, 378–385 (2003). conformation that would be similarly stabilized by minimization ReceivedWith 16 this December goal in 2010; mind, accepted we turned 4 April our 2011; attention to Still’s allyl tate salt, followed by treatment with paraformaldehyde in aceto- of local anaesthetics, such as lidocaine, suppresses the first phase of 28. Jann, M. W. & Slade, J. H. Antidepressant agents for the treatment of chronic of allylic strain (Fig. 2b)9. While providing a conformational bias publishedstannylmethyl online 23 ether-based May 2011 [2,3]-Wittig rearrangement14. Whereas pain and depression. Pharmacotherapy 27, 1571–1587 (2007). nitrile at 80 8C, delivered (+)-conolidine 1 in 95% yield. The the response, whereas non-steroidal anti-inflammatory drugs are 29. Shimoyama, N., Shimoyama, M., Davis, A. M., Inturrisi, C. E. & Elliot, K. J. to facilitate the establishment of the 1-azabicyclo[4.2.2]-core, the Referencesfew reports describe the application of this reaction to the establish- success of this process validated our proposed strategy to access effective in blocking the second phase. Few drugs, such as the 15–17 Spinal gabapentin is antinociceptive in the rat formalin test. Neurosci. Lett. 222, stereochemistry of this alkene was expected to impart kinetic 1.ment Melnikova, of stereodefined I. The pain market. exo-alkylideneNat. Rev. Drug Discov. cycloalkanes9, 589–590 (2010)., the unique 65–67 (1997). the strained azabicyclo[4.2.2] core of the C5-nor stemmadenine opioid narcotics, are effective in2. suppressing Crofford, L. J. Adverseboth phases effects of of chronic the opioid therapy for chronic family of natural products, and delivered the first synthetic sample pain response18. 30. Hunter, J. C. et al. The effect of novel anti-epileptic drugs in rat 450 musculoskeletalNATURE pain. CHEMISTRYNat. Rev.|Rheum. VOL 3 |6, JUNE191–197 2011 | (2010). www.nature.com/naturechemistry experimental models of acute and chronic pain. Eur. J. Pharmacol. 324, of conolidine by a synthesis pathway that proceeds in just nine Using a well-characterized mouse3. Kam, line T.-S., in Pand, this H.-S., particular Choo, Y.-M. assay, & Komiyama, K. Biologically active ibogan 153–160 (1997). steps and 18% overall yield from a readily available pyridine. the sulfate salts of (+)-, ( )- and (2and)-conolidine vallesamine derivatives were administered from Tabernaemontana divaricata. Chem. Biodivers. 31. Munro, G. Pharmacological assessment of the rat formalin test utilizing the 21 þ 1, 646–656 (2004). m clinically used analgesic drugs gabapentin, lamotrigine, morphine, duloxetine, Building on the successful synthesis of racemic 1, we explored the (10 mg kg , i.p.) 15 min before4. formalin Pratchayasakul, (5%, 25 W., Pongchaidecha,l, intraplantar) A., Chattipakorn, N. & Chattipakorn, S. 20 tramadol and ibuprofen: influence of low and high formalin concentrations. potential of this route to accomplish the first asymmetric synthesis challenge . Compared with vehicle,Ethnobotany all three & forms ethnopharmacology of conolidine of Tabernaemontana divaricata. Indian J. Eur. J. Pharmacol. 605, 95–102 (2009). of a C5-nor stemmadenine alkaloid. As illustrated in Fig. 4, resol- sulfate reduce the display of nocifensiveMed. Res. behaviours127, 317–335 in (2008). both Phase 32. Baillie, J. K. & Power, I. The mechanism of action of gabapentin in neuropathic ution of (+)-8 with a commercially available lipase provided opti- 1(P , 0.001) and Phase 2 (P , 0.001,5. Ingkaninan, two-way K., analysis Ijzerman, of A. varianceP., Taesotikul, T. & Verpoorte, R. Isolation of pain. Curr. Opin. Investig. Drugs 7, 33–39 (2006). cally enriched substrates ( 92% e.e.) that were advanced to ( )- (ANOVA)) (Fig. 5b). Further, the unnaturalopioid-active antipode compounds ( from2)-conoli-Tabernaemopntana pachysiphon leaves. 33. Goldstein, A. & Sheehan, P. Tolerance to opioid narcotics. I. Tolerance to the J. Pharm. Pharmacol. 51, 1441 (1999). “running fit” caused by levorphanol in the mouse. J. Pharmacol. Exp. Ther. 169, or (2)-conolidine ( 90%≥ e.e.) by a simple eight-step sequence.þ dine is effective in a dose-dependent manner (Fig. 5c), displaying ≥ 6. Scott, A. I., Yeh, C.-L. & Greenslade, D. Laboratory model for the biosynthesis of 175–184 (1969). Here, the only preparative modification required was a substitute potency similar to that shown forvallesamine, morphine apparicine, sulfate (seeand related Fig. alkaloids. 5d J. Chem. Soc. Chem. Commun. 34. Wise, R. A. & Bozarth, M. A. A psychomotor stimulant theory of addiction. for the Dess–Martin oxidation used to generate the b,g-unsaturated and Table 1)20. 947–948 (1978). Psychol. Rev. 94, 469–492 (1987). 7. Lim, D-H., Low, T-Y. & Kam, T-S. Biomimetic oxidative transformations of aldehyde en route to enantio-enriched carbinol 13. Whereas To investigate the duration of drugpericine: action, partial (2)-conolidine synthesis of apparicine sulfate and valparicine, a new pentacyclic indole Dess–Martin oxidation of optically active samples of 10 resulted was administered at increasing timealkaloid intervals from beforeKopsia. formalin Tetrahedron chal- Lett. 47, 5037–5039 (2006). Acknowledgements in inconsistent partial racemization, a modified Parikh–Doering lenge. Whereas one-way ANOVA8. reveals Ahond, A., no Cave time´, A., effect Kan-Fan, for C., either Langlois, Y. & Potier, P. The fragmentation of We gratefully acknowledge T.-S. Kam (University of Malaya, Kuala Lumpur, Malaysia) for providing authentic spectra of natural ( )-conolidine for comparison with our synthetic oxidation ensured a reproducible pathway to enantio-defined phase (P . 0.05), subsequent Student’sN,N-dimethyltryptaminet-test analysis reveals oxide and that related compounds: a possible implication þ in indole alkaloid biosynthesis. J. Chem. Soc. Chem. Commun. 517 (1970). samples (see Supplementary Information). samples of either enantiomer of conolidine. although conolidine significantly9. attenuates Hoffmann, the R. W. pain Allylic response 1,3-strain com- as a controlling factor in stereoselective With a concise asymmetric chemical pathway established, and pared with vehicle at 15 and 30 mintransformations. before injectionChem. in PhaseRev. 89, 1,1841–1860 and (1989). Author contributions ´ ´ ´ the first synthetic samples of ( )-, (2)- and (+)-conolidine in at 15 min before that in Phase 2,10. it was Bennasar, less efficaciousM.-L., Zulaica, when E., Sole admi-, D., Roca, T., Garcıa-Dıaz, D. & Alonso, S. G.C.M. conceived, initiated and directed the project. M.A.T. and A.K.B. conducted all þ Total synthesis of the bridged indole alkaloid apparicine. J. Org. Chem. 74, hand, hundreds of milligrams of synthetic material was prepared nistered 1 h before formalin challenge in Phase 1 or 30–60 min chemical experiments. L.M.B. initiated and directed the in vivo and in vitro 8359–8368 (2009). and our attention shifted to exploring the potential analgesic prop- before that in Phase 2 (Fig. 5e). pharmacological evaluation. L.M.B. and M.D.C. directed the pharmacokinetic 11. Amat, M., Dolors Coll, M., Passarella, D. & Bosch, J. An enantioselective experiments, and K.M.R. and C.G. conducted all biochemical and in vivo experiments. erties of these substances in vivo. Taken together, these data demonstrate that, like opioid analge-2 synthesis of the Strychnos alkaloid ( )-tubifoline. Tetrahedron: Asymmetry 7, Receptor binding profiles were generously provided by the National Institute of Mental Because related members of this natural product class have been sics, conolidine is effective in suppressing2775–2778 (1996). responses to both Health’s Psychoactive Drug Screening Program, Contract no. HHSN-271-2008-00025-C 12. Amat, M., Dolors Coll, M., Bosch, J., Espinosa, E. & Molins, E., Total syntheses postulated to have opioid properties4,5, we assessed the efficacy of chemically induced and inflammation-derived pain20. Further, (NIMH PDSP). The NIMH PDSP is directed by Bryan L. Roth (MD, PhD) at the University of the Strychnos indole alkaloids (2)-tubifoline, (2)-tubifolidine, and (2)- conolidine in several nociceptive tests in C57BL/6J mice. Unlike pharmacokinetic measures demonstrate that conolidine is present of North Carolina at Chapel Hill and Project Officer Jamie Driscol at NIMH, Bethesda, 19,20-dihydroakuammicine. Tetrahedron: Asymmetry 8, 935–948 (1997). Maryland, USA. G.C.M. and L.M.B. wrote the manuscript. opioids, ( )- and (2)-conolidine sulfate (1) do not produce antino- in the brain and plasma at relatively13. Kolundzic, high concentrationsF. & Micalizio, G. C. during Synthesis of substituted 1,4-dienes by direct þ ciception in the hot plate (51 8C) or in the warm water tail immer- the period of its analgesic efficacy andalkylation remains of allylic present alcohols. upJ. Am. to 4Chem. h Soc. 129, 15112–15113 (2007). 21 14. Still, W. C. & Mitra, A. A highly stereoselective synthesis of Z-trisubstituted Additional information sion assay (49 8C) following systemic injection of 10 mg kg , after injection (Fig. 5f)25. olefins via [2,3]-sigmatropic rearrangement. Preference for a pseudoaxially The authors declare no competing financial interests. Supplementary information and intraperitoneally (i.p.) (Supplementary Fig. S1). However, in a To further demonstrate that conolidinesubstituted transition differs state. fromJ. Am. opioid Chem. Soc. 100, 1927–1928 (1978). chemical compound information accompany this paper at www.nature.com/ visceral pain model, the acetic acid abdominal constriction analgesics, pharmacological studies15. Hart, were S. A.,performed. Trindle, C. O. On & Etzkorn, the basis F. A. Solvent-dependent stereoselectivity in naturechemistry. Reprints and permission information is available online at http://www. assay18, conolidine (10, 20 mg kg21) proved to be efficacious in of b-arrestin translocation assays, radioliganda Still–Wittig binding rearrangement: displacement an experimental and ab initio study. Org. Lett. 3, nature.com/reprints/. Correspondence and requests for materials should be addressed to L.M.B. preventing the acetic acid-induced writhing response (Fig. 5a). and G-protein coupling assays,1789–1791 conolidine (2001). has no affinity and G.C.M. Therefore, conolidine differs from morphine in that it does not for or efficacy at the m-opioid receptor, the primary target promote antinociception to acute thermal stimulation; however, of morphine (Supplementary Fig. S2 and Table S1). Moreover, like morphine, and a host of known analgesics, it does suppress radioligand displacement and b-arrestin translocation studies also the acetic acid-induced writhing response19,20. demonstrate a lack of affinity or efficacy for k- and d-opioid recep- NATURE CHEMISTRY | VOL 3 | JUNE 2011 | www.nature.com/naturechemistry 453 A pain model designed to assess both acute tonic and persistent tors (Supplementary Fig. S2a and Table S1). pain responses was also used to ascertain conolidine’s analgesic effi- In an attempt to determine the biological target of conolidine, we cacy. The injection of a dilute formalin solution into a rodent’s paw subjected (+)-conolidine sulfate to screening by the Psychoactive

NATURE CHEMISTRY | VOL 3 | JUNE 2011 | www.nature.com/naturechemistry 451 ARTICLES PUBLISHED ONLINE: 19 JUNE 2011 | DOI: 10.1038/NCHEM.1072

An efﬁcient synthesis of loline alkaloids Mesut Cakmak, Peter Mayer and Dirk Trauner*

Loline (1) is a small alkaloid that, in spite of its simple-looking structure, has posed surprising challenges to synthetic chemists. It has been known for more than a century and has been the subject of extensive biological investigations, but only two total syntheses have been achieved to date. Here, we report an asymmetric total synthesis of loline that, with less then ten steps, is remarkably short. Our synthesis incorporates a Sharpless epoxidation, a Grubbs olefin metathesis and an unprecedented transannular aminobromination, which converts an eight-membered cyclic carbamate into a bromopyrrolizidine. The synthesis is marked by a high degree of chemo- and stereoselectivity and gives access to several members of the loline alkaloid family. It delivers sufficient material to support a programme aimed at studying the complex interactions between plants, fungi, insects and bacteria brokered by loline alkaloids. fficiency continues to be one of the greatest challenges in total biosynthesis has been investigated in detail and the gene cluster synthesis. It can be addressed at the level of individual steps, for accounting for their production has recently been cloned9. Eexample through high-yielding and atom-economic reactions1, Despite their long history and intriguing biological activities, few or at the strategic level, for example through highly step-economic successful syntheses of loline alkaloids have been reported to date. synthetic schemes2. These approaches are closely intertwined, This may be due to their strained, heterotricyclic molecular skeleton, because new reactions allow for the redefinition of strategies, and which incorporates polar functionalities in close proximity and strategic needs often provoke the invention of new reactions. makes the lolines more challenging than they appear at first sight. Certain rules have emerged to maximize strategic efficiency. For The molecules feature a pyrrolizidine and a morpholine ring instance, protecting groups should be circumvented, or should be system, as well as four stereocentres, only one of which is indepen- cleaved in the course of strategic bond-forming operations3,4. If una- dent. As such, they are about as complex as oseltamivir (Tamiflu)12, voidable, they should at least become a part of the molecule after epibatidine13 or kainic acid14, which have received much attention in they have fulfilled their defensive role. Oxidation state adjustments the synthetic community. Loline itself has also been the subject of that are not directed towards the final oxidation state of the target several synthetic approaches, two of which were abandoned due molecule should also be avoided5. Generally, corrective measures to an inability to introduce the secondary amine at C1 via nucleo- should be kept to a minimum. All this requires highly selective philic substitution15,16. In 1986, Tufariello published a racemic syn- (in particular chemo- and stereoselective) reactions, which need to thesis of the alkaloid that was based on a nitrone cycloaddition17. be optimally arranged to make a synthesis as short and practical This was followed 14 years later by the first, and thus far only, asym- as possible. metric synthesis, which required 20 steps to reach the target mol- Loline (1), a small alkaloid that has been known for decades, ecule and establishedLoline hetero Diels–Alder alkoilds chemistry-new bole with old wine as an effective represents an interesting target for testing the state of synthetic effi- way to synthesize complex pyrrolizidines18,19. Very recently, a ciency (Fig. 1). It is the eponymous member of a family of racemic synthesis of N-acetyl norloline (3) has been reported20. alkaloids that were initially isolated from tall fescue grasses but were later found in many other plant families. Its congener temu- O line (2) was isolated from Lolium temulentum in 1892 and sub- 6,7 NH NH2 NH sequently renamed norloline . N-Acetyl norloline (3) occurs in 1 NHMe H Festuca arundinancea, together with several other loline members8. O 1 O O • Loline has been known over 100 years N O N N • Heterocyclic structure Its biosynthetic derivative N-formyl loline (4) is the most abundant N loline alkaloid. N-Senecioyl norloline (5) was found in the urine • No specific bioacvity reported of horses, which enjoy grazing on tall fescue grass9,10.Thestructure Loline (1) Temuline (2) -acetyl • Two total synthesis reported (norloline) norloline (3) of the unusual chlorine-containing alkaloid lolidine (6) was proposed based on mass-spectrometric data, but the compound could not be OH O 11 Cl H NHAc further evaluated due to a scarcity of material . N NH Introducon: Ideal synthesis (Baran) The loline alkaloids play a remarkable range of biological roles, O O many of which are poorly understood and are still being unravelled. O O N Ø Atom-economic (Trost, 1991) N Produced by endophytic fungi, these widely distributed alkaloids N N Ø Step-economic (Wender, 2009) appear to provide chemoprotection to their plant hosts, primarily O Ø Protecng group free (Baran, 2007) N against certain types of insects and aphids. Although most loline -formyl -senecioyl Lolidine (6) Ø Redox-economic (Baran, 2009) alkaloids are as toxic to these animals as nicotine, they seem to loline (4) norloline (5) have relatively few negative effects on mammalian herbivores, including horses. In fact, the toxic affects occasionally associated Figure 1 | Loline alkaloids. Members of the loline family of alkaloids have a with tall fescue have been traced to ergot alkaloids9. Emphasizing common heterotricyclic core and are distinguished by the substitution the interest that lolines have generated among biologists, their pattern at the nitrogen in Cakmak, M.; Mayer, P.; Trauner, D. position 1. Nat. Chem. 2011, 3, 543.

Department of Chemistry and Pharmacology, Ludwig-Maximilians-Universita¨t, Mu¨nchen, and Center for Integrated Protein Science, 81377 Munich, Germany. *e-mail: [email protected]

NATURE CHEMISTRY | VOL 3 | JULY 2011 | www.nature.com/naturechemistry 543 on ftesynthesis. the of point diene yrlzdns h atclrcmiaino lcrpieand knowledge. our electrophile of is best aminobromination the of deoxycarbonylative to this unprecedented, combination in used particular of nucleophile synthesis the the in before pyrrolizidines, used been hydrobromide have substitutions bromopyrrolizidine nucleophilic of yield 14 excellent an obtained alcohol observed. were 544 epoxidation Sharpless carbinol diastereoface-selective ( divinyl and achiral enantiotopos temuline of from available synthesis readily total asymmetric and an report now We Results pyrrolizidine intermediate key the of structure crystal the gives butyldicarbonate pyrrolizidine bicyclic the into transformed is compound this step, key the In | 2 Figure ARTICLES eetvt n ngo il.N rdcso S of products procedure No one-pot yield. good a in alcohol azido as and sulﬁte yielded selectivity of performed then Treatment azide be lithium Information). Supplementary also II (see ( Grubbs could sulﬁte a sequence cyclic of as presence ( stitution the diol resultant in The catalyst. metathesis ring-closing diene smooth yielded carbamate, benzyl a by lowed yencepii usiuin hsivrinwsachieved was formamide dimethyl inversion 15 chloropyrrolizidine in gave LiCl This which using Williamson- (DMF), reaction, a Finkelstein substitution. via a alkaloids through nucleophilic loline the of type bridge ether emblematic oasu abnt sabs e otesot omto fthe of 5- formation smooth only the azide ditions, to gave and led bridge base ether a as carbonate potassium alkaloids. of loline solution the a of Heating mysterious structurally and complex most the aiu oieaklis ipehdoeaingv temuline gave hydrogenation Simple alkaloids. loline synthesis our various streamlining further carried procedure, Information). be one-step Supplementary also (see a could reaction synthesis as Finkelstein ether out The Williamson observed. subsequent be and could products elimination Bromopyrrolizidine azido of solution a treated we synthesis, our of step key the In ulohlcoeigo epoxide of opening Nucleophilic Azide rf 3 secytlsrcuei i.2) lhuhtransannular Although 2c). Fig. in structure crystal (see 23) (ref. orsod oteclrproiiiesbnto oiie( lolidine of subunit chloropyrrolizidine the to corresponds N 10 fry oie( loline -formyl .Ar i n g - c l o s i n gm e t a t h e s i sc o n v e r t st h i sc o m p o u n di n t ot h ee i g h t - m e m b e r e dh e t e r o c y c l e 13 hr oa ytei flln alkaloids. loline of synthesis total Short 16 nsitu in Temuline ( b a ( (83%) O nmtao ih1euv fboiea 0 at bromine of equiv. 1 with methanol in norloline a ev sabacigpitfrtettlsnhssof synthesis total the for point branching a as serve can LiN OH 7 3 exo

DOI: 10.1038/NCHEM.1072 N rtcino h el omdscnayaieas amine secondary formed newly the of protection ARTICLES NATURE CHEMISTRY NH NATURE CHEMISTRY DOI: 10.1038/NCHEM.1072 2 2 ARTICLES ) ttncepii usiuinocre n no and occurred substitution nucleophilic -tet b ) a 4

NH • HCl ,Azide 15 2 H )(Fig.1).Itstartswithepoxyalcohol OH OH Cbz OH OH O Ref. 22 a -Pr NEt SOCl H 9 2 SAE SAE 2 NHGrubbs2 • HCl II H OH OH N OH SO (90%) 2 N 14 OH O namcoaeaprtsi h rsneof presence the in apparatus microwave a in

Et N SOCl N SAE 9 -Pr(86%)2NEt 3Grubbs II 2 , Pd-C

Ref. 22 into transformed further be can which compound, protected -Boc O CbzCl, Na CO Cbz O ARTICLES → 2 3 OH N N OH (99%) N SO 7 8 Ref. 22 10 Cbz (86%) H Et3N O (75%) O CbzCl, Na CO 13 → 2 3 OH Cbz N OH N 8 Cbz (99%) H 99% e.e. 7 (75%) 10 16 eursivrina 7t omthe form to C7 at inversion requires iue2| 2 Figure

12 diene 11 compound transformedthis step, key the is bicyclic the pyrrolizidineIn into 16 11 12 Cbz Cbz synthesis. the of point butyldicarbonate the gives 99% e.e. intermediate key pyrrolizidine the structurecrystal of Results enwrpr naymti oa ytei ftmln ( temuline of synthesis total asymmetric an report now We 11 12 and edl vial rmahrldvnlcarbinol divinyl achiral from available readily N enantiotopos diastereoface-selective and Sharpless epoxidation lowed by 3 carbamate, diene yielded benzyl a mohrn-lsn eahssi h rsneo rbsII Grubbs a of presence the in metathesis ring-closing smooth Br ( resultantcatalyst. diol The temuline to converted be can N ( sulﬁte cyclic as stitution OH Cl N eunecudas epromda n-o procedure one-pot a as performed be also could sequence N activation and ring-closing N This ). 3 3 3 sub- nucleophilic for activated was ) sulﬁte Supplementary of Information). Treatment (see H H K CO alcohol azido yielded then azide lithium 10 S of products No yield. good in and selectivity Br a N 2 3 Cl observed.were N Nucleophilic epoxide opening of

3 99% e.e. alcohol N necletyed ne hs con- these Under yield. excellent in H 3 K CO H bromopyrrolizidine hydrobromideof yield excellent an obtained 14

2 3 3 N synthesisnucleophilic of the substitutions in before used been have 3 O and electrophile of combination particular the pyrrolizidines, .Ar i n g - c l o s i n gm e t a t h e s i sc o n v e r t st h i sc o m p o u n di n t ot h ee i g h t - m e m b e r e dh e t e r o cO y c l e LiCl NaOH -wave deoxycarbonylative this aminobromination in nucleophile used is LiN3 Br2, MeOH N → µ knowledge. our of best unprecedented, the to -formyl ( loline 3 Williamson- a via alkaloids loline the of bridge emblematic ether O -wave b LiN Br , MeOH LiCl NaOH achieved was inversion This substitution. nucleophilic type µ synthesisShort total loline alkaloids. of (83%) → dimethylformamide in Finkelstein LiCl reaction, using a through azido of solution a treated synthesis, we our of step key the In OH 2 LiN 3 OH OH chloropyrrolizidine gave which (DMF), 15 + Temuline ( h most complexthe structurally and mysterious loline alkaloids. the of OH N Although transannular 2c). Fig. crystal structure (see in 23) (ref. 7 etn solutionHeating of a OH oasu abnt sabs e otesot omto fthe formation smooth of the to led base a potassium carbonate as O

+ 16 te rdeadgv azide gave and bridge ether N N OH (91%) ( N 5- only ditions, (86%) Bromopyrrolizidine (83%) (97%) N Finkelstein reaction The observed. be elimination could products (91%) norloline n usqetWlimo te ytei ol lob carried be also could synthesis subsequent Williamson ether and (86%) N u saoese rcdr,frhrsralnn u synthesis streamlining our further procedure, one-step a as out

(83%) 8 (97%) 3 seSupplementary(see Information). H – situin OH temuline gave hydrogenation Simple alkaloids. loline various H – OH ( chloropyrrolizidine lolidine the of corresponds subunit to N N 544 Br Br 10 NH N

N N 13 8 Br Cbz Cbz Azide 2 13 14 13 1514 16 15 16 Ref. 22 3 SAE 2 ) hscmon underwent compound This .

nmtao ih1euv fboiea 0 at bromine of equiv. 1 with methanol in Cbz 2 O ih3btnlmn ( 3-butenylamine with

) (97%) rtcino h newly formed secondaryprotection the amineas of , MeOH b H

N c ,Azide 16 CHO 2 (90%) b H 4 b NH N NHBoc CHO NHMec , Pd-C 2 15 3 )(Fig.1).Itstartswithepoxyalcohol 13 H , Pd-C, N

NH N NHBoc NHMe Boc

2 2

2 3 N (93%) H , Pd-C, synthesis total branching of the for point a as serve can Bcprotected-Boc compound, further transformed be which can into 2 H2, Pd-C Boc O LAHN AFA , Pd-C, 2 N O O exo 99% e.e. O O OH → H , Pd-C Boc O LAH AFA 13 16 2 2 3 O O ON (90%) N O (93%) N (96%) N (81%) O OH 8 ngo il.Notably, yield. good in O O no nucleophilic substitution and occurred -tet 2 (90%) (93%) N converted temuline be to can (96%) (81%) CbzCl, Na 15

N N N N O a 14 11 12 Br 16

N O N 9 N alcohol achiral an with starts Synthesis ,

namicrowave a in presence apparatus the of in regio- excellent with 2 a ciae o nucleophilic activated for sub- was ) .Ti igcoigadactivation and ring-closing This ). 3 (97%) Temuline (2) 16 17 Loline (1) -formyl loline (4) 14 , MeOH e ursivrina 7t omthe form to C7 at inversion requires 7

H (norloline) → (75%)

Temuline (2) .A F A ,a c e t i cf o r m i ca n h y d r i d e . 16 17 Loline (1) -formyl loline (4) 10 16 8 Boc 2

(93%) , Pd-C, CbzCl, Na Br

9 ih3-butenylaminewith ( hog highly a through (norloline) compound underwent This . NH

necletyed ne hs con- these Under excellent yield. in 2 -Pr O Figure 2 | Short total synthesis of loline alkaloids. a, Synthesis starts with an achiral alcohol 7 that can be easily desymmetrized to give epoxide 8 and then O N

(75%)

a NH

Br -Pr Synthesis achiral alcohol an starts, with

diene 10.Aring-closingmetathesisconvertsthiscompoundintotheeight-memberedheterocycle11, which is2 further processed to yield azidoalcohol 13. 2 2 2 Figure 2 | Short total synthesis of loline alkaloids. a, Synthesis starts with an achiral alcohol 7 that can be easily desymmetrized to give epoxide 8 and then H N H 2

14 2 • HCl 13 • HCl 2 NEt CO ′ NEt In the key step, this compound is transformed into the bicyclic pyrrolizidine 14. Establishment of the oxygen bridge then yields an azide (16), a branching DOI: 10.1038/NCHEM.1072 O CO NATURE CHEMISTRY H +

17 N H

.A F A ,a c e t i cf o r m i ca n h y d r i d e . N 14 ARTICLES 15 NHBoc 7 diene 10.Aring-closingmetathesisconvertsthiscompoundintotheeight-memberedheterocycle11, which is further processed to yield azidoalcohol 13. Br An eﬃcient synthesis of loline alkloid +

ihecletregio- excellent with 14 Br 3 point of the synthesis. b,Azide16 can be converted to temuline 2 by reduction of the azide. Alternatively, reductionsubstitution of the azide in the presence of di-tert- N 2 NHBoc hog highly a through N N

17 – 3 ngo il.Notably, yield. good in 3

In the key step, this compound is transformed into the bicyclic pyrrolizidine 14. Establishment of the oxygen bridge then yields an azide (16), a branching – 2 butyldicarbonate gives the N-Boc protected compound, which can be further transformed into N-methyl compound loline 1 and N-formyl loline 4. c,X-ray ( loline ), 8 3

a N point of the synthesis. b,Azide16 can be converted to temuline 2 by reduction of the azide. Alternatively, reduction of the azide in the presence of di-tert- 2 ,whichis crystal structure of the key intermediate pyrrolizidine 14.AFA,aceticformicanhydride. NH • HCl 2 di- of presence the in azide the of reduction Alternatively, azide. the of reduction by 2 2 H

OH OH

OH12 OH loline ( ), OH di- presence of the in azide Alternatively, azide. the the reduction of reduction of by ′ 8

8 OH O -Pr NEt SOCl (96%) butyldicarbonate gives the N-Boc protected compound, which can be further transformed into N-methyl compound loline 1 and9N-formyl loline 4. c,X-ray ,whichis 2 LAH SAE 2 Grubbs II LiCl substitution OH 9 and C SO Results a N Et N 39 crystal structure of the key intermediate pyrrolizidine 14.AFA,aceticformicanhydride. Ref. 22 (86%)– fol- ), Cbz 12 O (96%) CbzCl, Na CO (86%) O → → 2 3 Br OH fol- ), N N Cbz OH N LAH (99%) 21,22 H with We now report an asymmetric total synthesis of temuline7 (2), loline (1) 8 10 8 21,22 NaOH (75%) 14 and C 1

Cbz OH with Cbz ) Establishment azide( oxygen. bridge yieldsthe an then of and N-formyl loline (4)(Fig.1).Itstartswithepoxyalcohol8,whichis 99% e.e. O O LiCl . 10 6 a O + O Results N Br N 11 12 O Br 1 3 – 2 3 Loline ( OH readily available from), achiral divinyl carbinol 7 through a highly Br N OH OH 3 14 ) NHMe Br . N N We now report an asymmetric total synthesis of temuline (2), loline (1) 21,22 N H 3 Cl NH N3 K CO 6 2 3 enantiotopos and diastereoface-selective Sharpless epoxidation . NATURE CHEMISTRY (86%) Cl

N ), →

Grubbs II O 3 O LiCl NaOH -wave O ( azide an yields then bridge oxygen the of Establishment . and N-formyl loline (4)(Fig.1).Itstartswithepoxyalcohol8,whichis LiN3 O Br2, MeOH 13+ O → 18 µ 1 ) Nucleophilic opening of epoxide 8 with 3-butenylamine (9N), fol- Br + OHN OH (86%) Br N NaOH 3 2 3 H N | 3 Figure N (91%) Cbz (83%) bromine bromoniumaffords ion (86%) N N (97%) transannular carbamate the attack nitrogen of yield to a 19 O readily available from achiral divinyl carbinol 7 through a highly alkaloids ( loline the pyrrolizidine of the core and n h yrlzdn oeo h oieaklis( alkaloids loline the of structure of core pyrrolizidine the and yield19 to nitrogen carbamate the of attack transannular ion bromonium affords bromine | 3 Figure setal nvndrWascnatwt h abnwt hc teventually it which with bond. carbon a the forms with contact Waals der van in essentially uy yoabnt yielded pyrocarbonate butyl ( (norloline) ol ecenyrdcdwt ihu lmnu yrd oﬁnally to hydride ( aluminium itself lithium loline afford with reduced cleanly be could rd hngave then dride hs eotdi h ieaue(e upeetr Information). Supplementary (see literature the in reported those to corresponded compounds synthetic all of data spectral The interest. structure of H OH – OH (81%) lowed by in situ protection of the newly formed secondary amine as OH eventually Waals carbon whichessentially it contact der with the van with in om bond. a forms N 7 AFA 15 N (norloline) ( Br .S u b s e q u e n tc l e a v a g eb ym e t h a n o la f f o r d sac a r b o n a t eb y - p r o d u c t(

N N O u y pyrocarbonatebutyl yielded 21,22 cleanly lithiumreduced aluminium with ﬁnally be could hydride to afford loline (itself

dride then gave Loline ( 3 interest. spectral The synthetic all data of compounds corresponded to easily desymmetrized be can that epoxide give to enantiotopos and diastereoface-selectivea benzyl Sharpless carbamate, epoxidation yielded diene. 10. This compound literature reportedthe those Supplementary (see in Information). underwent N Cbz O b OMe .S u b s e q u e n tc l e a v a g eb ym e t h a n o la f f o r d sac a r b o n a t eb y - p r o d u c t( 13 14 15 16 -methyl compound loline OH

13 MeOH18 11 Nucleophilic opening of epoxide 8smoothwith 3-butenylamine ring-closing metathesis (9), fol- in the presence of a Grubbs II 21 10 hc sfurther processed azidoalcohol which yield is to , Cbz

-formyl loline ( O Mechanism step. key the of – Br N NHMe 3 N catalyst. The resultant diol (11) was activated for nucleophilic sub- OBr H CHO c O lowed by in situ protection of the newly formed secondary amine as b O N 13 O Cbz N O Br µ K NH N 20 NHBoc NHMe 2 3 N (91%) O -wave NATURE CHEMISTRY H , Pd-C, 3 2 stitution as cyclic sulﬁte (12). This ring-closing and activation 2 CHO 11 N N N CO OH 19 N N N a benzyl carbamate, yielded diene 10. This compound underwent sulfonate estera of , OH + O OMe 1 H , Pd-C 2 Boc O LAH AFA 3 2 2 N O N MeOH O b O O Br O O a sequence could also be performed as a one-pot procedure di- presenceof the hydrogenation whereas in ), OH 20 N 3 ) OH

AUECHEMISTRY NATURE H – MeOH + Cl 21 O OH smooth ring-closing metathesis in the presence of a Grubbs II (90%) (93%) (96%) 3 Br (81%) N N N N N N + (see Supplementary Information). Treatment step. key the of Mechanism of sulﬁte 12 with N O – Br 19 N 14 N 4 -formyl loline ( 3 OSO N Br H ) CHEMISTRY NATURE catalyst. The resultant diol (11) was activated for nucleophilic sub- OSO 3 SOCl 20 Grubbs II

O 3 1 lithium azide then yielded azido alcohol 13 with excellent regio- c (99%)

7 Et O O

.Formylation). lolineacetic-formic with of anhy- N Br (81%) H OH N N stitution as cyclic sulﬁte (12). This ring-closing and activation 3 3 (86%) 2 Temuline (2) N 2 ′ 16 17 Loline (1) -formyl loline (4) selectivity and in good yield. No products of S 2 substitution 16 – AFA OH Me Br N 2 htcnb aiydsmerzdt ieepoxide give to desymmetrized easily be can that b 2 OH + N N

ufnt se of ester sulfonate a , Me

O 3 N mty opudloline compound -methyl (norloline) N 15 3 ,weeshdoeaini h rsneo di- of presence the in hydrogenation whereas ), N 2 10.1038/NCHEM.1072DOI: Br Cbz sequence could also be performedwere as observed. a one-pot procedure X-ray crystal 1 + H – 18 and N

N Br O UY21 www.nature.com/naturechemistry | JULY 2011 | 3 VOL | Cbz N

which presumably, undergoes 11 In the key step of our synthesis, we treated a solution of azido OSO Me O (see Supplementary Information). Treatment of sulﬁte 12 with N 2 Br Figure 2 | Short total synthesis of loline alkaloids. a, Synthesis starts with an achiral alcohol 7 that can be easily desymmetrizedstructure of 14 to give epoxide 8 and then 3 19 N 14 N 13 N fry oie( loline -formyl 20 O N hc sfrhrpoesdt il azidoalcohol yield to processed further is which , alcohol 13 in methanol with 1 equiv. of bromine at 0 C and Cbz 13 8 temuline-Boc lithium azide then yielded azido alcohol 13 with excellent regio- a 19 -formyl loline

,w h i c hs h o w st h a tt h en i t r o g e ni s N diene 10.Aring-closingmetathesisconvertsthiscompoundintotheeight-memberedheterocycle4 N 11, which is further processed to yield azidoalcohol 13.

,E l e c t r o p h i l i ca c t i v a t i o no f OH

12 H

,wihi fconsiderable which of ), is biological -formyl loline ( 1 obtained an excellent yield of bromopyrrolizidine hydrobromide Br H

3 + selectivity and in good yield. No products of S 2′ substitution In the key step, this compound is transformed into the bicyclic pyrrolizidine 14. Establishment of the oxygen bridge then yields an azideO (16), a branching N O O .Fryaino oiewt ctcfri anhy- acetic-formic with loline of Formylation ). b N Br

3 N

14 (ref. 23) (see crystal structure in Fig. 2c). Although transannular O MeOH 3.1 Å SO H O 16 N H were observed. point of the synthesis. b,Azide16 can be converted to temuline 2 by reduction of the azide. Alternatively, reduction of the azide in the presenceO of di-tert- Cbz + 3 OSO nucleophilic substitutions have been used before in the synthesis of branching a ), O Br 14 Br N O 10 steps 18 N

14 N butyldicarbonate gives the N-Boc protected compound, whichN can be further transformed+ into N-methyl compound loline 1 and N-formyl loline 4. c,X-ray – 3 In the key step of our synthesis, we treated a solution of azido 3 – OSO Me 3.1 Å O pyrrolizidines, the particular combination of electrophileN and 2 N ). N O OSO Me OH OSO 4 O 2 One protecng group OH N crystal structure of the key intermediate pyrrolizidine 14.AFA,aceticformicanhydride. µ 3 CHO N N

N K O b 8 3 21

N . (91%)

alcohol 13 in methanol with 1 equiv. of bromine at 0 C and Cbz nucleophile used in this deoxycarbonylative8 aminobromination is N

X-ray crystal, -wave c -acylammonium ion OH 17 thenand O UY21 www.nature.com/naturechemistry | 2011 JULY | 3 VOL | 2 ,X-ray 2 OMe 11 3 CO 13 obtained an excellent yield of bromopyrrolizidineunprecedented, to hydrobromide the best of our knowledge. Me 20 a hscompound This .

Results tert – .

Br Br

Bromopyrrolizidine 14 requires inversion at C7 to form the 2 OH

14 (ref. 23) (see crystal structure in Fig. 2c). Although transannular 18 We now report an asymmetric total synthesis of temuline (2), loline (1) - Me Figure 3 | Mechanism of the key step. a,Electrophilicactivationof13 with 3 N emblematic ether bridge of the loline alkaloids via a Williamson- 4 nucleophilic substitutions have been used before in the synthesis of – O O O OH OH

hc rsmbyundergoes presumably which , and N-formyl loline (4)(Fig.1).Itstartswithepoxyalcohol8,whichis + O 13 O O type nucleophilic substitution. This inversion wasN achieved bromine affords bromonium ion 18, which presumably undergoes N3 Br Br N3 ) Bctemuline -Boc 3 3.1 Å 2 4

13 with pyrrolizidines, the particular combination of electrophile and readily available from achiral divinyl carbinol 7 through a highly OH Br OH OSO2Metransannular attack of the21 carbamate nitrogen to yield N-acylammoniumN ion N ,wihi fcnieal biological considerable of is which ), through a Finkelstein reaction, using LiCl in dimethylN formamide 21,22 c enantiotopos and diastereoface-selective Sharpless epoxidation) . nucleophile used in this deoxycarbonylative aminobromination,w h i c hs h o w st h a tt his en i t r o g e ni s 2 (DMF), which gave chloropyrrolizidine 15 in good yield. Notably, 19.Subsequentcleavagebymethanolaffordsacarbonateby-product(tert 21)

a 13 18 Nucleophilic opening of epoxide 8 with 3-butenylamine (9), fol- O unprecedented, to the best of our knowledge. SOCl 20 (99%) 15 corresponds to the chloropyrrolizidine,E l esubunit c t r o p h iof l i lolidine ca c t i v a t ( i6 o), no f and the- pyrrolizidine core of the loline alkaloids (14). b, X-ray crystal 1 O Et Bromopyrrolizidine 14 requires inversion at C7 to form the lowed by in situ protection of the newly formed secondary amine as 16 the most complex and structurally mysterious of the loline alkaloids. structure of 20, a sulfonate ester of 13,whichshowsthatthenitrogenis and a benzyl carbamate, yielded diene 10. This compound underwent 3

O OMe N N Heating a solution of 15 in a microwaveFigure apparatus 3 | Mechanism in the presence of the of keyessentially step. a,Electrophilicactivationof in van der Waals contact with13 thewith carbon with which it eventually N emblematic ether bridge of the loline alkaloids via a Williamson- MeOH smooth ring-closing metathesis in the presence of a Grubbs II 21 2 potassium carbonate as a base led to the smooth formation of the forms a bond. 3 type nucleophilic substitution. This inversion was achieved bromine affords bromonium ion 18, which presumably undergoes – Br N H 3 N catalyst. The resultant diol (11) was activated for nucleophilic sub- Br Br ether bridge and gave azide 16 in excellent yield. Under these3.1 Å con- O O transannular attack of the carbamate nitrogen to yield N-acylammonium ion 10.1038/NCHEM.1072 DOI: through a Finkelstein reaction, using LiCl in dimethyl formamide N loline -formyl 14 stitution as cyclic sulﬁte (12). This ring-closing and activation 3 OH ditions, only 5-exo-tet nucleophilic substitution occurred and no (norloline) (2), whereas hydrogenation in the presence of di-tert- Cbz 19.Subsequentcleavagebymethanolaffordsacarbonateby-product(21) OH N+ (DMF), which gave chloropyrrolizidine 15 in good yield. Notably, sequence could also be performed as a one-pot procedure Br N + ). H N H H elimination products could be observed. The Finkelstein reaction butyl pyrocarbonate yielded N-Boc temuline 17. This compound+ Br– 15 corresponds to the chloropyrrolizidine subunit of lolidine (6), and the pyrrolizidine core of the loline alkaloids (14). b, X-ray crystal Br + (see Supplementary Information). Treatment of sulﬁte 12 with N 14

b 19 14 and subsequent Williamson ether synthesis could also be carried could be cleanly reduced with lithiumBr aluminium hydride to ﬁnally 17 N structure of 20, a sulfonate ester of 13,whichshowsthatthenitrogenis O

-a crystal X-ray , lithium azide then yielded azido alcohol 13 with excellent regio- the most complex and structurally mysteriousout as a one-stepof the loline procedure, alkaloids. further streamlining our synthesis afford loline itself (1). Formylation of loline with acetic-formicN anhy- – ayamnu ion -acylammonium 12 16 18 selectivity and in good yield. No products of S 2′ substitution 3 Heating a solution of 15 ina compound microwave This . apparatus in the presence of essentially in van der Waals contact with the carbonN with which it eventually b

(see Supplementary Information). dride then gave N-formyl loline (4), which is of considerable biologicalN N 3 Br H were observed. H O ,abranching a ), potassium carbonate as a base led to theAzide smooth16 can formation serve as a branching of the pointforms for a bond. the total synthesis of interest. The spectral data of all synthetic compoundsOH corresponded to O – N O In the key step of our synthesis, we treated a solution of azido OSO Me O various loline alkaloids. Simple hydrogenation gave temuline those reported in the literature (see Supplementary Information).21 N 2 ether bridge and gave azide 16 in excellent yield. Under these con- O 3 alcohol 13 in methanol with 1 equiv. of bromine at 0 8C and Cbz SO ditions, only 5-exo-tet nucleophilic substitution occurred and no (norloline) (2), whereas hydrogenation in the presence of di-tert- 4

544 NATURE CHEMISTRY | VOL 3 | JULY 2011 | www.nature.com/naturechemistryOMe obtained an excellent yield of bromopyrrolizidine hydrobromide OH .

elimination products could be observed. The Finkelstein reaction butyl pyrocarbonate yielded N-Boc temuline 17. This compound 8

14 (ref. 23) (see crystal structure in Fig. 2c). Although transannular c and subsequent Williamson ether synthesis could also be carried could be cleanly reduced with lithium aluminium hydride to ﬁnally ,X-ray nucleophilic substitutions have been used before in the synthesis of O then and

13 O N3 3.1 Å out as a one-step procedure, further streamlining our synthesis afford loline itself (1). Formylation of loline with acetic-formic anhy- 13 pyrrolizidines, the particular combination of electrophile and OSO Me N 2 tert with

(see Supplementary Information). 21 nucleophiledride then gaveusedN in-formyl this deoxycarbonylative loline (4), which aminobromination is of considerable biological is . tert unprecedented, to the best of our knowledge. 20 ) Azide 16 can serve as a branching point for the total synthesis of interest. The spectral data of all synthetic compounds corresponded to - various loline alkaloids. Simple hydrogenation gave temuline thoseBromopyrrolizidine reported in the14 literaturerequires (see inversion Supplementary at C7 to Information). form the

- emblematic ether bridge of the loline alkaloids via a Williamson- Figure 3 | Mechanism of the key step. a,Electrophilicactivationof13 with 544 type nucleophilicNATURE CHEMISTRY substitution.| VOL 3 This | JULY inversion 2011 | www.nature.com/naturechemistry was achieved bromine affords bromonium ion 18, which presumably undergoes through a Finkelstein reaction, using LiCl in dimethyl formamide transannular attack of the carbamate nitrogen to yield N-acylammonium ion (DMF), which gave chloropyrrolizidine 15 in good yield. Notably, 19.Subsequentcleavagebymethanolaffordsacarbonateby-product(21) 15 corresponds to the chloropyrrolizidine subunit of lolidine (6), and the pyrrolizidine core of the loline alkaloids (14). b, X-ray crystal the most complex and structurally mysterious of the loline alkaloids. structure of 20, a sulfonate ester of 13,whichshowsthatthenitrogenis Heating a solution of 15 in a microwave apparatus in the presence of essentially in van der Waals contact with the carbon with which it eventually potassium carbonate as a base led to the smooth formation of the forms a bond. ether bridge and gave azide 16 in excellent yield. Under these conditions, only 5-exo-tet nucleophilic substitution occurred and no (norloline) (2), whereas hydrogenation in the presence of di-tert- elimination products could be observed. The Finkelstein reaction butyl pyrocarbonate yielded N-Boc temuline 17. This compound and subsequent Williamson ether synthesis could also be carried could be cleanly reduced with lithium aluminium hydride to ﬁnally out as a one-step procedure, further streamlining our synthesis afford loline itself (1). Formylation of loline with acetic-formic anhy- (see Supplementary Information). dride then gave N-formyl loline (4), which is of considerable biological Azide 16 can serve as a branching point for the total synthesis of interest. The spectral data of all synthetic compounds corresponded to various loline alkaloids. Simple hydrogenation gave temuline those reported in the literature (see Supplementary Information).

544 NATURE CHEMISTRY | VOL 3 | JULY 2011 | www.nature.com/naturechemistry ARTICLES NATURE CHEMISTRY DOI: 10.1038/NCHEM.1178

a O O OH Nu O HO HO– Nu Reaction O HO HO discovery-based H O H O 2 H O diversification O MeO MeO MeO 1 2 Fumagillin Fumagillol Natural product scaffold Common intermediate

b OH Atomic Ratio*,† Entry M(OTf)n radii (pm) 234:: Metal-dependent selectivity 100 N OMe No catalyst O 1 – 100 : 00: HO La(OTf) 75 H 2 3 195 0:: 955 M(OTf)n (10 mol%) MeO 3 Pr(OTf)3 185 5:: 887 Isoindole p-anisidine (1.1 equiv.) HO 50 HO Isoquinoline 3 4 Er(OTf)3 175 21:: 54 25 H O + OMe MeO DTBMP (60 mol%) 5 Yb(OTf)3 175 34:: 46 20 Product (%) 25 toluene OH 6 Sc(OTf)3 160 30:: 43 27 2 60 °C, 20 h N 0 7 Mg(OTf)2 150 52::867 Zn Mg Sc Yb/Er Pr La

HO 8 Zn(OTf)2 135 71::281 130 140 150 160 170 180 190 200 H OH Atomic radii (pm) MeO *Ratios were determined by 1H NMR 4 †Average of three runs

Figure 1 | Natural product remodelling using fumagillol. a,Fumagillolcanbetransformedintomultiplechemotypesbymeansofapanelofrelatedreaction conditions. Fumagillol was obtained by hydrolysis of crude fumagillin isolated from the fermentation broth of Aspergillus fumigatus.Nucleophilesaddfirstto the terminal epoxide to form a common intermediate that can be diversified by further reaction under a variety of conditions b,Thesecondepoxidering opening produces either an isoindole or an isoquinoline as product depending on the regioselectivity of the reaction. There is a linear correlation between the atomic radius of the metal in the metal triflate catalyst and the observed regioselectivity. DTBMP, 2,6-di-tert-butyl-4-methylpyridine.

Table 1 | MetalDOS as a reacon discovery approach triﬂate catalysed reactions of fumagillol with primary and secondary amines. O OH OH R N M(OTf)n N R amine HO HO + HO H O H H MeO DTBMP MeO MeO OH toluene HO 2 60 °C 5 6

Entry Amine M(OTf)n Reaction % yield Entry Amine M(OTf)n Reaction %yield (reaction time 5:6 (reaction time 5:6 conditions) conditions) ‡ 1 La(OTf)3*7h 91:3 6 La(OTf)3 20 h 62:25 † † MeO NH2 Zn(OTf) 29 h 9:76 MeO Zn(OTf) 60 h 0:57 (a) 2 2 NH (f) § 2 Mg(OTf)2 16 h 13:82

2 MeO La(OTf)*3h 84:0 7 La(OTf)‡ 24 h 66:32 3 NH 3 † 2 (g) † Zn(OTf)2 24 h 6:80 Zn(OTf)2 60 h 0:2 MeO NH2 § (b) Mg(OTf)2 36 h 18:74

MeO

‡ 3 La(OTf)3*29h 89:2 8 O La(OTf)3 20 h 49:20 NH † NH2 (h) † 2 Zn(OTf)2 29 h 8:65 Zn(OTf)2 60 h 0:11 (c) § Mg(OTf)2 20 h 12:37

‡ H 4 La(OTf)3*36h 87:0 9 La(OTf)3 † N OTf F NH2 Zn(OTf) 29 h 10:88 MeO NH (i) R PMP (d) 2 R 7 ¼(74%) O

‡ HO 5 La(OTf)3*4d 40:0 10 N La(OTf)3 H OH † H F3C NH2 Zn(OTf) 48 h 10:61 (j) R Bn MeO (e) 2 8 ¼(75%)

† ‡ § *La(OTf)3 (10 mol%), DTBMP (60 mol%), amine (1.1 equiv.). Zn(OTf)2 (10 mol%), DTBMP (60 mol%), amine (1.1 equiv.). La(OTf)3 (50 mol%), DTBMP (1.5 equiv.), amine (2.0 equiv.). Mg(OTf)2 (50 mol%), DTBMP (1.5 equiv.), amine (2.0 equiv.). DTBMP, 2,6-di-tert-butyl-4-methylpyridine; PMP, p-methoxyaniline. Porco, J. A.; Snyder, J. K. etc. Nat. Chem. 2011, 3, 969. regioselectivity, although toluene proved to be optimal, in which optimized using La(OTf)3, with 91% isolated yield (91:3 regioselec- case catalyst loading could be reduced to 10 mol% while maintain- tivity), and 4 could be obtained with Zn(OTf)2 in 76% yield (9:76 ing reasonable reaction times. Production of 3 was ultimately regioselectivity) (Table 1, entry 1).

970 NATURE CHEMISTRY | VOL 3 | DECEMBER 2011 | www.nature.com/naturechemistry NATURE CHEMISTRY DOI: 10.1038/NCHEM.1170 ARTICLES [3,3,1]nonane-triones-classical chemistry rules This work trans cis O O O O

R3 3 7 O O R O O O 1 O R HO Ph HO R4 HO 4 7 R1 R2 R HO HO R2 O O HO Oblongifolin A (1) Hyperibone L (3) No synthesis reported No synthesis reported Inhibition of tubulin assembly Inhibition of human tumour cell replication

O O O O

R3 R3 7 5 O O O O R1 O O O O R1 1 Ph HO HO R2 R2 7 R4 HO R4 HO Hyperpapuanone (4) Total synthesis reported No synthesis reported epi-Clusianone (2) • Selected antimicrobial activity against Bacillus cereus • Anti-HIV activity Staphylococcus epidermidis • Low cytotoxicity Micrococcus luteus • Inhibition of human tumour cell replication O O • Antispasmodic activity O R3 • Vascorelaxant activity R3 • Antianaphylactic activity O 7 • Selected antimicrobial activity against: O O Leishmania amazonensis (leishmaniasis) O O R1

Type A Type BStaphylococcus Type C aureus O 1 4 4 R R Staphylococcus mutans (dental caries) HO R 2 R2 7 HO R Trypanosoma cruzi (Chagas disease) HO

No synthesis reported regio-Hyperpapuanone (5) Total synthesis reported

Figure 1 | ClassificationNicole of PPAPsBiber and, Katrin total syntheses Möws accomplished., Bernd PlietkerTo tal synth Nature Chemistry es es of cis-type A and B PPAPs 2011 like,, 3 for, 938. example, garsubellin A (refs 16,17), nemoroson20,23, ent-hyperforin22, clusianone18–21,24,plukentione25 and hyperibone K (ref. 26) have been achieved. HIV human immunodeficiency virus. ¼ easy-to-operate methodologies and a minimization of protecting identical conditions independent of the structure of the starting group operations, only a small number of synthetic material. A regioselective 1,2-addition28 of methyllithium to the transformations may be necessary, which thus increases both the diesters 9 and 10 gave rise to the corresponding methyl ketones, practicability and the efficiency of the total synthesis. A brief retrosynthetic analysis is sketched in Fig. 2. With regard to the O framework construction, the Dieckmann reaction of a densely R3 substituted cyclohexanone III and a tandem Michael addition– O 1 Knoevenagel condensation to give a cyclohexenone IV are key O R 2 4 HO R steps that would allow quick access to the O O R bicyclo[3.3.1]nonatrione core II. Neither transformation had been I O used for the construction of strained bicyclic ring systems, so they VI R3 7 needed to be elaborated within this synthetic context. The O R1 iterative framework decorations via nucleophilic substitutions a-Methylenation 2 using alkylating, allylating or acylating conditions allow for a Allylation HO R Framework II regioselective generation of the correct substitution pattern in construction Dieckmann which the C7 stereocentre orchestrates the trajectory of the O condensation Framework C-acylation incoming electrophiles to set up the stereogenic centres with the decoration correct relative configuration (Fig. 2). R1 V O O R2 R3 Michael addition– RO2C Synthesis. Acetylacetone (6) serves as a common starting material Knoevenagel condensation and was allylated in the first step using sodium hydride (NaH) Alkylation 1 Cuprate addition and the corresponding allylating agent R –X (Fig. 3). This early Alkylation R1 stage of the synthesis represents our first diversification point O O R2 III (Fig. 3) and paves the way for the total synthesis of oblongifolin A RO2C (1), which possesses a geranyl side chain, and the total synthesis of the C7-prenylated compounds 2–5.Wesubjectedthe corresponding allylation products directly to a deacylating aldol-type R1 a-methylenation using potassium carbonate (K2CO3)inanaqueous IV formalin solution27.Thea,b-unsaturated ketones 7 and 8 were obtained in good yields starting from 6 and were employed in a Figure 2 | Retrosynthetic analysis. As the trans-type B PPAPs differ subsequent tandem Michael addition–Knoevenagel condensation to mainly in the arrangement of the substituents R1 to R4,theretrosynthetic give the cyclohexenone cores 9 and 10 in 86% and 89% yield, approach was designed to separate the steps that build the core structure respectively. Both a-methylenation and ring-forming condensation (framework construction) from those that introduce the substituents are framework-constructing operations and were performed under (framework decoration).

NATURE CHEMISTRY | VOL 3 | DECEMBER 2011 | www.nature.com/naturechemistry 939 NATURE CHEMISTRY DOI: 10.1038/NCHEM.1170 ARTICLES

This work trans cis O O O O

O O O O

Type A Type BStaphylococcus Type C aureus O 1 4 4 R R Staphylococcus mutans (dental caries) HO R 2 R2 7 HO R Trypanosoma cruzi (Chagas disease) HO

No synthesis reported regio-Hyperpapuanone (5) Total synthesis reported

Figure 1 | Classification of PPAPs and total syntheses accomplished. To tal synth es es of cis-type A and B PPAPs like, for example, garsubellin A (refs 16,17), nemoroson20,23, ent-hyperforin22, clusianone18–21,24,plukentione25 and hyperibone K (ref. 26) have been achieved. HIV human immunodeficiency virus. ¼ easy-to-operate methodologies and a minimization of protecting identical conditions independent of the structure of the starting group operations, only a small number of synthetic material. A regioselective 1,2-addition28 of methyllithium to the transformations may be necessary, which thus increasesRetrosynthec analysis-classical chemistry rules both the diesters 9 and 10 gave rise to the corresponding methyl ketones, practicability and the efficiency of the total synthesis. A brief retrosynthetic analysis is sketched in Fig. 2. With regard to the O framework construction, the Dieckmann reaction of a densely R3 substituted cyclohexanone III and a tandem Michael addition– O 1 Knoevenagel condensation to give a cyclohexenone IV are key O R 2 4 HO R steps that would allow quick access to the O O R bicyclo[3.3.1]nonatrione core II. Neither transformation had been I O used for the construction of strained bicyclic ring systems, so they VI R3 7 needed to be elaborated within this synthetic context. The O R1 iterative framework decorations via nucleophilic substitutions a-Methylenation 2 using alkylating, allylating or acylating conditions allow for a Allylation HO R Framework II regioselective generation of the correct substitution pattern in construction Dieckmann which the C7 stereocentre orchestrates the trajectory of the O condensation Framework C-acylation incoming electrophiles to set up the stereogenic centres with the decoration correct relative configuration (Fig. 2). R1 V O O R2 R3 Michael addition– RO2C Synthesis. Acetylacetone (6) serves as a common starting material Knoevenagel condensation and was allylated in the first step using sodium hydride (NaH) Alkylation 1 Cuprate addition and the corresponding allylating agent R –X (Fig. 3). This early Alkylation R1 stage of the synthesis represents our first diversification point O O R2 III (Fig. 3) and paves the way for the total synthesis of oblongifolin A RO2C (1), which possesses a geranyl side chain, and the total synthesis of the C7-prenylated compounds 2–5.Wesubjectedthe corresponding allylation products directly to a deacylating aldol-type R1 a-methylenation using potassium carbonate (K2CO3)inanaqueous IV formalin solution27.Thea,b-unsaturated ketones 7 and 8 were obtained in good yields starting from 6 and were employedRodeschini in a Figure, V., Ahmad, N. M. & Simpkins, N. S. Org. 2 | Retrosynthetic analysis. As the trans-type B PPAPsLe. differ 2006 8, 5283. subsequent tandem Michael addition–Knoevenagel condensation to mainly in the arrangement of the substituents R1 to R4,theretrosynthetic give the cyclohexenone cores 9 and 10 in 86% and 89% yield, approach was designed to separate the steps that build the core structure respectively. Both a-methylenation and ring-forming condensation (framework construction) from those that introduce the substituents are framework-constructing operations and were performed under (framework decoration).

NATURE CHEMISTRY | VOL 3 | DECEMBER 2011 | www.nature.com/naturechemistry 939 ARTICLES NATURE CHEMISTRY DOI: 10.1038/NCHEM.1170

O O Diversification 1

i 51%i 60% R1a = O O

R1b =

R1b 78R1a

ii 86% ii 89%

O O O O O O

MeO OMe MeO OMe

R1b 9 R1a 10 Diversification 2

iii, iv59% over two steps iii, iv 62% over two steps iii, iv 63% over two steps

2a O O O O O O O O O R = R2a R2b R2a

MeO MeO MeO R2b = Me

R1b 11 R1a 12R1a 13

vv95%v 97% 96%

O O O O O O O O O R2a R2b R2a

MeO MeO MeO Diversification 3

1a 16 R1b 14R1a 15 R R3a = vi 77%vi 90%vi 86%vii 81%

R3b = Me O O O O O O O O O O O O R2a R3a R2b R3a R2a R3a R2a R3b

MeO MeO MeO MeO

17 18 19 20 R1b R1a R1a R1a

ix 33%viii 72% viii 78% viii 84% viii 51% Diversification 4 OH OH O O O O

O HO O HO O HO O HO O O O O O HO O O

1 4 2 3 5 6% overall yield 21% overall yield 22% overall yield 22% overall yield 14% overall yield

Figure 3 | Total synthesis of oblongifolin A (1), hyperpapuanone (4), epi-clusianone (2), hyperibone L (3) and regio-hyperpapuanone (5). i, NaH 1 (1.1 equiv.), R X(1.5equiv.),ethanol,08C to room temperature (r.t.), 15 h, then K2CO3,formaldehyde,r.t.,5h;ii,methylmagnesiumchloride(2.0equiv.), dimethyl 1,3-acetonedicarboxylate (1.0 equiv.), methanol, 0 to 60 8C, 15 h; iii, NaH (1.1 equiv.), methyllithium (2.3 equiv.), THF, 0 8C, 5 h; iv, NaH (1.1 equiv.), 18-crown-6 (0.1 equiv.), R2X (1.5–2.5 equiv.), THF, 0 8C to r.t., 15 h; v, lithium chloride (2.02 equiv.), CuI (2.0 equiv.), methylmagnesium bromide (2.0 equiv.),

Me3SiCl (2.0 equiv.), THF, –78 8C, 5 h; vi, KO-t-amylate (2.0 equiv.), 1,3-dimesitylimidazolin-2-ylidene hexaﬂuorophosphate (0.1 equiv.), Bu4N[Fe(CO)3(NO)] (0.1 equiv.), 2-methyl-3-butene-2-yl methylcarbonate (2.0 equiv.), THF/methyl-t-butyl ether, 0 to 80 8C, 20 h; vii, KH (1.1 equiv.), 18-crown-6 (0.1 equiv.), MeI (2.0 equiv.), dimethoxyethane, 0 8C to r.t., 15 h; viii, KO-t-Bu (2.0 equiv.), R4–C( O)CN (3.3 equiv.), THF, 0 to 35 8C, 24 h; ix, same as viii, but 0 8Cto ¼ r.t. and additional basic work-up with K2CO3,methanol.

940 NATURE CHEMISTRY | VOL 3 | DECEMBER 2011 | www.nature.com/naturechemistry ARTICLES NATURE CHEMISTRY DOI: 10.1038/NCHEM.1170

O O Diversification 1

i 51%i 60% R1a = O O

R1b =

R1b 78R1a

ii 86% ii 89%

O O O O O O

MeO OMe MeO OMe

R1b 9 R1a 10 Diversification 2

iii, iv59% over two steps iii, iv 62% over two steps iii, iv 63% over two steps

2a O O O O O O O O O R = R2a R2b R2a

MeO MeO MeO R2b = Me

R1b 11 R1a 12R1a 13

vv95%v 97% 96%

O O O O O O O O O R2a R2b R2a

MeO MeO MeO Diversification 3

1a 16 R1b 14R1a 15 R R3a = vi 77%vi 90%vi 86%vii 81%

R3b = Me O O O O O O O O O O O O R2a R3a R2b R3a R2a R3a R2a R3b

MeO MeO MeO MeO

17 18 19 20 R1b R1a R1a R1a

ix 33%viii 72% viii 78% viii 84% viii 51% Diversification 4 OH OH O O O O

O HO O HO O HO O HO O O O O O HO O O

1 4 2 3 5 6% overall yield 21% overall yield 22% overall yield 22% overall yield 14% overall yield

940 NATURE CHEMISTRY | VOL 3 | DECEMBER 2011 | www.nature.com/naturechemistry Summary

• 5 publicaons in 2011 (mul-targets wl <10 steps, single target shd biological test) • 4 publicaons in 2012 • 5 publicaons in 2013 Selecve in-depth discussion • Conclusion and summary ARTICLESEnanoselecve synthesis of taxanesNATURE CHEMISTRY-scale it up DOI: 10.1038/NCHEM.1196 a Retrosynthetic analysis of Taxol by an 'oxidase phase pyramid' allowing for a gram-scale access to 6 in only seven steps, as well as a AcO O OHO Higher [O] state gram-scale synthesis of ( )-taxadiene (7). OH OH Me Me Same Me Me þ 10 [O] state B 2 Me 8 Me RO 13 5 HO A C 'Level 11' Results 1 Me 4 Me H O H O 3 In the forward direction, known compounds 10 (refs 35,40,41; made HO 2 HO 'Level 9' BzO AcO OH HO from 8 in a modified one-pot procedure; see Supplementary 1 2 4 Information) and 11 (ref. 45; made from 9 using a one-step pro- NATURE CHEMISTRY DOI: 10.1038/NCHEM.1196 'Level 7' ARTICLES Taxol Taxol [O] state cedure) were combined to generate enone 12 by means of a Lewis BzHN O (= 'level 11') 5 R = 'Level 6' acid-modified organocopper 1,6-addition (Fig. 2). This convergent Ph Me Me Me Me oligomers and polymers. This problem was solved using high 'Oxidase phase pyramid' synthesis of 12 is operationally simple and provides decagram quan- OH Me C: Shapiro/aldol/ Me dilution conditions (running the reaction at 0.01 M) and by 8 A: RCM Diels–Alder 8 tities1 Me per reaction batch (86%; over 50 g of this materialMe has beenthe slow addition of substrate 16 to the Lewis! acid solution (see H Me Me O b Representative taxanes of varying [O] state synthesizedH to date). This enone was deemed to be a suitable sub- O O Me Supplementary Information). OHHO OHHO HO Me 43 OH strate18 for the enantioselective conjugate addition of Alexakis20 to Developing a scalable route to the taxane core involved the study Me Me Me Me Me Me Me D: later-stage MeestablishMe the quaternary centreH (C8).1,4-addition However, at thisMe strategy was con-and modification of fundamental aspects of the described chemistry, Me Me Me B: aldol H 8 HO HO HO O Me C8 Me Me Me tingentMe on8 the feasibility of trapping the resultingMe aluminiumrather than a mere exercise in scaling up. The conciseness of the OH OH OH 1 Me (+)-6 X Me H H H enolate by trimethylsilyl chloride (TMSCl). Although Alexakis HO H H 'Taxadienone' 2 3 synthetic route is a direct result of trying to achieve a scalable OH OH OH 14 notedH thatO isolation of such silyl enol ethers is difficult due to O O synthesis, and the reliability of the yields attests to the small 3 4 5 43 50 2-acetoxybrevifoliol [O] state Taxinine E or J [O] state Yunnanxane [O] state facile19 desilylation , a modified quenching procedure21 involvingvariability of reactions run on a larger scale . (= 'level 9') (= 'level 7') (= 'level 6') dilution with tetrahydrofuran (THF), addition of TMSCl,X = H or R3Si followed Despite the efficiency of the described approach, there are two by addition onto Florisil in 1:10 Et3N:hexanes, allowed the isolationobvious limitations in the synthesis of taxadienone (6): (i) the Figure 4 | Initial synthetic investigations towards the synthesis of c Disconnection of 'taxadienone' (6) of trimethylsilyl (TMS) enol ether 14 in 89% yield and in 93% e.e.single functional group manipulation from cyclized diketone 17 to Me Al taxadienone (6). Disconnection A: an RCM approach would require many Me Br 3 (1.0 g scale; enantioselectivity measured for desilylated ketone 15the corresponding enol triflate (rendering the route 85% rather 42 Me Me Me Me moreon steps chiral in building high-performance the taxane framework. liquid Disconnection chromatography B: the required (HPLC); seethan 100% ideal) and (ii) the 2:1 diastereoselectivity in the aldol Me Me [H] Me aldol closure simply did not proceed. Disconnection C: a Shapiro reaction, reaction of 14 and acrolein. These issues are currently being Me Me Me Supplementary Information). Only 2 mol% of CuTC and 4 mol% followed by aldol and Diels–Alder reactions, is strategically similar43 to the addressed by (i) developing a scalable, one-step enol triflation/ O H H of chiral phosphoramidite ligand 13 were required , and this asym- H H successful synthetic route, but the stereochemistry at C8 could not be set O Me Me methyl coupling reaction and (ii) generating creative acrolein O Me2Zn metric reaction was routinely conducted on a gram scale (over 20 g (+)-6 stereoselectively. Disconnection D: conjugate addition at C8 to install the • Three-component coupling (+)-7 equivalents and/or reaction conditions with various additives to 'Taxadienone' Taxadiene of this material has been synthesized to date). TMS enol ether 14 • Scalable methyl unit did not proceed, because only the undesired conjugate addition increase the diastereoselectivity of the aldol step. (= 'level 4') (= 'level 2') was indeed quite prone to desilylation, and formation of undesired • Enantioselective onto C14 occurred. In summary, a scalable, enantioselective entry to the taxane • Minimizes concession steps ketone 15 was often the sole reaction pathway when attempting family of natural products was achieved in only seven steps from Mendoza, A.; Ishihara, Y.; BaranMukaiyama, P. S. Nat. Chem aldol reactions. 201246 , with4, 21 commonly used Lewis acids Figure 1 | Retrosynthetic analysis of Taxol (1) and other members of the because reaction first occurred at the less hindered C14, even. when a commercially available starting material (18–20% overall yield). (such as TiCl4, SnCl4, Sn(OTf)2, Sc(OTf)3,BF3 OEt2, TMSOTf, taxane family. a,Partial‘oxidasephasepyramid’fortheretrosynthetic large tert-butyldiphenylsilyl. group was appended at C14. After This triply convergent approach to taxa-4(5),11(12)-dien-2-one ZnCl2 or MgBr2 OEt2) or other catalysts (such as LiClO4, planning of the taxane family, including its key member, Taxol (1). many moret strategic revisions and success in forming the required (6) allowed for a minimization of concession steps, in which 6 of C2–C3Zr(O bondBu)4,Bi(OTf) from TMS3, enol AgOTf ether or14 SiClusing4), Gd(OTf) even when, the performed final 7 steps formed skeletal (C–C) bonds. Every one of these steps b,Representativetaxanesofvaryingoxidationstates,sharingaC2-hydroxyl 3 syntheticunder strategy strictly depicted anhydrous in Fig. conditions. 2 was realized. Surprisingly, water waswas performed on a gram scale, attesting to the scalability and group. c, Synthetic design for ‘taxadienone’ (6)andreductiontogenerate foundTactically, to although be the enabling quite efficient additive at present, that allowedmost steps for shown an aldolrobustness of the sequence. Furthermore, ( )-taxadiene (7)was taxadiene (7). Sites of oxidation installed onto taxadiene (7)areindicatedin 47 inreaction Fig. 2 were with initially acrolein. difficult Gratifyingly, to scale and Kobayashi suffered from conditions low and withsynthesized, enabling further structural confirmation,þ as well as red. The ‘oxidation level’ of taxanes is defined as the number of C Cand inconsistentGd(OTf) yields.in 1:10:4 Even H O:EtOH:PhMe the first transformation generated from the corresponding8 to 10 the first optical rotation determination of this natural product. 33 ¼ 3 2 C–O bonds installed onto the taxane carbon skeleton . (Fig.aldol 2) was product not trivial, smoothly despite as it a being 2:1 mixture a known of two-step diastereomers transform- that areThe simple chemical route to ( )-7 nicely complements the ationisomeric35,40,41. at Modifying C3. This the aldol reaction product stoichiometry was then oxidized and reaction in the samerecent pioneering studies of Stephanopoulosþ and co-workers22, all feature a C2-hydroxyl group and can be further simplified to timesreaction was necessary pot with Jones’ to coax reagent this process to generate into giving keto-enone good yields16 in 85%whose bioengineering strategy also delivers gram-scale quantities taxa-4(5),11(12)-dien-2-one (6), dubbed ‘taxadienone’, which consistentlyover two steps on a (3.2 decagram g scale, scale, 2:1 ratio with of eventual diastereomers success at as C3; a one- this inse-of ( )-7, albeit as a 9:1 mixture of olefin isomers (taxa- þ represents a benchmark intermediate for comprehensive access potparable operation. mixture The second of stereoisomers transformation, was a carried merging onwards of the two for the4(5),11(12)-diene (7) and taxa-4(20),11(12)-diene) (see to the taxane family. Furthermore, if taxadiene (7) were desired, similar-sizedensuing step). fragments A Diels–Alder10 and reaction11, can to be generate conducted the 6-8-6 in good tricyclicSupplementary Information). Studies are currently under way to one could simply deoxygenate 6 to generate the least oxidized yieldsskeleton (86%) has on previously a decagram been scale, demonstrated but initially on this similar reaction systems was 35–37make, use of the existing functional group handles in 6 to oxidize natural product in the taxane family (Fig. 1c). A full scholarly plaguedand mixture with inconsistent16 reacted yields predictably due to side-product with BF .OEt formation:to furnishvarious sites on the taxane skeleton and to create a pyramid-like 3 . 2 analysis of the taxane pyramid has been described previously, thetricyclic original compound reaction conditions17 (2.3 g of scale).tert-butyllithium, The desired BF diketone3 OEt2 and17 waslibrary of unnatural and natural taxanes en route to Taxol (1). together with the merits in choosing a C2-oxidized taxane such CuIobtained resulted in in 1,6-addition 47% yield, of tert together-butyllithium with (whereas its diastereomer sec-butyl- (see as 6 (ref. 33). lithiumSupplementary is not a very Information) competent nucleophile in 29% yield, for this with reaction), complete iodi- diaster-Received 22 August 2011; accepted 10 October 2011; . Strategic disconnections of 6 were built upon the landmark nationeoselectivity of 12 (whereas at C1. These the cyclized use of CuBr diketonesSMe2 displayedcircumvents very this differentpublished online 6 November 2011 b g syntheses of 1 (refs 11–20) and the many studies geared towards problem),polarities and and deconjugation were thus easily of 12 separatedto give by a silica, -unsaturated gel chromato- ketone (this problem was rectified by optimizing the work-up the taxane framework35–41. Of the many possible disconnections of graphy at this stage. Only one more carbon remained to be installedReferences procedure; see Supplementary Information). The third, asymme- 1. Heusler, F. The Chemistry of the Terpenes. (P. Blakiston’s Son & Co., 1902). the 6-8-6 tricyclic backbone, one was chosen such that (i) the to complete the43 taxane framework: this was achieved via enol triflate 42 try-inducing step from 12 to 14 was straightforward when run 2. Breitmaier, E. Terpenes: Flavors, Fragrances, Pharmaca, Pheromones forward synthesis would be short, convergent and scalable and onformation a small scale followed (,100 by mg) Negishi and deliveredcoupling tohigh generate enantioselectivity taxadienone (6) (Wiley-VCH, 2006). (ii) its asymmetric synthesis would only rely on one enantioselective within 0.5 84% mol% over CuTC two and steps 1 mol% (2.4 g ligand scale). loading. This material However, solidified on 3. Maimone, T. J. & Baran, P. S. Modern synthetic efforts toward biologically active reaction, after which the resulting stereochemical information could increasingquite readily, the reaction and single-crystal scale to a gram X-ray scale, analysis the reaction confirmed conversion both the terpenes. Nature Chem. Biol. 3, 396–407 (2007). be propagated to set all other stereocentres diastereoselectively. A absolute and relative stereochemistry of the synthesized ABC4. Schiff, P. B., Fant, J. & Horwitz, S. B. Promotion of microtubule assembly in vitro suffered significantly. Eventually, a higher catalyst loading (2 mol% by taxol. Nature 277, 665–667 (1979). vicinal difunctionalization/Diels–Alder strategy to forge the taxane CuTCring and system. 4 mol% This ligand) lowly and oxidized precise core temperature is the key control intermediate allowed for5. Suffness, M. TAXOLw: Science and Applications (CRC Press, 1995). AB ring system (see ring numbering in 2, Fig. 1a) seemed to fulfil thisour reaction future to research give reliable efforts yields to and elaborate consistent the enantioselectivity taxane pyramid.6. In Itokawa, H. & Lee, K.-H. Taxus: The Genus Taxus (Taylor & Francis, 2003). most of the above criteria. The Diels–Alder strategy has been used onsummary, a gram scale. the Also total worth synthesis mentioning of ( )-taxadienone is a modified (6 quenching) was achieved7. Wani, M. C., Taylor, H. L., Wall, M. E., Coggon, P. & McPhail, A. T. Plant þ previously in the context of taxane synthesis by Shea35, Jenkins36 procedurein a total that of was8 steps, developed with a to longest address linear the sequence troublesome of 7 TMS steps. A antitumor agents. VI. Isolation and structure of taxol, a novel antileukemic and 37 38–41 43 antitumor agent from Taxus brevifolia. J. Am. Chem. Soc. 93, 2325–2327 (1971). and Williams , as well as others . In fact, Williams reported trappingfew grams of the of aluminium taxadienone enolate (6) could(vide be supra synthesized). by one chemist8. Denis, J. N. et al. Highly efficient, practical approach to natural taxol. J. Am. the only total synthesis of (+)-taxadiene (7), in 26 steps, as long overThe fourththe course and the of 7most days, difficult with an reaction overall was yield the aldol of 18% reaction from 8 or Chem. Soc. 110, 5917–5919 (1988). ago as 1995 (ref. 37). Furthermore, the decision to implement a of 20%14 and from acrolein,9. which only returned desilylated ketone 15 under 9. Holton, R. A. Method for preparation of taxol. European patent EP0400971 vicinal difunctionalization strategy was underpinned by recent devel- most reactionWith the conditions. key taxane A6 varietysecured of Lewis in gram acid-mediated quantities, reactions taxadiene (7) (A2) (1990). 10. Ojima, I. et al. New and efficient approaches to the semisynthesis of taxol and its opments in the asymmetric formation of all-carbon quaternary (videwas supra then), targeted, as well primarily as anionic as silicon–metal a means of spectroscopic exchange reactions, comparison C-13 side chain analogs by means of b-lactam synthon method. Tetrahedron 48, centres43,44. Thus, a concise, enantioselective strategy was conceived, neverto previously led to the desired reported product. data, butThis also aldol to reaction demonstrate only proceeded the feasibility 6985–7012 (1992). when lanthanide triflates such as Yb(OTf)3 or Gd(OTf)3 and very 11. Nicolaou, K. C. et al. Total synthesis of taxol. Nature 367, 630–634 (1994). 22 specificNATURE solvent CHEMISTRY systems were| VOL used. 4 | JANUARY The final 2012 challenge | www.nature.com/naturechemistry was a scalable 12. Holton, R. A. et al. First total synthesis of taxol. 1. Functionalization of the B Diels–Alder reaction from 16 to 17, which has been known to ring. J. Am. Chem. Soc. 116, 1597–1598 (1994). 35–37 13. Holton, R. A. et al. First total synthesis of taxol. 2. Completion of the C and D proceed in moderate yields on similar substrates . Although rings. J. Am. Chem. Soc. 116, 1599–1600 (1994). efficient on a small scale, the yield decreased when the reaction 14. Masters, J. J., Link, J. T., Snyder, L. B., Young, W. B. & Danishefsky, S. J. A total was conducted on a gram scale, possibly due to the formation of synthesis of taxol. Angew. Chem. Int. Ed. 34, 1723–1726 (1995).

24 NATURE CHEMISTRY | VOL 4 | JANUARY 2012 | www.nature.com/naturechemistry NATURE CHEMISTRY DOI: 10.1038/NCHEM.1196 ARTICLES

Me Me Ph Me Me t Me a. CHBr3, BuOK; then O Me Me Me Br Me PhNMe2, 150 ºC P N O Me Me Me Me Me (67%) Ph O Me Me Me 8 (commercial) 10 (known) (+)-13 (+)-15 b. sBuLi, TMSCl, c. Me3Al (1.4 equiv.), CuBr •SMe 2 2 Me CuTC (2 mol%), Me Me O a'. vinylmagnesium ligand 13 (4 mol%), bromide (see ref. 45) Me then THF, TMSCl Me 8 Me Me (75%) (86%) (89%, 93% e.e.) OEt O [decagram scale] O [gram scale] OTMS 12 (–)-14 9 (commercial) 11 (known) d. Acrolein (20 equiv.), Gd(OTf)3 (10 mol%), 1:10:4 H 2O/EtOH/PhMe; then Jones' reagent (85%) [gram scale] Me Me Me Me Me e. BF •OEt , Me f. PhNTf2, KHMDS; 3 2 Me Me CH2Cl2 Me g. Me2Zn, Pd(PPh3)4 3 Me 1 Me Me 3 (47% (+)-17 2 H (84%, two steps) H H H H + 29% undesired O Me [gram scale] O O O O diastereomer) 16 (2:1 d.r., mixture of (+)-6 (+)-17 (>99:1 d.r. at C1) [gram scale] isomers, major 'Taxadienone' diastereomer shown)

Figure 2 | Enantioselective synthesis of key taxane 6. Conditions: a. 2,3-dimethyl-2-butene, CHBr3,potassiumtert-butoxide, hexanes, 2 h; evaporate volatile 45 materials, then N,N-dimethylaniline, 150 8C, 30 min (67%); a′. 3-ethoxy-2-cyclohexen-1-one, vinylmagnesium bromide, Et2O, 16 h (75%) ; . b. 10, sec-butyllithium, Et2O, –78 8C, 15 min; then CuBrSMe2,30min;thenTMSCl,5min;then11,2h;warmtoroomtemperature,8h;thenAcOH,30min; then 3 M HCl, 30 min (86%); c.CuTC(2mol%),phosphoramidite13 (4 mol%), Et2O, room temperature, 30 min; then 2.0 M Me3Al, enone 12,–308C, 24 h; then THF, TMSCl, 0 8C to room temperature, 8 h; then Et3N, Florisil, 2 h (89%, 93% e.e.); d.Gd(OTf)3 (10 mol%), acrolein, 1:10:4 H2O:EtOH:PhMe, 4 8C, 24 h; then evaporate volatiles, then Jones’ reagent, acetone, 10 min (85% over two steps, 2:1 d.r. at C3, inseparable mixture of diastereomers); e. BF .OEt ,CHCl ,08C, 6 h (47% 17 29% undesired diketone); f.0.4MKHMDS,PhNTf ,THF,08C, 1 h; g.1.2MMeZn, Pd(PPh ) (5 mol %), THF, 0 8C 3 2 2 2 þ NATURE CHEMISTRY2 DOI: 10.1038/NCHEM.11962 3 4 ARTICLES to room temperature, 5 h (84% over two steps). TMSCl, trimethylsilyl chloride; CuTC, copper(I)thiophene-2-carboxylate;PhNTf2, N-phenylbis(trifluoro methanesulfonimide); KHMDS, potassium hexamethyldisilazide; Pd(PPh3)4,tetrakis(triphenylphosphine)palladium. Me Me Ph Me Me t Me of a large-scale laboratory production of enantioenriched 7. epimerizea. theCHBr3, C1BuOK; stereocentre then into the desired configuration.O Me Me Me Br Me PhNMe2, 150 ºC P N Furthermore, taxadiene (7) is produced in negligible amounts in However, despite a plethora of attempted experiments, the desiredO Me Me Me Me Me nature (less than 1 mg can be obtained from 750 kg of tree bark cyclization from(67%)19 did not proceed. Thereafter, strategies involvingPh O 34 Me Me Me from T. brevifolia) and therefore its optical rotation has never been 8closure (commercial) of the AB ring by a10 Diels–Alder (known) reaction were envisioned.(+)-13 (+)-15 48 recorded. To this end, a three-step deoxygenation sequence was One attempt involved the formationb. sBuLi, of ketone TMSCl, 20 by a sequence c. Me3Al (1.4 equiv.), CuBr •SMe performed in 52% yield overall on a gram scale (Fig. 3), to generate involving a coupling of the enolate of 2,6-dimethylcyclohexanone2 2 Me CuTC (2 mol%), Me Me ( )-7, which was spectroscopically indistinguishable from natural Oand a primarya'. vinylmagnesium alkyl bromide, a Shapiro reaction onto acrolein, ligand 13 (4 mol%), þ bromide (see ref. 45) Me then THF, TMSCl Me 8 7 (ref. 34), previously synthesized (+)-7 (ref. 37) and bioengineered oxidation and Diels–Alder (disconnection C), much akinMe to the Me D (75%) (86%) (89%, 93% e.e.) ( )-7 (ref. 22). The optical rotation of [a] (CHCl3) 1658 (c 1.0) transformationOEt from 14 to 17O (Fig.[decagram 2). scale] However, ketone 20O [gram scale] OTMS þ ¼þ ¼ 12 (–)-14 is reported herein for the first time and is comparable to that 9already (commercial) required many steps11 (known) to construct, and the stereocentre at d. Acrolein (20 equiv.), obtained for a bioengineered sample of ( )-7 (see Supplementary C8 was challenging to control, despite existing methods in asym- Gd(OTf)3 (10 mol%), þ 49 Information). This represents the largest quantity of pure ( )-7 metric enolate alkylation . Finally, enones 21 were considered as 1:10:4 H 2O/EtOH/PhMe; þ then Jones' reagent (85%) isolated to date. viable intermediates in the formation of 6 (disconnection D) due [gram scale] to our initial difficultiesMe in forging the C2–C3 bondMe via the Me Me Me e. BF •OEt , Me f. PhNTf2, KHMDS; 3 2 Discussion Mukaiyama aldol reaction (videMe supra). Although enones 21Mewere CH2Cl2 Me g. Me2Zn, Pd(PPh3)4 3 Me 1 Me Me 3 In retrospect, the forward synthesis of ( )-taxadienone (6) appears formed in short order and already contained all but one carbon of (47% (+)-17 2 H (84%, two steps) H H þ H H + 29% undesired to be rather simple and perhaps intuitive; however, in practice, this the taxane framework, they didO notMe undergo[gram scale] [4 2] cyclization,O O O O 2 þ diastereomer) 16 (2:1 d.r., mixture of optimized approach required many rounds of strategic and tactical probably due to the rigidity(+)- of6 the sp carbons at(+)- C317 (>99:1 and d.r. C8. at C1) [gram scale] isomers, major revision21. Strategically, the retrosynthesis designed in Fig. 1c was Furthermore, a methyl'Taxadienone' 1,4-addition at C8 was not possible, diastereomer shown) one of many that were considered at the outset. A small snapshot Figure 2 | Enantioselective synthesis of key taxane 6. Conditions: a. 2,3-dimethyl-2-butene, CHBr ,potassiumtert-butoxide, hexanes, 2 h; evaporate volatile a. LiAlH , Et O (72%) 3 of the many evaluated blueprints is presented in Fig. 4. For Me Me 4 2 Me Me 45 materials, then N,N-dimethylaniline,b. 150 KH,8 AcClC, 30 (89%) min (67%); a′. 3-ethoxy-2-cyclohexen-1-one, vinylmagnesium bromide, Et2O, 16 h (75%) ; example, the known difficulties in forming the 6-8-6 tricyclic frame- Me . Me 11–20 b. 10, sec-butyllithium, Et2O, –78 8C,c. Na, 15 min; tBuOH then (82%) CuBrSMe2,30min;thenTMSCl,5min;then11,2h;warmtoroomtemperature,8h;thenAcOH,30min;First lab-prepared 7 (>1g) work of taxanes led us to first consider a ring-closing metathesis Me Me then 3 M HCl, 30 min (86%); c.CuTC(2mol%),phosphoramidite[gram scale] 13 (4 mol%), Et2O, room temperature, 30 min; then 2.0 M Me3Al, enone 12,–308C, H H 7 steps for 6 in 18% yield (RCM) strategy to close the central 8-membered ring (disconnec-24 h; then THF, TMSCl,H 0 C to room temperature, 8 h; then EtH N, Florisil, 2 h (89%, 93% e.e.); d.Gd(OTf) (10 mol%), acrolein, 1:10:4 H O:EtOH:PhMe, O 8 Me 3 Me 3 2 tion A). However, the realization that the required substrate4 C,18 24 h; then evaporate volatiles, then>1 g Jones’ of (+)- reagent,7 acetone, 10 min (85% over two steps, 2:1 d.r. at10 steps for C3, inseparable mixture7 of diastereomers); e. 8 (+)-6 has been (+)-7 would take many steps to build, and the fact that the stereocentresBF .OEt ,CHCl 'Taxadienone',08C, 6 h (47% 17prepared29% undesiredto date diketone);Taxadienef.0.4MKHMDS,PhNTf ,THF,08C, 1 h; g.1.2MMeZn, Pd(PPh ) (5 mol %), THF, 0 8C 3 2 2 2 þ 2 2 3 4 at C1 and C8 would have to be formed with two separate enantio-to room temperature, 5 h (84% over two steps). TMSCl, trimethylsilyl chloride; CuTC, copper(I)thiophene-2-carboxylate;PhNTf2, N-phenylbis(trifluoro selective reactions, dissuaded us from pursuing this route.methanesulfonimide); An Figure 3 | KHMDS,Elaboration potassium of (1)-taxadienone hexamethyldisilazide; (6) to Pd(PPh (1)-taxadiene3)4,tetrakis(triphenylphosphine)palladium. (7) by a aldol route was then conceived, partly due to the facile formation three-step reduction–deoxygenation sequence. Conditions: a.LiAlH4 of 19 via ketone 15 (disconnection B). Although installationof a of large-scale(3.0 equiv.), laboratory Et2O, –78 production8C to room temperature, of enantioenriched 12 h (72 %);7. b.KHepimerize the C1 stereocentre into the desired configuration. the stereocentres at C1 and C8 might still require two independentFurthermore,(7 equiv.), taxadiene acetyl (7 chloride) is produced (4 equiv.), in negligible THF, 60 8C, amounts 18 h (89%); in c.NaHowever, despite a plethora of attempted experiments, the desired t 48 enantioselective transformations, the hope was that the reactionnature (less(18 than equiv.), 1 mg Et2O, can HMPA, be obtainedBuOH, from room temperature,750 kg of tree 40 bark min (82%)cyclization.Sites from 19 did not proceed. Thereafter, strategies involving 34 conditions used for the aldol cyclization would concomitantlyfrom T. brevifoliaof oxidation) and installed therefore onto its taxadiene optical rotation (7)areindicatedinred. has never been closure of the AB ring by a Diels–Alder reaction were envisioned. recorded. To this end, a three-step deoxygenation sequence48 was One attempt involved the formation of ketone 20 by a sequence NATURE CHEMISTRY | VOL 4 | JANUARY 2012 | www.nature.com/naturechemistryperformed in 52% yield overall on a gram scale (Fig. 3), to generate involving a23 coupling of the enolate of 2,6-dimethylcyclohexanone ( )-7, which was spectroscopically indistinguishable from natural and a primary alkyl bromide, a Shapiro reaction onto acrolein, þ 7 (ref. 34), previously synthesized (+)-7 (ref. 37) and bioengineered oxidation and Diels–Alder (disconnection C), much akin to the D ( )-7 (ref. 22). The optical rotation of [a] (CHCl3) 1658 (c 1.0) transformation from 14 to 17 (Fig. 2). However, ketone 20 isþ reported herein for the first time and is comparable¼þ to¼ that already required many steps to construct, and the stereocentre at obtained for a bioengineered sample of ( )-7 (see Supplementary C8 was challenging to control, despite existing methods in asym- Information). This represents the largestþ quantity of pure ( )-7 metric enolate alkylation49. Finally, enones 21 were considered as isolated to date. þ viable intermediates in the formation of 6 (disconnection D) due to our initial difficulties in forging the C2–C3 bond via the Discussion Mukaiyama aldol reaction (vide supra). Although enones 21 were In retrospect, the forward synthesis of ( )-taxadienone (6) appears formed in short order and already contained all but one carbon of to be rather simple and perhaps intuitive;þ however, in practice, this the taxane framework, they did not undergo [4 2] cyclization, optimized approach required many rounds of strategic and tactical probably due to the rigidity of the sp2 carbonsþ at C3 and C8. revision21. Strategically, the retrosynthesis designed in Fig. 1c was Furthermore, a methyl 1,4-addition at C8 was not possible, one of many that were considered at the outset. A small snapshot a. LiAlH , Et O (72%) of the many evaluated blueprints is presented in Fig. 4. For Me Me 4 2 Me Me b. KH, AcCl (89%) example, the known difficulties in forming the 6-8-6 tricyclic frame- Me Me 11–20 c. Na, tBuOH (82%) work of taxanes led us to first consider a ring-closing metathesis Me Me [gram scale] H H (RCM) strategy to close the central 8-membered ring (disconnec- H Me H Me tion A). However, the realization that the required substrate 18 O >1 g of (+)-7 (+)-6 has been (+)-7 would take many steps to build, and the fact that the stereocentres 'Taxadienone' prepared to date Taxadiene at C1 and C8 would have to be formed with two separate enantioselective reactions, dissuaded us from pursuing this route. An Figure 3 | Elaboration of (1)-taxadienone (6) to (1)-taxadiene (7) by a aldol route was then conceived, partly due to the facile formation three-step reduction–deoxygenation sequence. Conditions: a.LiAlH4 of 19 via ketone 15 (disconnection B). Although installation of (3.0 equiv.), Et2O, –78 8C to room temperature, 12 h (72 %); b.KH the stereocentres at C1 and C8 might still require two independent (7 equiv.), acetyl chloride (4 equiv.), THF, 60 8C, 18 h (89%); c.Na t 48 enantioselective transformations, the hope was that the reaction (18 equiv.), Et2O, HMPA, BuOH, room temperature, 40 min (82%) .Sites conditions used for the aldol cyclization would concomitantly of oxidation installed onto taxadiene (7)areindicatedinred.

NATURE CHEMISTRY | VOL 4 | JANUARY 2012 | www.nature.com/naturechemistry 23 NATURE CHEMISTRY DOI: 10.1038/NCHEM.1330 ARTICLES

a R R R

TE OH R P P O O O H O O S HO O S HO O O HO H ACP SH TE OH O O ACP ACP TE ARTICLES Activation? Acyl transfer NATURE CHEMISTRYCyclizationDOI: 10.1038/NCHEM.1330 a 1. i. n-BuLi, 8 ii. 7, then MOM-Cl OH 1. MOM-Cl, i-Pr2EtN O OMOM 1. SmI2, THF TBSO OMOM b THF, −78 to 0 ºC S S CH2Cl2, –5 cºC CH3OH, RT O O IR νmax (C=O) + + SS OH 2. i. n-BuLi, then 9 2. I2, sat. NaHCO3 2. TBS-Cl Cl ii. CH2=CHMgBr MOMO CH3CN, –5 ºC MOMO 9 imidazole MOMO 7 48 9 CuI, 4THF 104 75% (2 steps) 11 DMF, RT 4 HO 78 ºC to RT 5O 3 NaO− 3 MeO 3 8 5 5 88% (2 steps) 2 77% (2 steps)2 2 OH OH OH O O H O O OH OH OH O OMe O OMe O OMe 8' 1 1 1 OH A B C 9' 1,661 cm–1 1,694 cm–1 1,729 cm–1 HO b 1. I , Ph P, imidazole 1. Zn, H O Cat. Bi(NO ) ·5H O O 2 3 2 3 3 2 OMe –1 –1O OMe THF, 80 ºC CH CHO8,9 8',9' HO ∆ν = 33 cmTHF, 0 ºC to RT ∆ν =I 35 cm OTES MarinomycinO A (1; trans3 ∆ , trans ∆ ) O O O 8,9 8',9' 2. TES-Cl, pyridine 2. Ph P=CHCOCH Marinomycin BCH (2Cl; cis, 0 ∆ ºC to, cis RT ∆ ) HO TESO 3 3 2 2 CH Cl , 0 ºC 12 2 2 13 toluene, 110 ºC 8,9 8',9' 5 Deactivation 14 Marinomycin C (3; trans88% ∆ , cis ∆ ) 77% (2 steps) 73% (2 steps) Marinomycin A-macrolide not my favorite d TBSO OMOM c 1. n-BuLi, (i-Pr)2NEt OH n-Bu3SnH 17, Pd2(dba)3, O O 1. PBr3 O O THF, 0 ºC to RT Ph As, LiCl Ester CH Cl , 0 ºC O O 3 2 2 MeO OHC formation O O HO NMe2 HO SnBu3 HO MeO P 2. NaBH4 DMF, 80 ºC MOMO 2. P(OCH3)3 O THF/H O (4:1), 0 ºC OH OH OH O94% O toluene O 15 O O OH 2 OH OH 16 4 5 18 110 ºC + 6 33% (2 steps) OH Ester O O 64% (2 steps) O O formation TfO O MeO 17 O HO Cross Olefination MeO P metathesis O Marinomycin A (1) Figure 2Potent MRSA and VREF anbioc (0.13 | Synthesis of fragments 4, 5 and 6. a,PreparationofdifferentiallyprotecteduM) anti,anti-1,3,5-triol 4. b, Preparation of the syn6 -1,3-dioxane 5. c, Preparation of dienyl phosphonate 6. THF, tetrahydrofuran; RT, room temperature; DMF, N,N-dimethylformamide; TES, triethylsilyl; Pd (dba) , Isolate from marine acnomycete near La Jolla Nicolaou, K. C. et al. J. Am. Chem. Soc. 20072 1293 , 1760. tris(dibenzylidineacetone)dipalladium(0).Figure 1 | Proposed role of salicylate as a molecular switch for macrolide construction and synthetic plan for marinomycin A. a,Proposedmechanismfor salicylate-type macrolactonization involving an ACP thioester and thioesterase. b,Infrareddataillustratingdeactivationoftheortho-carboxylate for the phenol and sodiumTBSO phenoxideOMOM versus the methyl aryl ether. c, Structure of marinomycin A–C (1–3). d,Retrosyntheticanalysis:triplyconvergentcouplingand

dimerization strategy. ACP, acyl carrier protein; TE, thioesterase; P, functionalization or hydrogen bond; IR, infrared spectrum; nmax, maximum absorption; MOM, methoxymethyl; TBS, tert-butyldimethylsilyl. NaH, DME MOMO 4 Cat. Hoveyda-Grubbs IITBSO OMOM O O EtO2CCH2P(O)(OEt)2 TBSO OMOM O O R1 resistance, which+ provides the impetus to develop a practical synthesis 7 with the lithium anion of 1,3-dithiane 8, followed by in situ CH2Cl2, 40 ºC O 0 ºC to RT of this agent to facilitate detailed SAR studies and elucidate the mech- protection of the resulting secondary alkoxide with methoxymethyl2 80% 63% 20a / 20% 20b R anism ofO action.O O MOMO 19 chloride furnishedMOMO the monosubstituted dithiane in quantitative yield. 1. n-BuLi, p-MeOC6H4SH,20 1 2 We envisioned that the macrodiolide could be prepared by The lithium anion of the dithiane was20a preformedR = CO2Et, and R = treatedH with 0 ºC, to RT sequential formation of the ester groups using the aforementioned (R)-(–)-epichlorohydrin2. m-CPBA,9 to−78 initiateºC to RT ring opening1 /2 closure, followed 5 20b R = H, R = CO2Et salicylate switch to modulate the reactivity of the esters (Fig. 1d). by treatment of the50% 20a terminal / 38% 20b epoxide with vinyl magnesium A potential problemO withO this strategy lies in the opportunity for bromide in the presence(2 ofsteps) copper iodide to afford the homoallylic competitiveMeO intramolecular cyclization of the dimeric seco-acid to alcohol 10 in 77% yield (2 steps). Protection of the homoallylic O 1. DIBAL-H, CH2Cl2, −78 ºC MeO P provide the macrolide. For example,TBSO the attemptedOMOM doubleO O Suzuki alcohol 10 as the methoxymethyl2. ether TPAP, andNMO, removal4Å MS, CH2Cl of2, theRT dithiane O 3. 6, NaHMDS, THF, −78 ºC dimerization to6 form the macrodiolide favours intramolecular with iodine buffered with sodium hydrogen carbonate furnished 17 O cyclization to provide the macrolide . We envisioned that a acyclic ketone 11 in 75% overall yield88% (2(3 steps) steps). Stereoselective 19 21 syn-1,3-dioxane would provide aMOMO conformational lock to enforce reduction ofO11 usingO samarium diiodide under Keck conditions the linear chain conformation of the monomer and thereby suppress21 furnished the secondary alcohol (diastereoselectivity 19:1 by their intramolecular cyclization. The monomeric seco-acid would in NMR), which was protected as the tert-butyldimethylsilyl≥ ether in Figure 3 | Synthesis of differentially protected monomer 21. Monomer 21 was prepared by a five-step sequence involving cross metathesis of fragments 4 andturn5 to be afford readily ketone derived19,chainelongationandsubsequentcouplingoftheresultingaldehydewithphosphonate from a triply convergent assembly of the 88% overall yield (two6. DME, steps) 1,2-dimethoxyethane; to complete them-CPBA, synthesis of the 3-chloroperbenzoicprotected anti,anti acid;-1,3,5-triol RT, room temperature;4 with the DIBAL-H,syn-1,3-dioxane diisobutylaluminium5 followed hydride;differentially TPAP, tetrapropylammonium protected anti perruthenate;,anti-1,3,5-triol NMO,4.N-methylmorpholine N-oxide;by homologation MS, molecular and sieves; coupling NaHMDS, with sodium the dienyl hexamethyldisilazide. phosphonate 6 using Figure 2b outlines the five-step sequence used for the construc- cross metathesis and olefination reactions. tion of the 1,3-dioxane fragment 5 from commercially available 1- subjected to a Wittig olefination to provide the a,b-unsaturated phosphiteO-methyl-2-deoxy- furnished theD dienyl-ribose phosphonate12. Iodination in 64% of the overall primary yield alcohol ketoneResults14 in and 77% discussion overall yield (E/Z 19:1). The synthesis of the (twoin steps).12 followed by protection of the secondary alcohol with a triethy- syn-The1,3-dioxane differentially5 was protected completedanti,anti≥ in-1,3,5-triol a highly4 stereoselectivewas prepared in Figuresilyl group 3 outlines furnished the fragment the iodofuranoside coupling sequence13 in 73% to provide yield. Vasella manner51% overall (diastereoselectivity yield using the19:1) six-step from sequence the d outlined-triethylsilyloxy in Fig. 2a.the monomericfragmentation precursor22 of the to demonstrateprimary alkyl the iodide dimerization13 with strategy. zinc powder a,bRegioselective-unsaturated ketone ring opening14,usinganovelbismuth-mediatedtwo-com-≥ of enantiomerically enriched epoxideCrossafforded metathesis the26 of corresponding the olefins, 4 and aldehyde,5, with Hoveyda–Grubbs which wasimmediately II cat- ponent hemiacetal/oxa-conjugate addition reaction with acetaldehyde23. alyst furnished 19 in 80% yield, favouring the E-isomer (E/Z 19:1). NATUREThe dienyl CHEMISTRY phosphonate| VOL 4 | AUGUST6 was prepared2012 | www.nature.com/naturechemistry from Zincke aldehyde Although the Horner–Wadsworth–Emmons homologation of≥ methyl 681 15 using the five-step sequence outlined in Fig. 2c. Stannylation24 ketone 19 provided the a,b-unsaturated esters 20a/20b with modest and reduction of the conjugated aldehyde in 15 furnished the dienyl selectivity (E/Z 3:1), the isomers were readily separable by stannane 16 with 19:1 E/Z selectivity. Stille cross-coupling of the column chromatography¼ and the Z-isomer 20b recycled by the follow- dienyl stannane 16≥ with the known aryl triflate 1725 provided ing two-step sequence27. Treatment of the a,b-unsaturated ester 20b dienyl salicylate 18 in 94% yield. Finally, bromination of primary with the anion of 4-methoxybenzenethiol followed by the oxidation alcohol 18 followed by an Arbuzov reaction with trimethyl of the resulting sulfide to the sulfoxide and syn-elimination, furnished

682 NATURE CHEMISTRY | VOL 4 | AUGUST 2012 | www.nature.com/naturechemistry NATURE CHEMISTRY DOI: 10.1038/NCHEM.1330 ARTICLES

–1 –1 νmax = 1,732 cm νmax = 1,655 cm 1. TBAF, NH4F TBSO OMOM O O THF, RT OH OMOM O O

O 2. NaHMDS OH TMSE-OH MOMO O O THF, 0 ºC MOMO O OTMSE NATURE CHEMISTRY21 DOI: 10.1038/NCHEM.1330 ARTICLES22 81% (2 steps)

–1 –1 νmax = 1,732 cm νmax = 1,655 cm 1. TBAF, NH4F TBSO OMOM O O THF, RT OH OMOM O O 22, NaHMDS, then 21 82% NATURE CHEMISTRY DOI: 10.1038/NCHEM.1330 O 2. NaHMDS THF, 0 ºC OH TMSE-OH ARTICLES MOMO O O MOMO O OTMSE 21 THF, 0 ºC 22 OMOM 81% (2 steps) –1 –1 νmax = 1,732 cm νmax = 1,655 cm HO 1. TBAF, NH4F 22, NaHMDS, then 21 TBSO OMOM O O 82% THF, RTO O MOMOOH OMOM O O THF, 0 ºC OTBS O O OMOM O O O 2. NaHMDS OMOM OH TMSE-OH MOMO HO O O MOMO O OTMSE 21 THF, 0 ºC 22 MOMO O O MOMO TMSEOOTBS C O O 2 OMOM O 81%O (223 steps) OH

Figure 4 | Salicylate molecular switch-mediated dimerization.MOMO Dimeric ester 23 was preparedTMSEO2C by coupling electrophilic monomer 21 with the dianion 23 22, NaHMDS, then 21 82% OH of deactivated monomer 22. TBAF, tetrabutylammonium fluoride; RT, room temperature;THF, 0 TMSE-OH, ºC 2-(trimethylsilyl)ethanol; NaHMDS, sodium Figure 4 | Salicylate molecular switch-mediated dimerization. Dimeric ester 23 was prepared by coupling electrophilic monomer 21 with the dianion bis(trimethylsilyl)amide. OMOM of deactivated monomer 22. TBAF, tetrabutylammonium fluoride; RT, room temperature; TMSE-OH, 2-(trimethylsilyl)ethanol; NaHMDS, sodium bis(trimethylsilyl)amide. HO OMOM OMOM OMOM OMOM O O MOMO OTBS O O HO OMOM O MOM-ClO MOMO HO MOM-Cl MOMO O O O MOMOO MOMOOTBSOTBS ii-Pr-Pr2NEt2NEt O O MOMO OOTBS O MOMO OTBS O O OMOM O O O OOMOM O O O OO OMOMO OOMOMO O O CH2Cl2, 0 ºC CH2Cl2, 0 ºC MOMO TMSEO2C 94% 23 MOMO TMSEO2C 94% MOMO OH TMSEO2C 24 23 OH OMOM MOMO TMSEO2C MOMO TMSEO2C Figure 4 | Salicylate molecular switch-mediated23 dimerization. Dimeric ester 23 was prepared by coupling electrophilic monomer24 21 with the dianion OH 1. TBAF, NH4F, THF, RT OMOM of deactivated monomer 22. TBAF, tetrabutylammonium fluoride; RT, room temperature; TMSE-OH,34% 2-(trimethylsilyl)ethanol;(2 steps) 2. 2-bromo-1-ethylpyridinium NaHMDS, sodium tetrafluoroborate 1. TBAF, NH4F, THF, RT bis(trimethylsilyl)amide. NaHCO3, CH2Cl2, RT OH 34% (2 steps) 2. 2-bromo-1-ethylpyridiniumOMOM tetrafluoroborate OMOM HO OMOM MgBr2 MOMO n-BuSH NaHCO3, CH2Cl2, RT OH OH OH O OHO O O MOMO O O OMOM O O OH OH OH O O OMOM O O HO MOM-ClCH3NO2 MOMO OH Et O, RT OMOM i-Pr2 NEt O O MOMO HO O O MOMO OTBS MgBr2 2 MOMO OTBS O O 61% O O OMOM HO O O n-BuSH MOMO OMOM O O OH MarinomOH ycin A (1) OH 25 O O MOMO O O O O CH2Cl2, 0 ºC O O OH OH OH O O OMOM O O Figure 5 | Completion of the total synthesis of marinomycin A. The C -symmetricalCH94%3NO2 macrodiolide 25 was prepared by a three-step sequence involving OH2 OMOM protection of the deactivated phenols in dimer 23, selective cleavage of theEt2 silylO, RT ether, and a modified Mukaiyama macrolactonization. Final deprotection MOMO TMSEO2C MOMO TMSEO2C afforded the natural product, marinomycin A. 24 23 OH 61% OMOM HO MOMO 20a/20b Marinomin 84% overallycin A yield(1) and with 1.3:1 E/Z selectivity. This with 2-(trimethylsilyl)ethanol, to provide the251. TBAF, deactivated NH F, THF, RT enabled the preparation of a,b-unsaturated ester 20a in 76% monomer 22 in 81% overall yield. The differentiation in reactivity4 overall yield after recycling the Z-isomer 20b twice. Reduction of is evident from a comparison of34% the (2 infrared steps) stretch2. for2-bromo-1-ethylpyridinium the two Figure 5 | Completion of the total synthesis of marinomycin A. The C -symmetrical macrodiolide2 25 was prepared by a three-step sequence involving the a,b-unsaturatedP. A. Evans et al. ester 20a with diisobutylaluminiumNat. Chem. hydride,2 esters2007 (Dn 12977 cm , 1760. 1), which provided the impetus totetrafluoroborate test the ¼ protection of the deactivatedfollowed phenols by oxidation in ofdimer the allylic23, selectivealcohol with cleavage tetrapropylammo- of the silylhypothesis ether, and for a the modified phenoxide Mukaiyama as outlined macrolactonization. earlier. Gratifyingly,NaHCO3, CH2Cl Final2, RT deprotection afforded the natural product,nium perruthenate marinomycin (TPAP) A.28, afforded the a,b-unsaturatedOH alde- treatment of electrophilic monomer 21 with the dianion of the deac- OMOM hyde, which was subjected to fragment coupling with dienyl tivated monomer 22 furnished the dimeric ester 23 in 82% overall HO phosphonate 6 to provide the differentially protected monomerMgBr2 yield.MOMO Interestingly, the removal of the tert-butyldimethylsilyl 20a/20b in 84% overall21 in 88% yield overall and yield with for three 1.3:1 steps,E with/Z selectivity.19:1 E/Z geometry. Thisn-BuSHgroupwith from 2-(trimethylsilyl)ethanol,21 provided the corresponding secondary to alcohol, provide the deactivated In accord with ourOH hypothesis,OH we envisionedOH ≥ O thatO the salicylate which undergoes base-mediated decomposition underO analogousO MOMO O O enabledO theO preparation of a,b-unsaturated ester 20a in 76% monomerO O22 in 81% overall yield. The differentiation in reactivity OH wouldOH attenuateOH the reactivity of the esters and thereby facilitate the conditions. This is in sharpOMOM contrastO to theO deactivated ester 22, CH3NO2 overall yield after recyclingtransesterification the ofZ21-isomerwith 22 to20b providetwice. dimeric Reduction ester 23OH(Fig. 4). of whichis evidentis stable to strong from base, aconfirming comparison the necessity of the to deactivate infrared stretch forOMOM the two Et2O, RT 21 the a,b-unsaturatedIn ester practice,20a the salicylatewith diisobutylaluminium esters were differentiated via ahydride, two-step theesters salicylate ( esterDn of the77 pronucleophile cm ), which for acylation. provided the impetus to test the sequence, which involved the removal of the tert-butyldimethylsilyl61% Figure 5 outlines¼ the completion of the total synthesis of ( )-mar- followed by oxidation of the allylic alcohol with tetrapropylammo- hypothesisMOMO for the phenoxide as outlined earlier. Gratifyingly, HO ether in 21 with ammonium fluoride buffered tetrabutylammonium inomycin A (1). Protection of the deactivated phenols in theþ dimeric Marinom28 ycin A (1) 25 nium perruthenatefluoride (TPAP) and transesterification, afforded the ofa the,b salicylic-unsaturated acid acetonide alde-29 estertreatment23 as methoxymethyl of electrophilic ethers furnished monomer the bis-MOM21 arylwith ether the dianion of the deac- hyde, which was subjected to fragment coupling with dienyl tivated monomer 22 furnished the dimeric ester 23 in 82% overall Figure 5 | CompletionNATURE of the CHEMISTRY total synthesis| VOL 4 | AUGUST of 2012 marinomycin | www.nature.com/naturechemistry A. The C -symmetrical macrodiolide 25 was prepared by a three-step683 sequence involving phosphonate 6 to provide the differentially protected monomer2 yield. Interestingly, the removal of the tert-butyldimethylsilyl 21protectionin 88% of overall the deactivated yield for phenols three steps,in dimer with23, selective19:1 E cleavage/Z geometry. of the silyl ether,group and from a modified21 Mukaiyamaprovidedthe macrolactonization. corresponding Final secondary deprotection alcohol, afforded the natural product, marinomycin A. In accord with our hypothesis, we envisioned≥ that the salicylate which undergoes base-mediated decomposition under analogous would attenuate the reactivity of the esters and thereby facilitate the conditions. This is in sharp contrast to the deactivated ester 22, 20a/20b in 84% overall yield and with 1.3:1 E/Z selectivity. This with 2-(trimethylsilyl)ethanol, to provide the deactivated transesterificationenabled the preparation of 21 with of22ato,b-unsaturated provide dimeric ester ester20a23in(Fig. 76% 4). monomerwhich is stable22 in to 81% strong overall base, yield. confirming The differentiation the necessity in to reactivity deactivate Inoverall practice, yield the after salicylate recycling esters the Z were-isomer differentiated20b twice. via Reduction a two-step of isthe evident salicylate from ester a comparison of the pronucleophile of the infrared for acylation. stretch for the two sequence, which involved the removal of the tert-butyldimethylsilyl Figure 5 outlines2 the1 completion of the total synthesis of ( )-mar- the a,b-unsaturated ester 20a with diisobutylaluminium hydride, esters (Dn 77 cm ), which provided the impetus to testþ the etherfollowed in 21 bywith oxidation ammonium of the fluoride allylic alcohol buffered with tetrabutylammonium tetrapropylammo- hypothesisinomycin A¼ for (1). the Protection phenoxide of the as deactivated outlined earlier. phenols Gratifyingly, in the dimeric 29 fluoridenium perruthenate and transesterification (TPAP)28, afforded of the the salicylica,b-unsaturated acid acetonide alde- treatmentester 23 as of methoxymethyl electrophilic monomer ethers furnished21 with the the dianion bis-MOM of the aryl deac- ether hyde, which was subjected to fragment coupling with dienyl tivated monomer 22 furnished the dimeric ester 23 in 82% overall NATUREphosphonate CHEMISTRY6 |to VOL provide 4 | AUGUST the 2012 differentially | www.nature.com/naturechemistry protected monomer yield. Interestingly, the removal of the tert-butyldimethylsilyl683 21 in 88% overall yield for three steps, with 19:1 E/Z geometry. group from 21 provided the corresponding secondary alcohol, In accord with our hypothesis, we envisioned≥ that the salicylate which undergoes base-mediated decomposition under analogous would attenuate the reactivity of the esters and thereby facilitate the conditions. This is in sharp contrast to the deactivated ester 22, transesterification of 21 with 22 to provide dimeric ester 23 (Fig. 4). which is stable to strong base, confirming the necessity to deactivate In practice, the salicylate esters were differentiated via a two-step the salicylate ester of the pronucleophile for acylation. sequence, which involved the removal of the tert-butyldimethylsilyl Figure 5 outlines the completion of the total synthesis of ( )-mar- ether in 21 with ammonium fluoride buffered tetrabutylammonium inomycin A (1). Protection of the deactivated phenols in theþ dimeric fluoride and transesterification of the salicylic acid acetonide29 ester 23 as methoxymethyl ethers furnished the bis-MOM aryl ether

NATURE CHEMISTRY | VOL 4 | AUGUST 2012 | www.nature.com/naturechemistry 683 ARTICLES PUBLISHED ONLINE: 23 SEPTEMBER 2012 | DOI: 10.1038/NCHEM.1458 ARTICLES ARTICLES PUBLISHED ONLINE: 23 SEPTEMBER 2012 | DOI: 10.1038/NCHEM.1458 PUBLISHED ONLINE: 23 SEPTEMBER 2012 | DOI: 10.1038/NCHEM.1458 Synthesis of highly strained terpenes by non-stopARTICLES tail-to-headSynthesis of polycyclization highly strainedPUBLISHEDSynthesis terpenes ONLINE: 23 SEPTEMBER of by 2012 highly | DOI: non-stop 10.1038/NCHEM.1458 strained terpenes by non-stop Sergeytail-to-head V. Pronin and Ryan polycyclization A. Shenvi* tail-to-head polycyclization SynthesisSergey V. Pronin and of Ryan highly A. Shenvi strained terpenes by non-stop Non-stop carbocationic polycyclizations of isoprenoids* have beenSergey called the V. Pronin most complex and Ryan chemical A. Shenvi reactions* occurring in nature. We describe a strategy for the initiation of tail-to-head polycyclization that relies on the sequestration of the counteraniontail-to-headNon-stop carbocationic away from polycyclizations the carbocation, polycyclization of which isoprenoids allows have full been propagationNon-stop called the carbocationic of most the complex cationic polycyclizations chemical charge. reactionsIf theof isoprenoids anion occurring is mobile, have in been called the most complex chemical reactions occurring in Coulombicnature. We forces describe hold a this strategy species for in the close initiation proximity of tail-to-head to thenature. carbocation polycyclization We describe and that cause a relies strategy preemptive on for the the sequestration termination initiation of of tail-to-headthrough the polycyclization that relies on the sequestration of the elimination.Sergeycounteranion V. Anion Pronin away sequestration and from Ryan the carbocation, A. is Shenvi crucial which for* effecting allows full the propagation biomimeticcounteranion of synthesis the away cationic from of the complex charge. carbocation, If and the unstable anion which is allows mobile, terpenes, full propagation of the cationic charge. If the anion is mobile, includingCoulombic the forces highly hold strained this species funebrenes. in close This proximity study illustrates to theCoulombic carbocation the deleterious forces and cause hold role this preemptiveof speciesthe counterion in termination close inproximity tail-to-head through to the carbocation and cause preemptive termination through carbocationicelimination. polycyclization Anion sequestration reactions, is crucial which for to effecting the best the of biomimetic ourelimination. knowledge synthesis Anion has sequestration not of complex been rigorously is and crucial unstable explored.for effecting terpenes, These the biomimetic synthesis of complex and unstable terpenes, observationsNon-stopincluding carbocationic the are highly also strainedexpected polycyclizations funebrenes. to find of use isoprenoids This in study the design have illustrates been andincluding the called control deleterious the the of most highly cationic role complex strained of polycyclization the chemical funebrenes. counterion reactions along This in tail-to-head study biosynthetic occurring illustrates in the deleterious role of the counterion in tail-to-head carbocationic polycyclization reactions, which to the best of our knowledge has not been rigorously explored. These pathwaysnature.carbocationic We that describe have polycyclization previously a strategy been reactions, for inaccessible the which initiation to in bulk the of tail-to-head best solvent. of our polycyclization knowledge has that not been relies rigorously on the sequestration explored. These of the counteranionobservations away are also from expected the carbocation, to find use which in the allows design full andpropagationobservations control of of are cationic the also cationic expectedpolycyclization charge. to find If along the use anion in biosynthetic the is design mobile, and control of cationic polycyclization along biosynthetic pathways that have previously been inaccessible in bulk solvent. Coulombicpathways that forces have hold previously this species been inaccessible in close proximity in bulk solvent. to the carbocation and cause preemptive termination through elimination.or decades, Anion chemists sequestration have been fascinated is crucial by the for manipulation effecting the2-epi- biomimetica-cedrene synthesis (4a,b, Fig. 2a) of20,23 complex, but these and protonation unstable conditions terpenes, includingof carbocationic the highly charge strained that occurs funebrenes. during the course This study of terpene illustratesare non-selective, theor decades, deleterious chemists low yielding role have of been and the fascinated lead counterion to by complex the in manipulation tail-to-head mixtures of2-epi-a-cedrene (4a,b, Fig. 2a)20,23, but these protonation conditions a 20,23 carbocationicbiosynthesisor decades,1–5 polycyclization chemists. The biosynthetic have been reactions, fascinated pathways which by of the terpenes manipulationto the can best be ofvarious2-epi- ourof-cedrene knowledge skeletal carbocationic ( types4a,b has, charge Fig. (Fig. not 2a) that3). been Cryogenic occurs, but these rigorously during protonation protonation the course explored. conditions of of terpene polyenes Theseare non-selective, low yielding and lead to complex mixtures of F 1–5 observationsof carbocationic are also charge expected that occurs to during find the use course in the of terpene designare and non-selective, controlbiosynthesis of low cationic. The yielding biosynthetic polycyclization and lead pathways to complex ofalong terpenes mixtures biosynthetic can be of various skeletal types (Fig. 3). Cryogenic protonation of polyenes broadly categorized1–5 into two major branches, which differ in the withF super-acid delivers a novel series of thermodynamically pathwaysFbiosynthesis that have. The previously biosynthetic been pathways inaccessible of terpenes in bulk can solvent. be variousbroadly skeletal categorized types (Fig.into two 3). Cryogenic major branches, protonation which differ of polyenes in the with super-acid delivers a novel series of thermodynamically direction of charge propagation along the polyisoprene chain. The stable carbocycles, but these structures are not relevant to biosyn- broadly categorized into two major branches, which differ in the withdirection super-acid of charge24,25 delivers propagation a novel along series the polyisoprene of thermodynamically chain. The stable carbocycles, but these structures are not relevant to biosyn- first branch, comprising the head-to-tail (HT) cyclization thetic pathways . Naturally occurring strained terpenes (for 24,25 direction6,7 of charge propagation along the polyisoprene chain. The stablefirst carbocycles, branch, comprising but these structures the head-to-tail are not relevant (HT) cyclizationto biosynthetic pathways . Naturally occurring strained terpenes (for pathway , involves electrophilic activation of a terminal prenyl 6,7 24,25 firstor decades, branch, chemists comprising have the been head-to-tail fascinated by (HT) the manipulation cyclization thetic2-epi-pathway pathwaysa-cedrene, involves (4a., Naturallyb electrophilic, Fig. 2a)20,23 occurring activation, but these strained of protonation a terminal terpenes conditions prenyl (for unitpathway (head)6,7 and, involves gives rise electrophilic to the steroids, activation non-steroidal of a terminal triterpenes prenyl a unitHead-to-tail (head) polycyclization and gives rise to the steroids, non-steroidal triterpenes of carbocationic charge that occurs8 during the course of terpene are non-selective, low yielding and lead to complex• Non-stop: mixtures complete ofa Head-to-tail polycyclization and other poly-decalin frameworks (Fig. 1a). The second, the 8 R R • Non-stop: complete unit (head) and1–5 gives rise to the steroids, non-steroidal triterpenes a Head-to-tailand other polycyclization poly-decalin frameworks (Fig. 1a). The charge second, propagation the biosynthesis . The biosynthetic9 pathways of terpenes can be various skeletal types (Fig.9 3). Cryogenic protonation• of polyenes charge propagation tail-to-headFand other (TH) poly-decalin pathway frameworks, is initiated8 (Fig. by ionization 1a). The of second, an allylic the tail-to-head (TH) pathway+ , is initiated byR ionizationNon-stop: of complete an allylic + pyrophosphatebroadly categorized (tail) and into results two9 major in diverse branches, polycyclic which frameworks differ in the withpyrophosphateH super-acid (tail) delivers+ andX results a novel in diverse series polycyclic of charge thermodynamically• Reproducible propagation frameworks in H + X • Reproducible in tail-to-head (TH) pathway , is initiated by ionization of an allylic R + bulk solvent n R bulk solvent n or [O] • or [O] suchdirection as funebrene of charge (1a propagation), cumacrene along (2) the and polyisoprene taxadiene chain.(Fig. 1b). The stablesuchH carbocycles, as funebrene but+ ( 1aX ), these cumacrene structures (2) are and not taxadieneReproducible relevant (Fig. toin biosyn-1b). + pyrophosphate (tail) and results in diverse polycyclic frameworks R 24,25 then H+ then H Polyisoprenesn or [O] + bulk• solvent Polyisoprenes + • Extensively studied Interrogationfirstsuch branch, as funebrene of the comprising mechanisms (1a), cumacrene the of the head-to-tail two (2) pathwaysand taxadiene (HT) is not cyclization (Fig. straight- 1b). theticInterrogation pathways of the. mechanisms Naturally ofX occurring the two pathways strainedExtensively is not terpenes straight- studied (for X 6,7 Strained then Hterpene+ synthesis-Tail-to-Head cyclizaon and used in synthesis10,11 and used in synthesis pathway , involves electrophilic activation of a terminal prenyl10,11 forwardPolyisoprenes and is the focus of considerable+ research• Extensively and debate studied . forwardInterrogation and is the of the focus mechanisms of considerable of the two research pathways and is not debate straight-. X Head + Tail 10,11 Although the fundamentals ofHead HT+ polycyclizationTail and have used been in synthesis eluci- unitforward (head) and and is gives the focus rise to of the considerable steroids, non-steroidal research and debate triterpenes. a Head-to-tail polycyclization Although the fundamentals of HT polycyclization8 have been eluci- Head + Tail • 12–17 and other poly-decalin frameworks (Fig. 1a). The second,12–17 the dated by studying these polycyclizations inR bulk solventNon-stop: complete,TH OH datedAlthough by studying the fundamentals these polycyclizations of HT polycyclization in bulk solvent have been eluci-,TH OH charge propagation H 9 12–17 polycyclizations have remained relatively unexplored. polycyclizationstail-to-headdated by studying (TH) have pathway theseremained polycyclizations, is relatively initiated unexplored. by in ionization bulk solvent of an allylic,TH OH + H OH H Here, we show the+ first X biomimeticOH cationic polycyclization• ReproducibleH along in OH H pyrophosphatepolycyclizations (tail) have and remained results relatively in diverse unexplored. polycyclic frameworks R H H Here, we show the first biomimetic cationic polycyclization along a TH biosyntheticn pathway.or [O] TheOH successOH of these polycyclizations bulk solventH such as funebrene (1a), cumacrene (2) and taxadiene (Fig. 1b). H H Br Here, we show the first biomimetic cationic polycyclization along then H+ OH H H H a TH biosynthetic pathway. The success of these polycyclizations reliesPolyisoprenes onH a counteranion sequestrationH + strategy similar• Extensively to that occur- studied H Interrogationa TH biosynthetic of the mechanisms pathway. The of success the two of pathways these polycyclizations is not straight- Br X H relies on a counteranion sequestration strategy similar to that occur- ringH in cyclase enzymes.H As an initial proof of principle, and used we in synthesis also HO 10,11 Br H Albicanol Aplysin-20 H Lupeol forwardrelies on and a counteranion is the focus sequestration of considerable strategy research similar and to that debate occur-. H H ring in cyclase enzymes. As an initial proof of principle, we also report the first total synthesesHead of+ theHO strainedTail funebrene and Althoughring in cyclase the fundamentals enzymes. As of an HT initial polycyclization proof of principle, have been we eluci- also Albicanol Aplysin-20 HO H Lupeol cumacreneAlbicanol terpenes. Aplysin-20 H Lupeol report the first total syntheses of the strained funebrene12–17 and b Tail-to-head polycyclization datedreport by the studying first total these syntheses polycyclizations of the in strained bulk solvent funebrene,TH and MimicryOH of TH polycyclizations using non-enzymatic methods • Non-stop: complete cumacrene terpenes. H polycyclizationscumacrene terpenes. have remained relatively unexplored. b hasTail-to-head proven polycyclization challenging. Unlike HT cascades, the pathways of TH – charge propagation Mimicry of TH polycyclizations using non-enzymatic methods b Tail-to-head polycyclization OH • Non-stop: complete – OPP Mimicry of TH polycyclizations using non-enzymatic methods • Non-stop: complete + • Here, we show the first biomimetic cationic polycyclization along polycyclizations contain multipleOH branch points andlead charge to propagation numer-H H H Never reproduced in has proven challenging. Unlike HT cascades, the pathways of TH H – H charge propagation PPO bulk solvent ahas TH proven biosynthetic challenging. pathway. Unlike The HT success cascades, of these the pathways polycyclizations of TH ous skeletons in low– yields,OPP– or they terminate with the formation of n n polycyclizations contain multiple branch points and lead to numer- HBr – OPP + • Never reproduced in • polycyclizations contain multiple branch points and lead to numer- only a singleH ring. WithinH the+ sesquiterpene family,H H• Never cyclizationH reproduced in of Polyisoprenyl Under-explored relies on a counteranion sequestration strategy similar to that occur- PPO H bulk solvent ous skeletons in low yields, or they terminate with the formation of PPO n n bulk solvent diphosphates ringous in skeletons cyclase in enzymes. low yields, As or an they initial terminate proof with of principle, the formation we also of farnesyl or nerolidyln derivatives to a six-memberedHO n ring (bisabolene Tail + Head only a single ring. Within the sesquiterpene family, cyclization of skeleton)Albicanol has been extensivelyAplysin-20 investigated in bulk• solventH• Under-explored18–22Lupeol, but reportonly a the single first ring. total Within syntheses the sesquiterpene of the strained family, funebrene cyclization and of Polyisoprenyl Under-explored farnesyl or nerolidyl derivatives to a six-membered ring (bisabolene thediphosphates charge has never beenTailTail shown+ to propagateHeadHead beyond these cumacrenefarnesyl or terpenes. nerolidyl derivatives to a six-membered ring (bisabolene + H skeleton)skeleton) has has been been extensively extensively investigated investigated in in bulk bulk solvent solvent18–2218–22,, but but binitialTail-to-head monocycles polycyclization (Fig. 2a). Subsequent cyclizations require either Mimicry of TH polycyclizations using non-enzymatic methods strained ring formation, anti-Markovnikov alkene addition• Non-stop: or inter- complete thethe charge charge has has never never been been shown shown to to propagate propagate beyond beyond these these HH charge propagation H H has proven challenging. Unlike HT cascades, the pathways of TH mediate Wagner–Meerwein– rearrangement, none of which has H initialinitial monocycles monocycles (Fig. (Fig. 2a). 2a). Subsequent Subsequent cyclizations cyclizations require require either either – OPP polycyclizations contain multiple branch points and lead to numer- + • strained ring formation, anti-Markovnikov alkene addition or inter- proven to beH facile, and therefore quenchingH of theNever resultant reproduced in β-Funebrene (1a) Cumacrene (2) Taxa-4,11-diene strained ring formation, anti-Markovnikov alkene addition or inter- PPO H bulk solventH ous skeletons in low yields, or they terminate with the formation of cation via eliminationH n (E1) or anion capture (SNn1) occursH before mediate Wagner–Meerwein rearrangement, none of which has H mediate Wagner–Meerwein rearrangement, none of which has • H only a single ring. Within the sesquiterpene family, cyclization of anyPolyisoprenyl subsequent rings are formed. To circumvent thisUnder-explored problem, Figure 1 | Different branches of terpene biosynthesis. a,Conventional proven to be facile, and therefore quenching of the resultant -Funebrene (1a) Cumacrene (2) Taxa-4,11-diene proven to be facile, and therefore quenching of the resultant βstrong-Funebrenediphosphates acid (1a can) be used to reprotonateCumacrene (2) unsaturated bisaboleneTaxa-4,11-diene inter- cationic polycyclizations in bulk solvent proceed in the HT direction. farnesylcation or via nerolidyl elimination derivatives (E1) or to anion a six-membered capture (S 1) ring occurs (bisabolene before Tail + Head cation via elimination (E1) or anion capture (SNN1) occurs18–22 before mediates (3) and access natural terpenes such as a-cedrene and b,THpolycyclizationsarenotwellexplored. skeleton)any subsequent has been rings extensively are formed. investigated To circumvent in bulk solvent this problem,, but Figure 1 | Different branches of terpene biosynthesis. a,Conventional anythe subsequent charge has rings never are been formed. shown To to circumvent propagate this beyond problem, these Figure 1 | Different branches of terpene biosynthesis. a,Conventional strongstrong acid acid can can be be used used to to reprotonate reprotonate unsaturated unsaturated bisabolene bisabolene inter- inter- cationic polycyclizations polycyclizations in in bulkS. V. bulk solvent solventProninH proceed proceed, R. A. in inShenvi the the HT HT direction. Nat. Chem direction. . 2012 4, 915. initialmediates monocycles (3) and (Fig. access 2a). natural Subsequent terpenes cyclizations such as a-cedrene require either and b,THpolycyclizationsarenotwellexplored. mediatesstrained (ring3) and formation, access anti-Markovnikov natural terpenes such alkene as additiona-cedrene or inter- and b,THpolycyclizationsarenotwellexplored.Department of Chemistry, The Scripps Research Institute, 10550 North Torrey Pines Road, La Jolla, California 92037, USA. *e-mail: [email protected] H H mediate Wagner–Meerwein rearrangement, none of which has H NATURE CHEMISTRY | VOL 4 | NOVEMBER 2012 | www.nature.com/naturechemistry 915 proven to be facile, and therefore quenching of the resultant Cumacrene (2) Department of Chemistry, The Scripps Research Institute, 10550 North Torrey Pinesβ Road,-Funebrene La Jolla, (1a) California 92037, USA. *e-mail: [email protected],11-diene Departmentcation via of elimination Chemistry, The (E1) Scripps or anion Research capture Institute, (SN 105501) occurs North before Torrey Pines Road, La Jolla, California 92037, USA. *e-mail: [email protected] anyNATURE subsequent CHEMISTRY rings| VOL 4 are | NOVEMBER formed. 2012 To | www.nature.com/naturechemistry circumvent this problem, Figure 1 | Different branches of terpene biosynthesis. a,Conventional915 NATUREstrong CHEMISTRY acid can be| VOL used 4 | NOVEMBER to reprotonate 2012 |unsaturated www.nature.com/naturechemistry bisabolene inter- cationic polycyclizations in bulk solvent proceed in the HT direction. 915 mediates (3) and access natural terpenes such as a-cedrene and b,THpolycyclizationsarenotwellexplored.

Department of Chemistry, The Scripps Research Institute, 10550 North Torrey Pines Road, La Jolla, California 92037, USA. *e-mail: [email protected]

NATURE CHEMISTRY | VOL 4 | NOVEMBER 2012 | www.nature.com/naturechemistry 915 ARTICLES NATURE CHEMISTRY DOI: 10.1038/NCHEM.1458 example, 1a or 2) have not been observed in any earlier studies and trifluoroacetic acid (TFA) in n-decane23. This report recognized we show that they cannot be formed using this iterative the intermediacy of g-bisabolene (via b-bisabolene), and no protonation strategy. spectra were published. Careful reproduction of the Andersen In contrast, cyclase enzymes allow propagation of cationic charge conditions provides a mixture of terpenes similar to those without elimination (E1) or capture (SN1) through multiple ring observed in Ohta’s study, including a-cedrene and 2-epi-a- formations and rearrangements. This non-stop propagation is cederene (Trace C), which occur in 6% yield each relative to difficult to achieve in bulk solvent because Coulombic forces volatiles. In both of these examples, proton transfer terminates the hold the counteranion in close proximity to the intermediate cyclization after formation of a single ring (the bisabolyl cation), carbocation. Because the Hammett acidity (H0) of unstabilized and non-selective reprotonation of polyene intermediates leads to carbocations is close to 217, E1 elimination is rapid with most mixtures of mono-, bi- and tricyclic products. counteranions26 including triflate (see below). Cyclase enzymes The ability of a non-dissociating anion to prevent E1 elimination ionize their isoprenyl pyrophosphate substrate with a trinuclear of the bisabolyl cation was tested by ionization of vinyl epoxide 7, metal complex that is immobilized relative to the changing position which was synthesized in a short, scalable four-step sequence of the carbocation (5), and proton transfer to the basic phosphate (shown in Fig. 6). The epoxide function is an effective initiator of oxygens cannot occur3,27. cationic HT polycyclization, particularly when used in combination We reasoned that a similar anion sequestration could be achieved with aluminium Lewis acids28, and we also found that aluminium using a Lewis acid with non-dissociating ligands, which might Lewis acids were superior to other reagents (Table 1, Fig. 3) for prevent preemptive proton transfer and allow other reaction paths TH polycyclization. In addition to gas chromatography (GC) to predominate. Charge-separated species 6 could be derived from analysis, we found that the ratios of the integrals of aldehydic the Lewis acid-mediated ionization of a vinyl epoxide mimic of ner- protons (d 9.55–9.35 ppm) to integrals of methylene groups of olidol (7). Because the carbocation and Lewis base are distal to one allylic alcohol products (d 4.00–3.70 ppm) in crude 1H NMR another in 6, we expected proton transfer to be retarded relative to spectra were effective in assessing the relative efficiency of a Wagner–Meerwein rearrangement or ring closure. In principle, this polycyclization reaction with respect to the production of a aprotic polycyclization would allow the synthesis of acid-sensitive cedrene/funebrene carbon framework. Entries with low ratios terpenes that are inaccessible to an iterative protonation strategy. (entries 2–6, 8, 11, Table 1) also exhibited a high content of allylic methyl peaks, indicating the formation of acyclic or Results and discussion monocyclic products. Utilization of alkyl aluminium chlorides in Comparison of polycyclization strategies. To evaluate this dichloromethane as solvent (entries 9, 10) delivered the highest hypothesis, a reinvestigation of previous TH cyclizations was relative content of tricyclic aldehydes, and these reagents were undertaken to establish a benchmark in efficiency and product further investigated on a preparative scale. A mixture of MeAlCl2 distributions, because the primary experimental data were not and Me2AlCl provided the best overall yield of compounds with a available. Ohta and Hirose were the first to demonstrate that cedrene/funebrene carbon framework (entry 14). Increasing the complex mixtures of polycyclic sesquiterpenes could be formed in substrate concentration proved detrimental to the reaction (entries . bulk solvent by ionization of farnesol with BF3 Et2O (Fig. 3, Trace 12 and 13), and a single-digit millimolar range was found to be A)20. The products observed in this reaction include a-cedrene optimal with respect to both operational convenience and yield. and 2-epi-a-cedrene, which each occur in 6% yield, relative to As shown in Trace D (Fig. 3), a modest mechanistic adjustment total integration of volatiles, and not accounting for non-volatile of anion sequestration causes a dramatic change in the distribution polymers. These tricyclic skeletons are highlighted in Traces A–D. of terpenes produced (the cedrene/funebrene skeleton is high- However, these products appear to arise not by non-stop lighted). Ionization of 7 at low temperature with several Lewis processes, but by iterative protonations of unsaturated acids yielded varying amounts of polycyclic products, but, remark- . intermediates, because treatment of bisabolenes with H2O BF3, ably, no monocyclic products were observed by GC. After extensive which is initially formed in the previous reaction, promotes the analysis, we found that aluminium Lewis acids (for example, entry formation of polycycles, including cedrenes (Trace B). Andersen 14, Table 1) maximized the production of a single scaffold—the and Syrdal reported very similar cyclizations of nerolidol using cedrene/funebrene skeleton (highlighted in Trace D)—which a Previous observations H

– HO X CH3 + H + – H H Farnesol (ref. 20) H X –HX or + Nerolidol (ref. 23) H H Mono-, bi-, and tricyclic sesquiterpenes 3: Bisabolenes Non-selective H (see traces A-C, OH Coulombic attraction, rapid E1 elimination reprotonation Fig. 3) α-Cedrenes 4a,b b Immobilized anion to allow full charge propagation Aspartate-rich motif CH3 H + – Four steps Br – ~ M + + Mg3P2O6 O O H H Multigram scale OH (42% overall) 5: Cyclase active site: 6: Frustrated Coulombics: 7: Epoxy nerolidol mimic anion immobilization no E1 elimination? (farnesene oxide)

Figure 2 | Mechanistic comparisons of bulk solvent and cyclase active sites. a,BrønstedacidcyclizationleadstoanionpairthatrapidlyundergoesSN1 elimination if cyclization is disfavoured. b, Cyclase active sites sequester the pyrophosphate anion, a strategy that could be mimicked in bulk solvent by non-dissociating ligands on a Lewis acid.

916 NATURE CHEMISTRY | VOL 4 | NOVEMBER 2012 | www.nature.com/naturechemistry

S. V. Pronin, R. A. Shenvi Nat. Chem. 2012 4, 915. ARTICLES NATURE CHEMISTRY DOI: 10.1038/NCHEM.1458

Table 1 | Screen of Lewis acids that promote non-stop TH polycyclizations.

Lewis acid O Solvent, temperature O 7 8–10 Entry Lewis acid Solvent, temperature (88888 C) C, mM Ratio of aldehydes to alcohols† Isolated yield (%)

1 TfOH CH2Cl2, 278 1 Alcohols only ND 2TiCl4 CH2Cl2, 278 1 1:11.6 ND 3TiCl3(Oi-Pr) CH2Cl2, 278 1 1:12.7 ND 4TiCl (Oi-Pr)* CH Cl , 278 1 1:10.0 ND DOI: 10.1038/NCHEM.1458 ARTICLES 3 2 2 NATURE CHEMISTRY 5 SnCl4 CH2Cl2, 278 1 1:7.0 ND 6 SnCl4 C6H14, 278 1 1:15.0 ND 7 SiCl4 CH2Cl2, 278 1 Alcohols only ND 8Sc(OTf) CH Cl , 278 1 1:9.8 ND Table 1 | Screen of Lewis acids3 that promote2 non-stop2 TH polycyclizations. 9 EtAlCl2 CH2Cl2, 278 1 1:3.1 ND 10 EtAlCl2/Me2AlCl CH2Cl2, 278 1 1:3.3 ND Lewis acid 11 EtAlCl2 O CF3Cl, 2110 1 1:16.0 ND 12 EtAlCl2 CH2Cl2, 278 5 1:2.8 35 13 EtAlCl CH Cl , 278 25Solvent, 1:3.5 30 2 2 2 temperature 14 MeAlCl2/Me2AlCl CH2Cl2, 278 2.5 1:2.6 44 O See Supplementary Information for a detailed experimental procedure. *4A˚ molecular sieves added. †According to 1H NMR integration. ND, not determined. 7 8–10 Entry Lewis acid Solvent, temperature (88888 C) C, mM Ratio of aldehydes to alcohols† Isolated yield (%) ‡ C6–C7 MeAlCl , H H 1 TfOH 2 CH2Cl2, 278H 1hydride Alcohols only ND Me2AlCl Identical – + shift 2TiCl 7 AlL + CH Cl , 278 1 1:11.6to 7(S) ND 4 3 2 2 – + CH2Cl2, O – pathway L3AlO 3TiCl3(Oi-Pr)–78 °C CH2Cl2, 278 1 1:12.7L3AlO ND 4TiCl3(Oi-Pr)* CH2Cl2, 2786(S)-Bisabolyl cation 1 1:10.07(R)-Homobisabolyl cation ND

5 SnCl4 CH2Cl2, 278 1C6–C7 1:7.0 ND 6 SnCl C H , 78 1hydride 1:15.0 ND 4 6 14 2 shift H 7 SiCl4 CH2Cl2, 278 1 Alcohols onlyH ND H H Hydride 8Sc(OTf) H C–C bondCH Cl , 278H C–C bond H 1C–C 1:9.8bond transfer ND 3 2 2 H H H 9 EtAlCl CH Cl , 278+ 1 1:3.1 ND 2 ––2 2 – + + H L AlO L AlO – – + 10 EtAlCl2L3/AlOMe2AlCl+ CH3 2Cl2, 2783 1 1:3.3 ND L3AlO L3AlO 11 EtAlCl2 CFα3-AcorenylCl, 2 110cation 7(S)-Homobisabolyl 1 cation 1:16.0β-Acorenyl cation ND 12 EtAlCl Elimination CH Cl , 278 5 1:2.8 35 2 or 2 2 C–C bond, Hydride hydride shift then shift 13 EtAlCl2 CH2Cl2, 278 25 1:3.5hydride shift 30 14 MeAlCl /Me AlCl CH Cl , 278 2.5 1:2.6 44 2 H 2 2 2 H 2,4-DNP, 2,4-DNP, H H ˚ † 1 H See Supplementary Information for a detailed experimental procedure. *4AMgSOmolecular4 sieves added. According to H NMRMgSO integration.4 ND, not determined. H + H + CH2Cl2 CH2Cl2 H H H H CHO OH CHO CHO 8a 9a ‡ 12 13 C6–C7 10a 11a MeAlCl , H H 2 H hydride Me2AlCl Figure 4 | Proposed reaction pathway. This cascade constitutes the ﬁrst mechanistic reproduction of the bisabolyl to homobisabolyl to cedryl/funebryl cation Identical – + shift 7 AlL pathway+ in bulk solvent. Key to its success is sequestration of the counteranion away from the unstable carbocation. If the counteranion is mobile, rapid to 7(S) 3 – + CH2Cl2, O – pathway termination of the cascade occurs through E1 elimination.L3AlO –78 °C L3AlO

twice with n-BuLi.TMEDA and monoalkylated with6(S)-Bisabolyl geranyl cationAs observed previously, the b7(-funebrenesR)-Homobisabolyl can be cation isomerized34 bromide to give alcohol 18. Chemoselective epoxidation was into a-funebrenes 21a,b with dilute acid, in this case 0.05 M effected by vanadyl acetylacetonate to provide 19. Oxidation of TFA.C6–C7 According to our calculations, a-funebrene is destabilized hydride 21 the alcohol function using Swern conditions produced an aldehyde by 15.5shift kcal mol relative to the diastereomeric a-cedrene (see that could be converted to vinyl epoxide 7 using a phosphonium Supplementary Information). As a result, we have found that H H ylide. It was important to generate this ylide using a sodium 21a,b will undergo rapid decomposition under mildly acidic con- H H Hydride H amideC–C bond base, because more Lewis-acidicH C–C bond alkali cations such asH ditions (0.5 M TFA,C–C PhH,bond 22 8C; t1/2 , 3 min),transfer whereas the lithium appeared to decompose the acid-sensitive product. cedrenes are stable for 72 hH under the same conditions. H H The aldehydes from the polycyclization+ of 7 were reduced with Therefore, the strongly Brønsted acidic conditions previously –– – + + H lithium aluminiumL AlO hydride, and the correspondingL AlO alcohols were used for iterative TH cyclizations– cannot produce the funebrenes,– + L3AlO + 3 3 eliminated using Grieco’s protocol, yielding a 2:1 mixture of and indeed they were notL3AlO reported in these earlier studies.L3AlO cedrenes 20a,b and funebrenes 1a,b, which are separable by Formation of the funebrene skeleton from the corresponding α-Acorenyl cation 7(S)-Homobisabolyl cation β-Acorenyl cation SiO /AgNO chromatography. b-acorenyl cation puts to rest the 50-year-old question ﬁrst Elimination 2 3 or C–C bond, Hydride hydride shift 918 NATURE CHEMISTRY | VOL 4 | NOVEMBER 2012 | www.nature.com/naturechemistrythen shift hydride shift

H H 2,4-DNP, 2,4-DNP, H H H MgSO4 MgSO4 H + H + CH2Cl2 CH2Cl2 H H H H CHO OH CHO CHO 8a 9a 12 13 10a 11a

Figure 4 | Proposed reaction pathway. This cascade constitutes the ﬁrst mechanistic reproduction of the bisabolyl to homobisabolyl to cedryl/funebryl cation pathway in bulk solvent. Key to its success is sequestration of the counteranion away from the unstable carbocation. If the counteranion is mobile, rapid termination of the cascade occurs through E1 elimination. twice with n-BuLi.TMEDA and monoalkylated with geranyl As observed previously, the b-funebrenes can be isomerized34 bromide to give alcohol 18. Chemoselective epoxidation was into a-funebrenes 21a,b with dilute acid, in this case 0.05 M effected by vanadyl acetylacetonate to provide 19. Oxidation of TFA. According to our calculations, a-funebrene is destabilized the alcohol function using Swern conditions produced an aldehyde by 15.5 kcal mol21 relative to the diastereomeric a-cedrene (see that could be converted to vinyl epoxide 7 using a phosphonium Supplementary Information). As a result, we have found that ylide. It was important to generate this ylide using a sodium 21a,b will undergo rapid decomposition under mildly acidic con- amide base, because more Lewis-acidic alkali cations such as ditions (0.5 M TFA, PhH, 22 8C; t1/2 , 3 min), whereas the lithium appeared to decompose the acid-sensitive product. cedrenes are stable for 72 h under the same conditions. The aldehydes from the polycyclization of 7 were reduced with Therefore, the strongly Brønsted acidic conditions previously lithium aluminium hydride, and the corresponding alcohols were used for iterative TH cyclizations cannot produce the funebrenes, eliminated using Grieco’s protocol, yielding a 2:1 mixture of and indeed they were not reported in these earlier studies. cedrenes 20a,b and funebrenes 1a,b, which are separable by Formation of the funebrene skeleton from the corresponding SiO2/AgNO3 chromatography. b-acorenyl cation puts to rest the 50-year-old question ﬁrst

918 NATURE CHEMISTRY | VOL 4 | NOVEMBER 2012 | www.nature.com/naturechemistry NATURE CHEMISTRY DOI: 10.1038/NCHEM.1458 ARTICLES

38 39 a Reaction of 7 with TfOH Total syntheses of cumacrene 2 and dunnienoic acid 22 served to confirm the identity of the cyclobutanes 17a–d by Trace F No 10% polycycles comparison to literature values. 2 and 22 were accessible by a TfOH observed short sequence depicted in Fig. 6. Chlorination of (E)-geranyl O acetone, followed by conjugate displacement with trimethylsilane 7 provided allylsilane 23, which could be converted to the kinetic TfOH silyl enol ether 24. Bromination and attack of the nascent ketone with vinylmagnesium bromide was followed by epoxide H closure to give 16. Although this linear allylsilane precursor was + –TfOH synthesized as a mixture of geometrical isomers, cyclization of – TfO HO the all trans version of 16 had no effect on the ratio of diastereomers produced. Polycyclization with ethylaluminium chloride OH 14 15 produced a mixture of diastereomers 17, which could be separ- b Reaction of 16 with TfOH ated as their t-butyldimethylsilyl (TBS) silylethers 26, and then Trace G Trace deprotected to alcohols 17 (see Supplementary Information). 10% cyclobutanes Conversion of the diastereomer of 17 corresponding to cumacrene TfOH 2 was effected by mesylation and reduction. The structures of two O TMS other diastereomers were confirmed by conversion to the corre- 16 sponding carboxylic acids 22, one of which is a known terpene, named here dunnienoic acid. Figure 5 | Effect of a mobile anion. a,Triflicacid-mediatedionization The acid stabilities of these terpenes were compared in varying prevents polycyclization of 7 and leads instead to bisabolols. b,Onlytrace concentrations of TFA. Epi-cumacrene 26, which contains a cyclo- cyclobutanes are observed when 16 reacts with TfOH. butane, decomposes in 0.5 M TFA at a rate comparable to funebrene. Even at 0.05 M TFA, both the cumacrenes and the advanced by Stork on whether this cyclization is feasible36. funebrenes decompose in a matter of minutes, which highlights Furthermore, the acid instability of 1 explains why Tomita and the instability of the trans-bicyclooctane embedded in 1 and is an Hirose were unable to observe 1 by protonolysis of b-acoradiene37. interesting example of the equal stability of a cyclopentane and a

a Total syntheses of β-funebrenes and β-cedrenes c Relative stabilities to acid

a. n-BuLi, H 0.05 M Isomerization TMEDA, THF b. V(O)(acac)2, H O TFA to 21a,b HO HO HO & slow Et O, 25 °C; PhH, 2 TBHP, PhH decomposition geranyl-Br (89%) 22 °C Methyallyl 18 19 alcohol (52%) c. (ClCO)2, 1a,b DMSO, Et3N (90%, 2 steps) d. H CPPh Br, 3 3 H 0.5 M NaHMDS H TFA Decomposition H e. MeAlCl / 2 PhH, (t1/2 < 3 min) H H f. LiAlH4 Me AlCl 2 O 22 °C H g. ArSeCN, (28% aldehydes 16% alcohols) 21a,b Bu3P; H2O2 + ΔEstrain = (78%) O 7 15.49 kcal mol–1

(β-Funebrenes) (β-Cedrenes) (2.0:1.0 H 1a,b 20a,b 20a,b:1a,b) 9,10 0.5 M TFA Stable H over 72 h b Total syntheses of cumacrene and dunnienoic acids PhH, 22 °C

a. PhSeCl, 4a NCS O c. LDA, TMSCl OTMS (E)-Geranyl acetone TMS TMS b. TMSLi, CuI, 0.05 M HMPA TFA Slow (65%, 2 steps) 23: E/Z = 1:1 24: E/Z = 1:1 PhH, decomposition H 22 °C d. NBS; (73%, 2 steps) vinyl-MgBr, HMPA 26 h. MsCl, Et3N; then LAH (67%) 0.5 M OR e. EtAlCl2 O TFA Decomposition H TMS H H PhH, (t1/2 < 3 min) i. TEMPO, (40%) H 22 °C NaClO2, NaClO CO H 2 OR 17: R = OH (63%) f. TBSCl 22: Dunnienoic 2: Cumacrene 25: R = OTBS 16: E/Z = 1:1 26 g. TBAF acids 17: R = OH

Figure 6 | Biomimetic total syntheses. a,b,ReproductionofTHpolycyclizationinbulksolventenablestheﬁrsttotalsynthesesoftheacid-labilefunebrene terpenes (a) and cumacrene and dunnienoic acid terpenes (b). c, Demonstration of the acid lability of the funebrene and cumacrene terpenes.

NATURE CHEMISTRY | VOL 4 | NOVEMBER 2012 | www.nature.com/naturechemistry 919 Summary

• 5 publicaons in 2011 (mul-targets wl <10 steps, single target shd biological test) • 4 publicaons in 2012 (old TMs play efficiency; simple TMs play concept; bioacvity unness. ) • 5 publicaons in 2013 Selecve in-depth discussion • Conclusion and summary ARTICLES PUBLISHED ONLINE: 24 MARCH 2013 | DOI: 10.1038/NCHEM.1597 ARTICLES EnantioselectivePUBLISHED ONLINE: 24 MARCH 2013 | DOI: synthesis 10.1038/NCHEM.1597 of tatanans A–C and reinvestigation of their glucokinase- activatingEnantioselective properties synthesis of tatanans A–C ARTICLES 1 1 1 2 2 Qingand Xiao , reinvestigation Jeffrey J. JacksonPUBLISHED,AshokBasak ONLINE: 24 MARCH of, Joseph their 2013 M. | DOI: Bowler 10.1038/NCHEM.1597 glucokinase-,BrianG.Miller* ARTICLES 1 10.1038/NCHEM.1597 DOI: | 2013 MARCH 24 ONLINE: PUBLISHED andactivating Armen Zakarian * properties The tatanans are members of a novel class of complex sesquilignan natural products recently isolated from the rhizomes ofQingAcorus Xiao tatarinowii1, JeffreySchott J.Enantioselective Jackson plants. Tatanans1,AshokBasak A, B and1, C Joseph have previously M. synthesis Bowler been2,BrianG.Miller reported to have of2 potent* tatanans glucokinase- A–C activating properties that exceed the in vitro activity of known synthetic antidiabetic agents. Here, using a series A–C of tatanans of synthesis Enantioselective ARTICLES1 sequentialand Armen [3,3]-sigmatropic Zakarian * rearrangements, we report the total synthesis of tatanan A in 13 steps and 13% overall yield. PUBLISHED ONLINE: 24 MARCH 2013 | DOI: 10.1038/NCHEM.1597 We also complete a conciseand enantioselective reinvestigation total synthesis of more complex, of atropisomeric their tatanans glucokinase- B and C via a glucokinase- their of reinvestigation and ARTICLESdistinct convergent strategy based on a palladium-catalysed diastereotopic aromatic group differentiation (12 steps, 4% The tatanans are members of a novel class of complex sesquilignan natural products recently isolated from the rhizomes PUBLISHED ONLINE:and 8% 24 MARCH overall 2013 |yield,DOI: 10.1038/NCHEM.1597 respectively). A plausible biosynthetic relationship between acyclic tatanan A and spirocyclic properties activating tatanansof Acorus B and tatarinowii C is proposedSchottactivating and plants. probed Tatanans experimentally. A, Bproperties and With C have sufficient previously quantities been of reported the natural to have products potent in glucokinase-hand, weactivating undertake properties a detailed that functional exceed characterization the in vitro activity of the of biological known synthetic activities ofantidiabetic tatanans A–C. agents. Contrary Here, to using previous a series of Enantioselective synthesis of tatanans A–C 2 ,BrianG.Miller 2 Bowler M. Joseph , 1 ,AshokBasak 1 Jackson J. Jeffrey , 1 Xiao Qing reports,sequential our assays [3,3]-sigmatropic utilizing pure rearrangements, recombinant1 human we report enzyme the total demonstrate1 synthesis that of tatanans tatanan1 A do in not 13 steps function and2 as 13% allosteric overall* yield. 2 Qing Xiao , Jeffrey J. Jackson ,AshokBasak, Joseph M. Bowler ,BrianG.Miller* 1 EnantioselectiveactivatorsWe also of complete glucokinase. a synthesis concise enantioselective of tatanans total synthesis A–C of more complex, atropisomeric tatanans B and C via a * Zakarian Armen and distinctand convergent reinvestigation strategyand based Armen on a Zakarian palladium-catalysed of1* their diastereotopic glucokinase- aromatic group differentiation (12 steps, 4% and 8% overall yield, respectively). A plausible biosynthetic relationship between acyclic tatanan A and spirocyclic and reinvestigationtatanans B and C is proposed of their and probed glucokinase- experimentally.a With sufficient quantitiesb of the rhizomes natural the from products isolated in hand, recently products natural sesquilignan complex of class novel a of members are tatanans The mall-moleculeactivating allosteric activators properties of human glucokinase have glucokinase- potent have to reported been previously have C and B A, Tatanans plants. Schott tatarinowii Acorus of wegenerated undertake considerable a detailed interestThe functional as tatanans potential characterization are therapeutic members agents of of a the novel biological classOMe of activities complex sesquilignanof tatanans A–C. naturalOMe Contrary products to recently previous isolated from the rhizomes 4' of series a using Here, agents. antidiabetic synthetic known of activity vitro in the exceed that properties activating activating properties1 1 1 1 2 OMe 2 OMe reports,for theQing treatment our assaysXiao of, type Jeffrey utilizing 2 diabetesof J.Acorus Jackson pure. These recombinant tatarinowii,AshokBasak molecules areSchott human effec-, Joseph plants. enzyme M. Tatanans Bowler demonstrate,BrianG.Miller A, B that and tatanans C have+ previouslydo not function been as reported allosteric to have potent glucokinase- S * H yield. overall 13% and steps 13 in A tatanan of synthesis total the report we rearrangements, [3,3]-sigmatropic sequential tiveactivators at stimulating of glucokinase. insulin secretion1 from pancreatic b-cells and 9'' 1 1 activating1 properties that2 exceed theMeO2 in vitro activityOMe of known a via synthetic C and B antidiabetic tatanans agents. atropisomeric Here, complex, using more a of series of synthesis total enantioselective concise a complete also We Qing Xiao , Jeffrey J.and Jackson Armen,AshokBasak Zakarian *, Joseph M. Bowler ,BrianG.Miller2 MeO OMe thereby rapidly reducing plasmasequential glucose levels [3,3]-sigmatropic in animal models rearrangements,. * we report the total 4% synthesis steps, (12 of tatanan A differentiation in 13 group steps and aromatic 13% overall diastereotopic yield. palladium-catalysed a on based strategy convergent distinct 1 8 8' 1'' and ArmenAt Zakarian least one* glucokinase activator,We also piragliatin, complete has advanced a concise to enantioselective7' total synthesis3'' of more spirocyclic complex, and A atropisomeric tatanan acyclic tatanans between B and relationship C via a biosynthetic plausible A respectively). yield, overall 8% and 3 MeO MeO hand, in products natural the of quantities sufficient With experimentally. probed and proposed is C and B tatanans phase IImall-molecule clinicalThe tatanans trials allosteric in are humans members activators. The of afirst novel of glucokinase human class of glucokinase complex activator sesquilignan have a natural7 9' products recentlyOMe isolated fromb the rhizomes 4 distinct convergent strategy based on a palladium-catalysed diastereotopic aromaticOMe group differentiation (12 steps, 4% of Acorus tatarinowii Schott plants. Tatanans A, B and C have previously beenOMe reported to have potent previous to glucokinase- Contrary OMe A–C. tatanans of activities biological the of characterization functional detailed a undertake we The tatananswas are described membersgenerated of in a 2003 considerablenovel, and class since of complexinterest that time sesquilignan as pharmaceutical potential natural therapeutic chemists products agents recently isolated from theOMe rhizomes OMe and 8% overall yield, respectively).MeO 4 A plausibleOMe 4' biosynthetic relationship between acyclic tatanan A and spirocyclic activating properties that exceed1 the in vitro activity of known synthetic antidiabeticOMe agents.MeO Here, using allosteric as OMe a series function of not OMe do tatanans that demonstrate enzyme human recombinant pure utilizing assays our reports, of Acorus tatarinowiihave identifiedforSchott the treatment a plants. variety Tatanans of of unique type A, 2 chemical diabetes B and C scaffolds. have These previously molecules capable been of are sti- reported effec- to have potent glucokinase- + S sequential [3,3]-sigmatropictatanans5 rearrangements, B and C is we proposed report the total and synthesis probed of experimentally. tatanan A in 13 steps With and sufficient 13%H overall quantities yield. of the natural products in hand, glucokinase. of activators activating properties that exceed the in vitro activity of known synthetic antidiabetic agents. Here,Tatanan using A (1) a series of mulatingtive at glucokinase stimulating activity insulinin secretion vivo . Although from all pancreatic glucokinaseb-cells acti- and 9'' sequential [3,3]-sigmatropicWe rearrangements, also complete weawe concise report undertake the enantioselective total synthesis a detailed total of tatanan synthesis functional A in 13 of steps morecharacterization and complex,MeO 13% overall atropisomeric ofyield.OMe the biological tatanans B andactivities C via a of tatanans A–C. Contrary to previous vatorsthereby investigated rapidly reducingto date are plasma synthetic, glucose the recent levels discovery in animal of models the 2. MeO OMe We also complete a concisedistinct enantioselective convergent strategyreports, total synthesis based our on of assays a more palladium-catalysed complex, utilizing atropisomeric pure diastereotopic recombinant tatanansMeO aromatic B8 human and8' C group via1'' enzyme a differentiation demonstrate (12 steps, that 4% tatanans do not function as allosteric distinct convergenttatanans,At least strategy a novel one based glucokinase class on of a naturally palladium-catalysed activator, occurring piragliatin, lignan diastereotopic derivatives has advanced aromatic that group to differentiation (127' steps, 4% 3'' b a and 8% overall yield,activators respectively). of glucokinase. A plausible biosynthetic relationshipOMe 7' between acyclic tatanan A and spirocyclic have glucokinase human of activators allosteric mall-molecule reportedly possess potent glucokinase-activating3 capabilities, MeO OMe and 8% overallphase yield, II respectively).tatanans clinical trials B and A in plausible C humans is proposed biosynthetic. The and first probed relationship glucokinase experimentally. between activator acyclic With tatanan sufficient A quantities and7 spirocyclic of the natural productsMeO OMe in hand, agents therapeutic potential as interest considerable generated 9' OMe OMe 4' tatanans B andsuggests C is proposed that natural and products probed4 experimentally. might represent With a rich sufficient resource quantities for of the natural products inOMe hand, OMe OMe 1 we undertake a detailed functional characterization of the biologicalC1'' activities7 of tatanans A–C. Contrary to previous+ effec- are molecules These . diabetes 2 type of treatment the for was described in 2003 , and since that time pharmaceutical6,7 chemists OMe OMe H OMe S we undertake a detailed functional characterization of the biological activities of tatanans A–C. Contrary to previous structurallyreports, and mechanistically our assays utilizing new antidiabetic pure recombinant agents human.Asa enzymeO demonstrateMeO 4 1 OMe that tatanans do not functionMeO as allostericOMe 9'' and -cells b pancreatic from secretion insulin stimulating at tive have identified a variety of unique chemical scaffolds capable of sti- OMe OMe MeO reports, our assays utilizing pure recombinant humanmall-molecule enzyme demonstrate allosteric that activators tatanans of do human not function glucokinaseA as allosteric have a OMe MeO b 2 first step inactivators exploring of tatanans glucokinase. and the derivatives5 thereof as poten- OMe Tatanan A (1) . models animal in levels glucose plasma reducing rapidly thereby activators of glucokinase.mulating glucokinase activity in vivo . Although all glucokinase acti- OMe 1'' 8' 8 OMe tial diabetes therapeutics, we undertookgenerated the total considerable chemical synthesis interest as potentialMeO therapeuticOMe agents 3'' 7' to advanced has piragliatin, activator, glucokinase one least At vators investigated to date are synthetic, the recent discovery of the 4' OMe 1 MeO OMeMeO 3 and detailed mechanistic characterizationfor the of treatment the glucokinase-acti- of type 2 diabetes . TheseTatanan molecules B (2) are effec- OMe + 9' 7 activator glucokinase first The . humans in trials clinical II phase S MeO OMe H 4 tatanans, a novel class of naturally occurring lignan derivatives thata b 9'' mall-moleculevating allosteric properties activatorsmall-molecule of tatanans of human allosteric glucokinaseA,tive B and activators at C. have stimulating ofa human insulin glucokinase secretion haveTatanans fromb pancreaticOMe A-C-enough work b-cells and OMe OMe chemists pharmaceutical time that since and , 2003 in described was OMe 7' MeO OMe OMe 4 MeO generated considerablereportedly interest possessgenerated as potential considerable potent therapeutic glucokinase-activating interest agents as potential therapeuticOMe capabilities, agents OMe 2 OMe OMe MeO MeO OMe sti- of capable scaffolds chemical unique of variety a identified have Lignans are a large class of polyphenolicthereby rapidly natural reducing products4' plasma related glucose levels in4' animalOMe models . 1 1 OMe MeO OMe 8OMe 8'MeO 1'' 5 for the treatmentsuggests of type that 2 diabetesfor natural8,9 the. treatment These products molecules of type might are 2 diabetes effec- represent. These moleculesa rich resource are effec-10 for + 1'' 7 OMe + ) (1 A Tatanan acti- glucokinase all Although . vivo in activity glucokinase mulating S to oligostilbenes but distinct in their basic monomeric unit . OMeH C H 7' 3'' S OMe At least one glucokinase activator,6,7 piragliatin, has advancedOMe to tive at stimulating insulin secretiontive at stimulating from pancreatic insulinb-cells secretion and from pancreatic b-cells9'' and 19'' the of discovery recent the synthetic, are date to investigated vators structurally and mechanistically new antidiabeticMeO agents OMe.Asa3 MeOO OMe OMe MeO Tatanans are structurally uniquephase sesquilignans II clinical2 trialsrecently in humansisolated 2. The first glucokinaseMeO A activatorOMe MeO 7 OMe MeO MeO thereby rapidly reducing plasmathereby glucose rapidly levels reducing in animal plasma models glucose. levels in animal models . OMe H O 9' OMe that OMe derivatives lignan occurring naturally of class novel a tatanans, first step in exploring tatanans and the derivatives thereof48 8' as1'' poten- 8 OMe8' 1'' 2 OMe from the ethyl acetate extracts of rhizomes of Acorus tatarinowii7' 3'' OMe At least one glucokinase activator, piragliatin, haswas advanced described to in 2003 , and since that time pharmaceutical7' chemists3'' OMe 7' At6 least one glucokinase activator, piragliatin, has advanced to MeO OMe capabilities, glucokinase-activating potent possess reportedly tial diabetes therapeutics,3 we undertook theMeO total chemical synthesis O MeO MeO OMe OMe MeO +4O MeOOMe OMe phase II clinicalSchott trials in plants humans. Tatanan. The first A glucokinase incorporates activator3 three consecutive7 9' tertiary OMe MeO MeO MeO OMe 4 phase II clinical trials inhave humans identified. The first a variety glucokinase of unique activator chemical scaffolds7 9' capableOMe ofOMe sti- OMe 7 1'' for resource rich a represent might products natural that suggests and detailed mechanistic characterization of the glucokinase-acti- OMe TatananOMe B (2) Me OMe OMe OMe C was described instereocentres 2003 , and since and thatthree time pharmaceutical aryl4 substituents chemists appended to an acyclicOMe 5 OMe 6,7 was described in 2003 , and since that timeMeO pharmaceutical4 OMe chemists OMe Tatanan A (1) 1 .Asa agents antidiabetic new mechanistically and structurally have identified a variety of unique chemical scaffoldsmulating capable of sti- glucokinase activity in vivo . AlthoughMeO allOMe glucokinaseOMe acti- OMeOMe OMe O framework.vating propertieshave Tatanans identified of B tatanans and a variety C, on of A, the unique B other and chemical C. hand, scaffolds are cyclic capable atropo- of sti- MeO 4 OMe MeO OMe A poten- as thereof derivatives the and tatanans exploring in step first mulating glucokinase activity in vivo5. Although all glucokinasevators investigated acti- toTatanan date A are (1) synthetic, the recentTatanan discovery C (3) of the OMe Lignans are a large class of polyphenolic5 natural products related isomers characterizedmulating glucokinase by complex activity in spirocyclic vivo . Although architectures all glucokinase that acti- Tatanan A (1) MeO MeO OMe MeO synthesis chemical total the undertook we therapeutics, diabetes tial vators investigated to date are synthetic,8,9 the recent discoverytatanans, of the a novel class of naturally occurring10 lignan derivatives that MeO to oligostilbenes but distinct in their basic monomericMeO unit . OMe ) 2 ( B Tatanan tatanans, a novelcombine class of naturally avators cyclohexane occurring investigated lignanring to date substituted derivatives are synthetic, that at each the recent position discovery with of a the Figure 1 | Tatanans A–C are structurally unusual novelOMe sesquilignans7' OMe glucokinase-acti- the of characterization mechanistic detailed and reportedly possessOMe potent glucokinase-activating capabilities, reportedly possessTatanans potenttatanans, are glucokinase-activating structurally a novel class uniqueof naturally capabilities, sesquilignans occurring lignan recently derivatives7' isolated that isolated from theMeO rhizomes of Acorus tatarinowii plants. These natural C. and B A, tatanans of properties vating 2,5-cyclohexadienone subunit. The existence of atropoisomerism OMe 13 steps for A in 13% yield 7' OMe H2O suggests that natural productsreportedly might represent possess a rich potentsuggests resource glucokinase-activating for that natural products capabilities, mightOMe represent a rich resource for 1'' 7 OMe OMe related products natural polyphenolic of class large a are Lignans infrom tatanans the B ethyl and acetate C is evidently extracts a of consequence rhizomes of ofCAcorus1'' the7 restricted tatarinowiiproducts have been reported to potently activate glucokinase.C a, Structure of MeO 6,7 OMe 6,7 MeO OMe 10 8,9 structurally and mechanisticallysuggests new6 that antidiabetic natural products agentsstructurally.Asa might represent12 steps for B (4% yield) & C and mechanistically a rich1 resource for new antidiabeticO agentsOMe.Asa + 1 OMe . unit monomeric basic their in distinct but oligostilbenes to rotationSchott around plants . the Tatanan C1–C7 A bond incorporates that links three ringO consecutiveA, proximalOMe tertiary to tatanans. bC,ProposedbiosyntheticrelationshipbetweenacyclictatananA1'' 7 MeO OMe O O OMe MeOOMe OMe A 6,7 OMe A first step in exploring tatanansstructurally and the derivatives and mechanistically thereof as poten- new antidiabeticOMe agents .Asa 1 OMe isolated recently sesquilignans unique structurally are Tatanans thestereocentres quaternary stereogenic and three centre arylfirst substituents at step the inspirocyclic exploring(8% yield) appended ring tatanans junction, to an and acyclic theand derivatives spirocyclicO tatanansOMe thereofOMe B as and poten- C. OMeMe OMe OMe tial diabetes therapeutics, we undertook the total chemical synthesis MeO OMe A OMe O 2 H OMe first step in exploring tatananstial diabetes and theExtensive biological study therapeutics,derivatives thereof we as undertook poten- the totalOMe chemical synthesisOMe MeO OMe OMe tatarinowii Acorus of rhizomes of extracts acetate ethyl the from and detailed mechanisticframework. characterization Tatanans of B the and glucokinase-acti- C, on the other hand, areTatanan cyclic B (2) atropo- 6 tial diabetes therapeutics, we undertook the total chemical synthesis MeO OMe OMe MeO O + OMe MeO O tertiary consecutive three incorporates A Tatanan . plants Schott vating properties of tatanans A, B and C. and detailedNATUREproved no mechanistic CHEMISTRYandiabec characterizationDOI: 10.1038/NCHEM.1597 acvity of the glucokinase-acti-Tatanan C (3) Tatanan B (2) ARTICLES isomersand characterized detailed mechanistic by complex characterization spirocyclic of the architectures glucokinase-acti- that Tatanan B (2) OMe OMe Me OMe OMe acyclic an to appended substituents aryl three and stereocentres Lignans are1 a large class of polyphenolic natural productsvating related properties of tatanansMeO A, B and C. 2 OMe OMe Department8,9combinevating of a Chemistry cyclohexane properties and of Biochemistry, ring tatanans substituted A, B University and10 C. at of each California, position Santa with Barbara, a CaliforniaFigure 1 93106, | Tatanans USA,MeO A–CDepartment are structurally of Chemistry unusual and novel Biochemistry, sesquilignans atropo- cyclic are hand, other the on C, and B Tatanans framework. to oligostilbenes but distinct in their basic monomeric unit . OMe a Retro OMe OMe OMe OMe ) 3 ( C Tatanan Florida2,5-cyclohexadienone State University,Lignans are Tallahassee, a large subunit. class Florida ofLignans The polyphenolic 32306-4390, existence are a natural large USA. of atropoisomerism classproducts*e-mail: of [email protected] natural from; [email protected] the rhizomes related of Acorus tatarinowii plants.MeO These natural that architectures spirocyclic complex by characterized isomers Tatanans are structurally unique sesquilignans recently isolated OMe MeO OMe MeO OMe MeO 8,9 8,9 OMe10 H O 10 from the ethyl acetatein tatanans extractsto oligostilbenes of B rhizomes and C of isAcorus evidentlybutto distinct tatarinowii oligostilbenes a in consequence their basicbut monomeric of distinct the restricted unit in. theirproducts basic2 OMe monomeric have been reported unitOMe to. potentlyOMe activateOMe glucokinase. sesquilignans novel a, unusual Structure of structurally are A–C OMeTatanans | 1 Figure a with position each at substituted ring cyclohexane a combine 6 [3,3]-sigmatropic+ [3,3]-sigmatropic Schott plants .410 Tatananrotation A incorporatesTatanans around theare three structurally C1–C7 consecutiveTatanans bond unique tertiary that sesquilignans are links structurallyO ringMeO MeO recently A, proximal unique isolatedOMeOMe sesquilignans to tatanans.NATUREO MeOb CHEMISTRYrecently,ProposedbiosyntheticrelationshipbetweenacyclictatananAMeO OMe isolated| VOL 5 | MAY 2013 | www.nature.com/naturechemistry natural These MeOplants. tatarinowii Acorus of rhizomes the from isolated atropoisomerism of existence The subunit. 2,5-cyclohexadienone transposition OMe H O transposition stereocentres and three arylfrom substituents the ethyl appended acetate extracts to an acyclic of rhizomes of OMeAcorus tatarinowiiOMe Me OMe OMe OMe2 of Structure , a OMe glucokinase. OMe activate potently to H2O reported been have products OMe restricted the of consequence a evidently is C and B tatanans in the quaternary stereogenic centrefrom the at the ethyl spirocyclic acetate extracts ring junction, of rhizomesand spirocyclic of Acorus tatanans tatarinowii B and C. OSiMe3 6 OMe O MeO OMe + framework. Tatanans B and C,Schott on the plants other. hand, Tatanan are cyclic A incorporates atropo- MeO6 three consecutive tertiary MeO OMe O O nA a n a MeO t a ct i l c MeO y c OMe na e e w t e pb i h s n o i t a l e cr i t e h t n y s o i + db e s o p o r ,P b tatanans. to proximal A, ring links that bond C1–C7 the around rotation Schott plants . TatananTatanan A C incorporates(3) OMe three consecutive tertiaryO OMe MeO OMeO O MeO OMe isomers characterized by complexstereocentres spirocyclic and three architectures aryl substituents that appended to an acyclic OMe OMe Me OMe OMe C. and B tatanans spirocyclic and OMe OMe OMe Me OMe OMe junction, ring spirocyclic the at centre stereogenic quaternary the combine a cyclohexane ringframework. substituted Tatanans at each B position andstereocentres C, with on the a otherMeOFigure and hand, 1 three | Tatanans areOMe cyclic aryl A–C atropo- substituents are structurally unusual appendedMeO novel sesquilignansOMe to an acyclic MeOOMe OMe OMe OMe 2,5-cyclohexadienone1 subunit. The existence of atropoisomerism isolated from the rhizomes of Acorus tatarinowii Tatananplants. CThese (3) natural 2 Departmentisomers of Chemistry characterized and Biochemistry, byframework. complex spirocyclic University Tatanans of architectures B California, and C, on Santa that the Barbara, other hand, California are cyclic 93106, atropo- USA, Department of Chemistry and(Z)-4 Biochemistry, in tatanans B and C is evidently a consequence of the restricted products have been reported to potently activate glucokinase. a, Structure of Tatanan C (3) combine a cyclohexaneisomers ring substituted characterized at each position by complex with Xiao a , Q. spirocyclicFigureZakarian 1 | Tatanans, A. et. al architectures A–C are. Nat. Chem structurally that unusual. 2013 novel, 5 sesquilignans, 410. 2 1 rotation around theFlorida C1–C7 State bond University, that links Tallahassee, ring A, proximal Florida to 32306-4390,tatanans. b,ProposedbiosyntheticrelationshipbetweenacyclictatananA USA. *e-mail: [email protected]; [email protected] Biochemistry, and Chemistry of Department USA, 93106, California Barbara, Santa California, of University Biochemistry, and Chemistry of Department 2,5-cyclohexadienone subunit. The existence of atropoisomerism isolated from the rhizomesHPh Ph of Acorus tatarinowiiFigureplants. 1 |TheseTatanans natural A–C are structurally unusual novel sesquilignans the quaternary stereogenic centre at the spirocyclic ringcombine junction, ab cyclohexaneand spirocyclicOMe tatanans ring B substituted and C. at each position with a [email protected] ; [email protected] e-mail: * USA. 32306-4390, Florida Tallahassee, University, State Florida in tatanans B and C is evidently a consequence of theOMe restricted productsOMe have been reportedO to potently activateOMe glucokinase. a, Structure of OMe 410 2,5-cyclohexadienone subunit. The existence ofNATURE atropoisomerismN B CHEMISTRY | VOLisolated 5 | MAY 2013 from | www.nature.com/naturechemistry the rhizomes of Acorus tatarinowii plants. These natural OMe OMe OMe rotation around the C1–C7 bond that links ring A, proximal1. TiCl , Et toN tatanans. b,ProposedbiosyntheticrelationshipbetweenacyclictatananAwww.nature.com/naturechemistry | 2013 MAY | 5 VOL | CHEMISTRY NATURE 410 in tatanans B and C is evidently4 3 a consequence of the restricted products have been reported to potently activate glucokinase. a, Structure of MeO BH Me S CH3CH2COCl 1 the quaternary stereogenic centre at the spirocyclic ring2. junction, MsCl, Et2 3N; and spirocyclic tatanans3 B2 and C. Department of Chemistry and Biochemistry, University of California, Santa Barbara, California 93106, USA, DepartmentMeO of Chemistry and Biochemistry, MeO o MeO rotation around the C1–C7 DBU bond that links ringTHF, A, –10 proximal oC to tatanans. Py,b,ProposedbiosyntheticrelationshipbetweenacyclictatananA 0 C Florida State University, Tallahassee, Florida 32306-4390, USA. *e-mail: [email protected] ; [email protected] + 6 O the quaternary stereogenic centre70% at the spirocyclic ring98% junction, and spirocyclic99% tatanans B and C. MeO MeO MeO MeO O O e.e. 93% OH O 410 1Department of Chemistry and Biochemistry, UniversityNATURE of California, CHEMISTRY Santa| VOL Barbara, 5 | MAY California 2013 | www.nature.com/naturechemistry 93106, USA, 2Department of Chemistry and Biochemistry, Florida State University, Tallahassee, FloridaMeO 32306-4390,OMe USA. *e-mail: [email protected] OMe ; [email protected] OMe MeO OMe 1Department of Chemistry5 and Biochemistry,7 University of California, Santa8 Barbara, California 93106, USA,9 2Department of Chemistry and Biochemistry, 410 NATURE CHEMISTRY | VOL 5 | MAY 2013 | www.nature.com/naturechemistry Florida State University, Tallahassee, FloridaOMe 32306-4390, USA. *e-mail: [email protected] ; [email protected] 1. LiAlH4, THF 1. H2, Pd-CaCO3-Pb, MeOH OMe OMe 2. (COCl) , Me SO; Et N 2. PPh , I , ImH, CH Cl LDA, THF, –78 oC; 2 2 3 3 2 2 2 410 3. CBr4, PPh3, CH2Cl2 NATURE CHEMISTRY3. 3,4-dimethoxyphenol,| VOL 5 | MAY 2013 | www.nature.com/naturechemistry Me3SiCl MeO O MeO OH o –78 to 25 oC 4. LDA, (CH2O)n, THF K2CO3, acetone, 50 C OH 70% MeO 68% MeO 84% d.r. 7:1

MeO OMe MeO OMe

10 11

OMe MeO OMe OMe OMe 1. NaH, MeI, THF OMe 2. Pd-CaCO -Pb Me3Al, PhMe + 3 o O – 100 C, 2 h [3,3]-shift MeO OH H2, EtOAc AlMe3 MeO Tatanan A OMe Me H 42% 83% H Ar MeO MeO (60% OMe O OMe b.r.s.m.) OMe Me MeO OMe MeO OMe d.r. 3:1 12 Ar TS-12 13 Minimization of A1,3-strain

Figure 2 | Description of the total synthesis of tatanan A using consecutive sigmatropic rearrangements as a key element of the synthesis design, setting the three adjacent stereogenic centres of the natural product. a,OutlineofthesynthesisplanfortatananA,illustratingtheapplicationofthe[3,3]- sigmatropic transforms. b, Full reaction sequence for the enantioselective synthesis of tatanan A, 15 steps from aldehyde 6 and ketone 5.Ms, methanesulfonyl; DBU, 1,8-diazabicycloundec-7-ene; THF, tetrahydrofuran; Py, pyridine; LDA, lithium diisopropylamide; ImH, imidazole.

to the cyclohexane subunit. This type of molecular structure is ketone (5) with 2,4,5-trimethoxybenzaldehyde promoted by unprecedented among known lignans11,12, enhancing our interest titanium tetrachloride provided ketone 7 in 70% yield15. in developing a concise chemical synthesis of these natural products. Enantioselective reduction of this to allylic alcohol 8 was We postulated that acyclic tatanan A is biosynthetically interrelated accomplished using (R)-2-methyl-CBS-oxazaborolidine (CBS Corey- to its spirocyclic congeners through a stereospeciﬁc electrophilic Bakshi-Shibata; 98% yield, 93% e.e.)16. The formation¼ of the ring-closing dearomatization initiated by a proton donor, as illus- propionyl ester of 8 in nearly quantitative yield was followed by trated in Fig. 1b13. Thus, an additional goal of our synthesis the Ireland–Claisen rearrangement17, which proceeds through an efforts was to investigate the facility of such a transformation. initial Z-selective enolization with lithium diisopropylamide, trapping of the lithium enolate with chlorotrimethylsilane to give Results and discussion the ketene acetal (Z)-4, and its relatively rapid rearrangement via Total synthesis of tatanans A–C. Tatanan A, with its three a chair-like transition structure at 25 8C over the course of 1 h. consecutive all-carbon tertiary stereogenic centres, became the This key constructive transformation18,19 establishes the requisite initial goal of our synthesis efforts. Iterative application of [3,3]- conﬁguration at the C7′ and C8′ stereocentres with good sigmatropic rearrangements formed the basis of the synthesis stereocontrol (d.r. 7:1, 70% yield). Propargyllic alcohol 11 was design (Fig. 2a)14, and its concise implementation is presented in accessed in four steps that included a reduction–oxidation Fig. 1b. Aldol condensation of ethyl 2,4,5-trimethoxyphenyl sequence to reduce the carboxy group in 10 to the aldehyde,

NATURE CHEMISTRY | VOL 5 | MAY 2013 | www.nature.com/naturechemistry 411 NATURE CHEMISTRY DOI: 10.1038/NCHEM.1597 ARTICLES NATURE CHEMISTRY DOI: 10.1038/NCHEM.1597 ARTICLES

a OMe DOI: 10.1038/NCHEM.1597 OMe OMe NATURE CHEMISTRYOMe ARTICLES a OMe OMe OMe OMeOMe OMe OMe OMe MeO OMe [3,3]-sigmatropic [3,3]-sigmatropic a OMe MeO OMe MeO OMe MeO OMe [3,3]-sigmatropictransposition [3,3]-sigmatropictransposition OMe MeO OMe OMe MeO OMe transposition OSiMe transposition OMe 3 MeO MeO OSiMe OMe [3,3]-sigmatropic MeO 3 MeO OMe [3,3]-sigmatropic MeO O OMe O MeO MeO MeO OMe transposition MeO OMe transposition O OMeOMe O MeO OMe MeO OMe MeO OMe OMe OSiMe3 MeOMeO OMe MeOMeO OMe MeOMeO OMe OMe O OMe (Z)-4O OMe (Z)-4 MeO OMe MeO OMe MeO OMe HPh Ph b OMe (Z)-4 Ph Ph b OMe OMe H O OMe OMe OMe Synthesis of N B tantanans A OMe OMe O OMe OMe OMeOMe OMe1. TiCl4, Et3N N HPhB Ph b MeO OMe 2. MsCl, Et N; OMe BH3 Me2S OMe CH3CH2COCl OMe 1. TiCl4, Et3N3 MeO OMe O MeO OMe o MeO OMe OMe DBU THF, N–10 oC CHPy,CH 0 COClC MeO O 2. MsCl, Et3N; BH3 MeB2S 3 2 MeO OMe MeO OMe o MeO OMe + 6 DBU1. TiCl4, Et3N o Py, 0 C O O 70% THF,98% –10 C 99% MeO BH Me S CH3CH2COCl MeO + 6 2. MsCl, EtMeO3N; 3 2 MeO MeO O O MeO O e.e. 93% MeO OH o MeO O 70% DBU THF,98% –10 oC Py,99% 0 C MeO O MeO MeO MeO e.e. 93% MeO OMeO + 6 MeO OMeO MeO OMeOH MeO OMeOO 70% 98% 99% MeO MeO MeO MeO MeO 5 OMe O MeO 7 OMeO e.e. 93% MeO 8 OMeOH MeO 9 OMeO

MeO 5 OMe MeO 7 OMe MeO 8 OMe MeO 9 OMe OMe OMe 1. LiAlH4, THF 1. H2, Pd-CaCO3-Pb, MeOH 5 OMe7 8 OMe 9 o OMe 2. (COCl)2, Me2SO; Et3N OMe 2. PPh3, I2, ImH, CH2Cl2 LDA, THF, –78 C; 1. LiAlH4, THF 1. H2, Pd-CaCO3-Pb, MeOH OMe 3. CBr4, PPh3, CH2Cl2 OMe 3. 3,4-dimethoxyphenol, Me3SiClo MeO OMe O 2. (COCl)2, Me2SO; Et3N MeO OMe 2. PPh3, I2, ImH, CH2Cl2 LDA, THF, –78 C; 1. LiAlH4, THF OH 1. H2, Pd-CaCO3-Pb, MeOHo –78 to 25 oC 4. LDA, (CH2O)n, THF K2CO3, acetone, 50 C OMe 3. CBr4, PPh3, CH2Cl2 OMe 3. 3,4-dimethoxyphenol, Me3SiCl o MeO O OH 2. (COCl)2, Me2SO; Et3N MeO 2. PPh3, I2, ImH, CH2Cl2 LDA, THF, –78 C; OH o –78 to 2570% oC 4. LDA, (CH68%2O)n, THF K2CO3, 84%acetone, 50 C MeO 3. CBr4, PPh3, CH2Cl2 MeO 3. 3,4-dimethoxyphenol, Me3SiCl MeO OOH MeO d.r. 7:1 OH o o 4. LDA, (CH2O)n, THF K2CO3, acetone, 50 C –7870% to 25 C MeO 68% MeO 84% OH MeO OMe d.r. 7:1 MeO OMe 70% MeO 68% MeO 84% d.r. 7:1 MeO OMe 10 MeO OMe11 MeO OMe MeO OMe10 11 OMe 10 MeO 11 OMe OMe 1. NaH, MeI, THF OMe OMe OMe OMe Me Al, PhMe MeO 2. Pd-CaCO3-Pb OMe 3 OMe + O 1. NaH, MeI, THF o – [3,3]-shift MeO OMe OMeOH H , EtOAc OMe 100 C, 2 h MeO AlMe 2 MeO OMe 3 1.2. NaH, Pd-CaCO MeI, THF-Pb Me3Al, PhMe + OMe 3 Tatanan A OMe OMe o O – 100Me 3C,Al, 2PhMe h Me [3,3]-shift MeO OH 2. Pd-CaCO H2,83% EtOAc3-Pb MeO H +AlMe3 42% o H Ar O – MeO Tatanan A MeO 100 C, 2 h [3,3]-shift(60% MeO OH OMe H2, EtOAc OMe AlMe3 MeO O OMe 83% Tatanan A Me H b.r.s.m.)42% OMe OMe MeO H MeAr MeO OMe 83% MeOMeO OMe Me H d.r.(60%42% 3:1 OMe O OMe H Ar MeO MeO 12 Ar TS-12 b.r.s.m.)(60% 13 OMe OMe O OMe Me b.r.s.m.)MeO OMe MeO OMe Minimization of A1,3d.r.-strain 3:1 OMe Me MeO OMe MeO OMe 12 Ar TS-12 d.r. 3:1 13 Figure 2 | Description of the total12 synthesis of tatanan A using consecutiveAr TS-12Minimization sigmatropic of A1,3-strain rearrangements as a key element13 of the synthesis design, setting Minimization of A -strain the three adjacent stereogenic centres of the natural product. a,OutlineofthesynthesisplanfortatananA,illustratingtheapplicationofthe[3,3]-1,3 Figuresigmatropic 2 | Description transforms. of theb, Full total reaction synthesis sequence of tatanan for the A enantioselective using consecutive synthesis sigmatropic of tatanan rearrangements A, 15 steps from as a aldehyde key element6 and of ketone the synthesis5.Ms, design, setting Figure 2 | Description of the total synthesis of tatanan A using consecutive sigmatropic rearrangements as a key element of the synthesis design, setting themethanesulfonyl; three adjacent stereogenic DBU, 1,8-diazabicycloundec-7-ene; centres of the natural THF, product. tetrahydrofuran;a,OutlineofthesynthesisplanfortatananA,illustratingtheapplicationofthe[3,3]- Py, pyridine; LDA, lithium diisopropylamide; ImH, imidazole. sigmatropicthe three transforms. adjacent stereogenicb, Full reaction centres sequence of the natural for the product. enantioselectivea,OutlineofthesynthesisplanfortatananA,illustratingtheapplicationofthe[3,3]- synthesis of tatanan A, 15 steps from aldehyde 6 and ketone 5.Ms, methanesulfonyl;sigmatropic transforms. DBU, 1,8-diazabicycloundec-7-ene;b, Full reaction sequence for THF, the tetrahydrofuran; enantioselectivePy, synthesis pyridine; of tatanan LDA, lithium A, 15 stepsdiisopropylamide; from aldehyde ImH,6 and imidazole. ketone 5.Ms, tomethanesulfonyl; the cyclohexane DBU, subunit. 1,8-diazabicycloundec-7-ene; This type of molecular THF, tetrahydrofuran; structure Py,is pyridine;ketone LDA, (5) lithium with diisopropylamide; 2,4,5-trimethoxybenzaldehyde ImH, imidazole. promoted by unprecedented among known lignans11,12, enhancing our interest titanium tetrachloride provided ketone 7 in 70% yield15. toin the developing cyclohexane a concise subunit. chemical This synthesis type of of molecular these natural structure products. is ketoneEnantioselective (5) with reduction 2,4,5-trimethoxybenzaldehyde of this to allylic alcohol promoted8 was by to the cyclohexane subunit. This type of molecular structure is ketone (5) with 2,4,5-trimethoxybenzaldehyde promoted by unprecedentedWe postulated among that acyclic known tatanan lignans A is11,12 biosynthetically, enhancing our interrelated interest titaniumaccomplished tetrachloride using (R)-2-methyl-CBS-oxazaborolidine provided ketone 7 in (CBS 70%Corey- yield15. unprecedented among known lignans11,12, enhancing our interest titanium tetrachloride provided ketone 7 in 70% yield15. into developing its spirocyclic a concise congeners chemical through synthesis a stereospecific of these natural electrophilic products. EnantioselectiveBakshi-Shibata; 98% reduction yield, 93% of this e.e.)16 to. The allylic formation alcohol¼ of8 thewas in developing a concise chemical synthesis of these natural products. Enantioselective reduction of this to allylic alcohol 8 was Wering-closing postulated dearomatization that acyclic tatanan initiated A is biosynthetically by a proton donor, interrelated as illus- accomplishedpropionyl ester using of 8 (Rin)-2-methyl-CBS-oxazaborolidine nearly quantitative yield was (CBS followedCorey- by We postulated that acyclic tatanan A is biosynthetically interrelated accomplished using (R)-2-methyl-CBS-oxazaborolidine (CBS Corey- trated in Fig. 1b13. Thus, an additional goal of our synthesis the Ireland–Claisen rearrangement17, which16 proceeds through¼ an to itsto its spirocyclic spirocyclic congeners congeners through through a a stereospecific stereospecific electrophilic electrophilic Bakshi-Shibata;Bakshi-Shibata; 98% yield, 93% 93% e.e.) e.e.)16.. The The formation formation¼ of of the the efforts was to investigate the facility of such a transformation. initial Z-selective enolization with lithium diisopropylamide, ring-closingring-closing dearomatization dearomatization initiated initiated by by a a proton proton donor, donor, as as illus- illus- propionylpropionyl ester ester of 8 in nearly nearly quantitative quantitative yield yield was was followed followed by by 13 trapping of the lithium enolate with17 chlorotrimethylsilane to give tratedtrated in in Fig. Fig. 1b 1b.13 Thus,. Thus, an an additional additional goal goal of of our our synthesis synthesis thethe Ireland–Claisen Ireland–Claisen rearrangement17,, which which proceeds proceeds through through an an Results and discussion the ketene acetal (Z)-4, and its relatively rapid rearrangement via effortsefforts was was to toinvestigate investigate the the facility facility of of such such a a transformation. transformation. initialinitial ZZ-selective-selective enolization with with lithium lithium diisopropylamide, diisopropylamide, Total synthesis of tatanans A–C. Tatanan A, with its three trappinga chair-like of the transition lithium structure enolate withat 25 chlorotrimethylsilane8C over the course of to 1 give h. trapping of the lithium enolate with18,19 chlorotrimethylsilane to give ResultsconsecutiveResults and and discussion all-carbon discussion tertiary stereogenic centres, became the theThisthe ketene ketene key constructive acetal acetal (Z)-4 transformation, and and its its relatively relatively rapidestablishes rapid rearrangement rearrangement the requisite via via initial goal of our synthesis efforts. Iterative application of [3,3]- configuration at the C7′ and C8′ stereocentres with good TotalTotal synthesis synthesis of of tatanans tatanans A–C. A–C.TatananTatanan A, A, with with its its three three aa chair-like chair-like transition transition structure at at 25 2588CC over over the the course course of of 1 1 h. h. 18,19 consecutivesigmatropicconsecutive all-carbon rearrangements all-carbon tertiary tertiary formed stereogenic stereogenic the basis centres, centres, of the became became synthesis the the ThisstereocontrolThis key key constructive constructive (d.r. 7:1, transformation 70% yield). Propargyllic18,19 establishesestablishes alcohol the the requisite11 requisitewas design (Fig. 2a)14, and its concise implementation is presented in accessed in four steps that included a reduction–oxidation initialinitial goal goal of of our our synthesis synthesis efforts. efforts. Iterative Iterative application application of of [3,3]- [3,3]- configurationconfiguration at the C7′′ andand C8 C8′′ stereocentresstereocentres with with good good sigmatropicFig.sigmatropic 1b. Aldol rearrangements rearrangements condensation formed formed of ethyl the the basis 2,4,5-trimethoxyphenyl basis of of the the synthesis synthesis stereocontrolsequencestereocontrol to reduce (d.r. (d.r. 7:1, the 70% carboxy yield). yield). group Propargyllic Propargyllic in 10 to alcohol alcohol the aldehyde,1111waswas designdesign (Fig. (Fig. 2a) 2a)14,14 and, and its its concise concise implementation implementation is is presented presented in in accessedaccessed in in four four steps that that included included a a reduction–oxidation reduction–oxidation NATURE CHEMISTRY | VOL 5 | MAY 2013 | www.nature.com/naturechemistry 411 Fig.Fig. 1b. 1b. Aldol Aldol condensation condensation of of ethyl ethyl 2,4,5-trimethoxyphenyl 2,4,5-trimethoxyphenyl sequencesequence to to reduce reduce the carboxy carboxy group group in in 1010toto the the aldehyde, aldehyde,

NATURENATURE CHEMISTRY CHEMISTRY| VOL| VOL 5 | 5 MAY | MAY 2013 2013 | www.nature.com/naturechemistry | www.nature.com/naturechemistry 411411 ARTICLES NATURE CHEMISTRY DOI: 10.1038/NCHEM.1597

OMe with 2.5 mM CF3SO3H/CH2Cl2 at 0–25 8C resulted initially in no OMe Acidic or oxidative MeO reaction, and then decomposition to an intractable mixture of pro- OMe electrophilic or radical ducts at higher temperature. No reaction was observed under milder MeO OMe conditions OMe conditions with pyridinium tosylate in CH2Cl2 or aqueous CH3OH. Next, an approach was explored based on reversed polarity in the O MeO OMe MeO cyclization, which again met with no success. Electrophilic or OR OMe OMe OMe OMe radical activation of ring C in 15 by oxidation with hypervalent MeO OMe 3 iodine reagents (PhI(OAc)2, PhI(O2CCF3)2) in the presence of a i 14: R = Si( Pr)3 hydride donor (PhSiH ) only resulted in the formation of benzoqui- (PhSiH3) 3 15: R = H none products, and no spirocyclization to tatanans B or C was

PhI(OAc)2 observed. These observations suggest that the postulated biosyn-

CH2Cl2 thetic assembly of tatanans B and C by cyclization of tatanan A is OMe unlikely to occur without the assistance of an enzyme. OMe OMe Accomplishing the synthesis of the structurally complex tatanans OMe B and C required a reformulated approach. The key recourse in – AcO MeO OMe developing a new synthesis plan was to reposition the ring- MeO OMe opening bond scission from C7–C1′′ to C1′′–C7′′ (Fig. 4a). To

+ MeO enable this strategy, a phenol dearomatization through intramolecu- MeO O lar allylation by a p-allyl palladium complex was envisioned, which O MeO OAc OMe MeO OMe was expected to occur with diastereotopic group differentiation and MeO OMe set three consecutive stereogenic centres of the fully substituted cyclohexane ring in one step. A precursor of acyclic intermediate Figure 3 | Tatanans B and C can conceivably arise from cyclization of 17 was to be assembled in a convergent manner by asymmetric con- tatanan A during their biosynthesis. The total synthesis of tatanan A jugate addition of an (E)-crotyllithium reagent 18, where SDG is a enabled an exploration of biomimetic cyclization of tatanan A derivatives to stereodirecting group, to cinnamic ester 19. Alkylation of the tatanans B and C, which proved unsuccessful under a variety of cationic or ester enolate arising from the conjugate addition with methyl radical reaction conditions. iodide was expected to build the stereotriad found in 17 in an expedient manner. Implementation of the amended synthesis plan was initiated via conversion of the aldehyde to the 1,1-dibromoalkene upon the development of a suitable conjugate addition method (Fig. 4b). treatment with carbon tetrabromide and triphenylphosphine, and Addition of the lithiated crotyl phosphoramide to ester 19 with sub- formation of the lithium acetylenide followed by quenching with sequent alkylation of the intermediate enolate with iodomethane, paraformaldehyde, affording 11 in 68% overall yield. Selective following the protocol developed by Hanessian and co-workers23, hydrogenation with Lindlar’s catalyst and two-step aryl ether delivered the expected product in high yield and with outstanding formation provided 12 in 84% overall yield. diastereocontrol. However, various attempts to remove the vinylic Lewis-acid mediated [3,3]-sigmatropic rearrangement20 of (Z)- phosphoramide substituent proved to be prohibitively inefficient, allylic aryl ether 12 secured the stereochemistry of the remaining prompting us to seek a more practical alternative. Addition of the chiral centre while completing the carbon framework of lithiated (R)-tert-butyl (E)-2-butenyl sulfoxide24 21 occurred with tatanan A. Phenol 13, isolated in 42% isolated yield (60% based high diastereoselectivity, delivering ester 22 as a single diastereomer on recovered starting material, b.r.s.m.), was formed in a diastereo- in 50% yield after in situ methylation of the transient ester enolate25. selective process that favoured the desired stereoisomer with a 3:1 Diastereoisomeric products resulting from a-alkylation of the sulf- selectivity. The Z configuration of the double bond was necessary oxide accounted for the remainder of the mass balance. Oxidation of to enhance diastereocontrol. The major stereoisomer presumably the sulfoxide to sulfone followed by a palladium-catalysed desulfo- arises through a reaction pathway involving transition structure nation (Pd(acac)2, i-PrMgBr) afforded terminal alkene 24 in good TS-12, which is favoured due to minimization of allylic strain. yield26. The diastereotopic aryl groups were introduced at this Rearrangement of the corresponding (E)-allylic ether provided an stage by addition of 2 equiv. of the aryllithium reagent derived approximately 1:1 mixture of stereoisomers. Starting from from 25 through lithium–bromine exchange, followed by electro- 27 rearrangement product 13, the completion of enantioselective syn- philic reduction of 26 with Et3SiH in the presence of BF3-etherate . thesis of tatanan A required only two additional steps. Methylation The cross-metathesis reaction28 between 27 and methyl acrylate of the phenolic hydroxyl (NaH, CH3I, THF) followed by chemo- using the Hoveyda–Grubbs II catalyst effectively provided a,b- selective hydrogenation of the terminal alkene in the presence of unsaturated ester 28, which was converted to allyl methyl carbonate Lindlar’s catalyst delivered 26 mg of tatanan A (83% yield over in three steps and 86% overall yield. two steps). Synthetic tatanan A showed identical spectroscopic For the critical assembly of the spirocyclic ring system, a tran- data (1H and 13C NMR) to those published for the natural sition metal-catalysed cyclization with concomitant dearomatiza- product; the optical rotation for the synthetic material was of the tion and diastereotopic aryl group differentiation was selected as 20 29 same sign but of a notably higher value ([a]D 55 (c 0.1, the preferred tactical solution (Fig. 4a) . Although intramolecular 6 20 þ CH3OH); literature [a]D 10 (c 0.1, CH3OH)). metal-catalysed allylic dearomatization is a powerful approach to The synthesis of tatananþ A on the aforementioned scale enabled the synthesis of spirocyclic cyclohexanediones, to date there are a study exploring its conversion to spirocyclic tatanans B and no applications of this methodology in the total synthesis of C, emulating the proposed biosynthetic pathway depicted in complex natural products30. Very recently, two reports describing 21,22 Fig. 1b . The C4′′ triisopropylsilyl ether 15 was prepared via methodological studies on direct intramolecular C-allylation of straightforward modification of reactions used in the total synthesis phenols have appeared in the literature, demonstrating the feasibility 31 of tatanan A (Fig. 3). Exposing the C4′′ phenolic hydroxyl was of this process. In 2010, Hamada and co-workers described a expected to activate ring C as a nucleophlic counterpart in the cycli- palladium-catalysed spirocyclization forming a spiro[4.5]decane zation. Under a variety of acidic reaction conditions, 15 failed to ring system. In that study, a single example was reported that undergo cyclization to tatanans B or C. For example, treatment afforded a spirocyclic ring system containing two six-membered

412 NATURE CHEMISTRY | VOL 5 | MAY 2013 | www.nature.com/naturechemistry Redesign of synthesis

NATURE CHEMISTRY DOI: 10.1038/NCHEM.1597 ARTICLES

a OH MeO O DGD MeO 7'' MeO O PdLn SDG OMe Ar O 7' 7 OtBu 1" 7 7'' + O 8' 8' 8 1" Li O MeO OMe 7' 7 MeO Me I OMe OMe 9 OMe 9 O OH 18 OMe 19

3 17 SDG = stereodirecting group

Key features of design: Dearomatization with π-allyl palladium complex Diastereotopic group differentiation (DGD) Conjugate addition to set the acyclic stereotriad

OTIPS b MeO 25 OMe OMe OMe OMe MeO MeO MeO Br o LDA, LiBr, THF, –78 C; mCPBA, CH Cl Pd(acac) (10 mol%) n O 2 2 2 BuLi, THF, o o i then 19, –78 C; CH3I 0 C, 40 min PrMgBr, THF o S O OMe OMe OMe 25 C, 3 h S CO tBu t t CO tBu 21 50% 2 86% BuSO2 CO2 Bu 71% 2 89%

O 22 23 24 N P N 20

OTIPS OH OTIPS OTIPS i BF3 Et2O, Et3SiH CH2=CHCO2Me 1. ( Bu)2AlH, CH2Cl2 MeO MeO MeO MeO CH2Cl2 HGII (5 mol%), CH2Cl2 2. ClCO2Me, Py –78 oC, 30 min o 3. nBu NF, THF 40 C, 72 h 4 OCO Me Ar OMe Ar OMe Ar OMe Ar OMe 2 OH OMe OMe OMe MeO C OMe 88% 93% 2 93% 7

MeO OH MeO OTIPS MeO OTIPS MeO OTIPS 26 27 28 29

MeO 1. NaH, CH I, THF Pd(dba)2 (20 mol%) MeO MeO O 3 MeO MeO MeO P(OPh)3 (48 mol%) 2. H2, Lindlar cat. Tatanan B 1" o CH2Cl2, 25 C, 12 h EtOAc (24%) OMe 7 OMe R 1" 7 1" 7 OMe ++OMe 7" MeO OMe 78% 84% (92% b.r.s.m.) O MeO OMe O OMe A A OMe Tatanan C d.r. 6:3:2 OMe OMe OMe MeO (48%) see Table 1 OH MeO OH OR'

34: R = CH=CH2, R'=H 32 33 35: R = CH2CH3, R'=CH3 c

MeO MeO MeO MeO PdLn PdLn PdLn O PdLn O O 7" O

OMe OMe OMe OMe 1" 7 7 Ar Ar Ar OMe – OMe – OMe – 1 OMe –O O O O MeO A C E OMe G O O O O OH

vs vs vs vs L Pd MeO n MeO PdL MeO PdLn MeO n O Ar O PdLn O 7"

O OMe OMe 7 OMe OMe 7 1" OMe Ar 1 OMe Ar – OMe OMe – OMe O –O O –O O O O O MeO OH B D F H

Figure 4 | Total synthesis of tatanans B and C by a palladium-catalysed cyclodearomatization. a,Reformulatedsynthesisplancentredonaconvergent strategy enabled by a stereochemically complex palladium-catalysed cyclization featuring diastereotopic aromatic group differentiation with concomitant construction of the central quaternary centre. b,Successfulimplementationoftheconvergentsynthesisplanleadingtoaseparablemixtureofatropisomeric tatanans B and C in 12 steps from sulfoxide 21. c,Fourpairsoftransitionstructuresrationalizingallfourstereogeniceventsinthecomplexpalladium- catalysed cyclodearomatization of substrate 29.StructuresA and B explain the observed selectivity for the formation of the C1′′ quaternary centre. Structures C and D provide the basis for the observed high selectivity during the diastereotopic group differentiation establishing stereochemistry at C7. Structures E and F explain the preference for the observed selectivity at C7′′,favouringthevinylsubstituentattheequatorialposition.StructuresG and H rationalize the observed atropselectivity, with a moderate preference for G over H,whichplacestheortho-MeO group in ring A in a sterically crowded environment of the forming cyclohexane ring. LDA, lithium diisopropylamide; MCPBA, 3-chloroperbenzoic acid; acac, acetylacetonate; TIPS, triisopropylsilyl; HGII, Hoveyda– Grubbs II catalyst; dba, dibenzylideneacetone.

NATURE CHEMISTRY | VOL 5 | MAY 2013 | www.nature.com/naturechemistry 413 NATURE CHEMISTRY DOI: 10.1038/NCHEM.1597 ARTICLES

a OH MeO O DGD MeO 7'' MeO O PdLn SDG OMe Ar O 7' 7 OtBu 1" 7 7'' + O 8' 8' 8 1" Li O MeO OMe 7' 7 MeO Me I OMe OMe 9 OMe 9 O OH 18 OMe 19

3 17 SDG = stereodirecting group

Key features of design: Dearomatization with π-allyl palladium complex Diastereotopic group differentiation (DGD) Conjugate addition to set the acyclic stereotriad

NATURE CHEMISTRYO DOI: 10.1038/NCHEM.159722 23 24 N P ARTICLES N 20 OH a OTIPS OH OTIPSMeO OTIPS i BF3 Et2O, Et3SiH CHO =CHCO Me DGD 1. ( Bu)2AlH, CH2Cl2 MeO MeO 2 2 MeO MeO MeO MeO O 7'' CH2Cl2 HGII (5 mol%), CH2Cl2 2. ClCO2Me, Py o o n –78 C, 30 min PdLn 40 C, 72 h SDG3. Bu4NF, THF OMe Ar OMe Ar O OCO2Me Ar 7' OMe7 OtBu 1"Ar 7 OMe 7'' Ar OMe + OH OMe8' 1" O 8' OMe OMe 8 MeO2C OMe Li 7 O MeO OMe 88% 7' 93%7 93% MeO Me I OMe OMe 9 OMeMeO 9 OH MeO OTIPS MeO OTIPS O OH MeO OTIPS 18 OMe 19 26 27 28 29 3 17 SDG = stereodirecting group

Key features of design: Dearomatization with π-allyl palladium complex Diastereotopic group differentiationMeO (DGD) Conjugate addition to set the acyclic stereotriad 1. NaH, CH I, THF Pd(dba)2 (20 mol%) MeO MeO O 3 MeO MeO MeO P(OPh)3 (48 mol%) 2. H2, Lindlar cat. Tatanan B 1" OTIPS o b CH2Cl2, 25 C, 12 h EtOAc MeO(24%) OMe 7 OMe R 25 1" 7 1" 7 OMe ++OMe 7" OMe OMeMeO OMe OMe78% OMe 84% (92% b.r.s.m.) O MeO OMe O OMe A OMe Tatanan C d.r. 6:3:2 MeO A MeO MeO Br OMeo OMe OMe LDA, LiBr, THF, –78 C; mCPBA, CH Cl Pd(acac) (10 mol%) (48%)n O 2 2 MeO 2 BuLi, THF, see Table 1 o OH o MeO OH i then 19, –78 C; CH3I 0 C, 40 min PrMgBr, THF o S O OMe OMe OR' OMe 25 C, 3 h S CO tBu t 34: Rt = CH=CH2, R'=H CO tBu 21 50% 32 2 86%33 BuSO2 CO2 Bu 71% 2 89% 35: R = CH2CH3, R'=CH3 c O 22 23 24 N P MeO MeO MeO MeO PdLn O PdLn O PdLn O PdLn O N 20 7" OMe OMe OMe OMe 1" 7 7 Ar OTIPS Ar Ar OH –OTIPS OMe – OMe – OTIPS OMe i – 1 OMe O BF3 Et2O, Et3SiH O CH =CHCO Me O 1. ( Bu)2AlH, CH2OCl2 MeO MeO 2 2 MeO MeO A C MeO E OMe G O CH2Cl2 O HGII (5 mol%), CH2Cl2 O 2. ClCO2Me, Py O o o n OH –78 C, 30 min 40 C, 72 h 3. Bu4NF, THF Ar OMe OCO2Me Ar OMe Ar OMe vs vs Ar OMevs vs OH OMe OMe OMe MeO C LnPd OMe MeO 2 MeO7 88% PdL 93% MeO 93% PdLn MeO n O Ar O PdLn O 7" MeO OH MeO OTIPS MeO OTIPS MeO OTIPS O OMe OMe 7 OMe OMe 7 26 1" 27 28 29 OMe Ar 1 OMe Ar – OMe OMe –O OMe O –O –O O O O O MeO MeO OH B D F H 1. NaH, CH I, THF Pd(dba)2 (20 mol%) MeO MeO O 3 MeO MeO MeO P(OPh)3 (48 mol%) 2. H2, Lindlar cat. Tatanan B Figure 4 | Total synthesis of tatanans B and C by a palladium-catalysed cyclodearomatization. a,Reformulatedsynthesisplancentredonaconvergent1" o EtOAc CH2Cl2, 25strategy C, 12 h enabled by a stereochemically complex palladium-catalysed cyclization featuring diastereotopic aromatic group differentiation with concomitant(24%) OMe 7 OMe R 1" 7 1" 7 OMe construction of the central quaternary centre. b++,SuccessfulimplementationoftheconvergentsynthesisplanleadingtoaseparablemixtureofatropisomericOMe 7" MeO OMe 78% 84% (92% b.r.s.m.) O MeO OMe O OMe tatanans B and C in 12 stepsA from sulfoxide 21. c,Fourpairsoftransitionstructuresrationalizingallfourstereogeniceventsinthecomplexpalladium-A OMe Tatanan C d.r. 6:3:2 OMe OMe OMe catalysed cyclodearomatization of substrate 29.StructuresA and B explain the observed selectivityMeO for the formation of the C1′′ quaternary centre. Structures(48%) see TableC and 1 D provide the basis forOH the observed high selectivity duringMeO theOH diastereotopic group differentiation establishing stereochemistry at C7. Structures E OR' and F explain the preference for the observed selectivity at C7′′,favouringthevinylsubstituentattheequatorialposition.StructuresG and H rationalize the 34: R = CH=CH2, R'=H observed atropselectivity,32 with a moderate preference for G over33 H,whichplacestheortho-MeO group in ring A in a sterically crowded environment of the forming cyclohexane ring. LDA, lithium diisopropylamide; MCPBA, 3-chloroperbenzoic acid; acac,35: acetylacetonate; R = CH2CH3, R'=CH TIPS,3 triisopropylsilyl; HGII, Hoveyda– c Grubbs II catalyst; dba, dibenzylideneacetone. MeO MeO MeO MeO PdLn PdLn PdLn O PdLn ONATURE CHEMISTRY | VOL 5 | MAY 2013 | www.nature.com/naturechemistryO 7" O 413

OMe OMe OMe OMe 1" 7 7 Ar Ar Ar OMe – OMe – OMe – 1 OMe –O O O O MeO A C E OMe G O O O O OH

vs vs vs vs L Pd MeO n MeO PdL MeO PdLn MeO n O Ar O PdLn O 7"

O OMe OMe 7 OMe OMe 7 1" OMe Ar 1 OMe Ar – OMe OMe – OMe O –O O –O O O O O MeO OH B D F H

NATURE CHEMISTRY | VOL 5 | MAY 2013 | www.nature.com/naturechemistry 413 NATURE CHEMISTRY DOI: 10.1038/NCHEM.1528 ARTICLES Yohimbinoid-on the shoulder of giants a OMe OMe

AcO CO2Me OMe HO CO2Me

H O O CO2Me H 1. Cyclization E H H 2. Lactonization H Direct cyclization N O 3. Epimerization H D H N CN 3 H N MeO OMe N 12,13 Woodward9–11 C B Stork OMe A OMe

NH 2: Reserpine NH

MeO MeO 1 High blood pressure 3

b HO OH OH HO

CO2Me CO2Me CO2Me CO2Me H H H H

H H H H H H H H H H H H N N N N N N N N

OMe MeO 4: Yohimbine (α-CO Me) 2 6: Pseudoyohimbine 7: Excelsnin 8: 9-Methoxy-3-epi-α-yohimbine 5: Corynanthine (β-CO2Me)

HO HO HO HO 3 CO2Me CO2Me CO2Me R H H H H

H H H H H H H H H H H 1 N N N OR N N N N

OR2 1 2 R , R = CH2 or CH3 MeO MeO 3 R = CO2Me, H 9: Venenatine ( -CO Me) 12: Alstovenine ( -CO Me) β 2 11: Rauwolscine β 2 14: Berbanes 10: 16-Epivenenatine (α-CO2Me) 13: 16-Epialstovenine (α-CO2Me)

Figure 1 | Classic approaches to the synthesis of reserpine, and selected yohimbine and berbane alkaloids. a, Classic approaches to closure of the C-ring and setting of the C-3 stereocentre of reserpine. b,Selectedyohimbinoidnaturalproductsthatvaryinthesubstitutionpatternoftheindolering.Theability to access a variety of yohimbine natural products from a common precursor would allow for a unified approach to this group of alkaloids. diastereomers (the endo assignment is in reference to the more assigned unambiguously for substrate 23b by X-ray crystallographic electron-withdrawing ketone moiety), which were separable by analysis (see the ORTEP structure, Table 1; some hydrogen atoms column chromatography. The enantiomers of the endo adducts are removed for clarity). can be separated readily on a preparatory scale using supercritical More surprising, however, was that the inherent diastereoselec- fluid chromatography and a chiral ODH column (for further tivity of the thermal Pictet–Spengler cyclization (heating to details, see the Supplementary Information). Although access to 18 160 8C in acetonitrile, Method B in Table 1) of the substrates was sets the stage for preparing the natural products in an enantioen- influenced greatly by the nature of the indole fragment (entries 4–6, riched form, the syntheses reported herein were carried out in Table 1). Tryptamine-derived aminonitrile 16a cyclized on racemic form. Hydrogenation of 18, silyl ether cleavage and sulfona- heating in acetonitrile to afford a 1:3 (b:a) mixture of diastereomers, tion of the resulting primary alcohol group yielded benzenesulfonate whereas 4-methoxytryptamine substrate 16b and 6-methoxytrypta- 21 (82% yield over three steps). At this juncture, a-hydroxylation mine substrate 16c cyclized to give a 1:1.8 (b:a) and 1:8 (b:a) followed by oxidative cleavage with lead tetraacetate unveiled aldehyde mixture of diastereomers, respectively. The latter observation is in 17 (70% yield). Treatment of aldehyde 17 with potassium cyanide in line with the observed diastereoselectivity for the thermal cyclization the presence of various tryptamine derivatives then furnished the of aminonitrile 3 in refluxing acetonitrile reported by Stork12,13. desired aminonitriles (16a–16c) as single diastereomers and in As several yohimbinoid natural products possess the indole sub- excellent overall yields. The relative stereochemistry of the 4-OMe stitution pattern present in 16a and 16b, the stereoselectivities for tryptamine aminonitrile was established unambiguously by X-ray these cyclizations needed to be improved if a versatile approach to crystallographic analysis (see the ORTEP structure for 16b in the yohimbinoid natural products was to be realized. As such, we Fig. 2; some hydrogen atoms are removed for clarity)42. embarked on a campaign to optimize the diastereoselectivity of Substrates 16a–16c were subjected to Pictet–Spengler cyclization the Pictet–Spengler cyclization under thermal conditions (that is, conditions (Table 1), with special attention paid to the role that Method B) and chose to focus on substrate 16b as it represented arene nucleophilicity43,44 plays in both the rate and stereoselectivity the most challenging selectivity scenario. Previously, Stork proposed of the pentacycle formation45–47. Consistent with the elegant studies the formation of a tight ion pair between the departing cyanide and of the Stork group, treatment of aminonitriles 16a–16c with hydro- forming iminium ion under thermal Pictet–Spengler conditions for chloric acid (10% 1 N HCl in tetrahydrofuran at 23 8C) resulted in aminonitriles, which leads to an attack of the indole nucleophile the exclusive formation of the b-diastereomers (23a–23c; see entries from the b-face (see 26 24b, Fig. 3)12,13. On the basis of this 1–3, Table 1) for all the substrates. The relative stereochemistry was analysis, we reasoned that! a solvent with a lower static permittivity

NATURE CHEMISTRY | VOL 5 | FEBRUARY 2013 | www.nature.com/naturechemistry 127 ARTICLES Retrosynthesis and precursor assembly NATURE CHEMISTRY DOI: 10.1038/NCHEM.1528

a BnO BnO CO Me CO2Me BnO CO2Me 2 BnO BnO CO2Me CO2allyl CO2allyl O O H H CO2allyl MeO C H 2 8–13 CN + H H H H H N N H N N CHO H OBs OTBS TBSO

15 R 16 R 17 18 19 20

b BnO BnO CO2Me BnO CO2Me O O O i. TBSOTf, Et3N, CH2Cl2 i. Pd/C, H2, EtOAc MeO2C ii. Oxone, NaHCO3, PhMe,100 ºC ii. 1% HCl/MeOH (1:1:1) CH2Cl2:H2O:Me2CO + (>10:1 endo:exo) iii. BsCl, pyr., DMAP iii. 1% HCl/MeOH 89% H 82% (three steps) H 85% (three steps) TBSO OTBS OBs 19 20 rac-18 21

BnO CO2Me CO allyl BnO CO2Me BnO 2 H O CO2Me CO allyl MgSO4, CN 2 H OH Pb(OAc)4, CH2Cl2 H KCN, MeCN H N N H HO H NH2 R CHO Ar OBs 82% OBs

22 17 16a: Ar = indole, 90% 16b: Ar = 4-(OMe)-indole, 81% ORTEP representation of 16b 16c: Ar = 6-(OMe)-indole, 87%

Figure 2 | Retrosynthetic planTanllo and synthesis, D. J.; of theSarpong aminonitrile, R. et. al. Pictet-SpenglerNat. Chem substrates. a. ,Avarietyofyohimbinoidnaturalproductscanbeaccessed2013, 5, 126. from the common hydrindanone precursor 18. b,TheDiels–Aldercyclizationbetweendiene19 and dienophile 20 provides hydrindanone 18,settingfour contiguous stereocentres. Conversion of hydrindanone 18 into aldehyde 17 sets the stage for a rapid formation of aminonitriles 16a–16c with varying substitution on the indole moiety. Bs benzenesulfonyl, TBS tert-butyldimethylsilyl, PhMe toluene, pyr pyridine, TBSOTf tert-butyldimethylsilyl ¼ ¼ ¼ ¼ ¼ triﬂate, oxone potassium monopersulfate. ¼

Table 1 | Role of the indole nucleophile in the stereoselectivity of the Pictet–Spengler cyclization.

BnO BnO BnO CO2Me CO2Me CO2Me

CO2allyl CO2allyl CO2allyl H H H A: 10% 1 N HCl/THF H CN H H H H + H H H N B: 160 oC, MW, MeCN N N N N N

R R R (β)(α) 16a: Ar = indole 16b: Ar = 4-(OMe)-indole 23a–23c 24a–24c 16c: Ar = 6-(OMe)-indole

Entry Aryl amine Method* Diastereomeric ratio (b:a)†

1 16a (R H) A 1:0 ¼ 2 16b (R 4-OMe) A 1:0 ¼ 3 16c (R 6-OMe) A 1:0 ¼ 4 16a (R H) B 1:3 ¼ 5 16b (R 4-OMe) B 1:1.8 ¼ 6 16c (R 6-OMe) B 1:8 ¼ ORTEP representation of 23b·HCl

*Method A: 10% 1 N HCl/THF, room temperature (r.t.). Method B: MeCN, 160 8C, microwave. †Ratio determined by NMR analysis of the crude reaction mixture.

(dielectric constant)48 would lead to a tighter ion pairing and further a greater participatory role of the solvent. Several mechanistic pos- enhance the diastereoselectivity of the cyclization. In an initial sibilities that may exist are shown in Fig. 3. investigation of solvent effects (entries 3–7, Table 2), among sol- In one scenario, an acetonitrile solvent molecule could engage vents of similar polarity only acetonitrile and isopropanol provided iminium ion 26′ to form a different electrophilic species (see 27 the desired diastereomer (24b) as the major product, which suggests or 28, Fig. 3) that could then be displaced by the indole moiety in

128 NATURE CHEMISTRY | VOL 5 | FEBRUARY 2013 | www.nature.com/naturechemistry ARTICLES NATURE CHEMISTRY DOI: 10.1038/NCHEM.1528

a BnO BnO CO Me CO2Me BnO CO2Me 2 BnO BnO CO2Me CO2allyl CO2allyl O O H H CO2allyl MeO C H 2 8–13 CN + H H H H H N N H N N CHO H OBs OTBS TBSO

15 R 16 R 17 18 19 20

BnO CO2Me CO allyl BnO CO2Me BnO 2 H O CO2Me CO allyl MgSO4, CN 2 H OH Pb(OAc)4, CH2Cl2 H KCN, MeCN H N N H HO H NH2 R CHO Ar OBs 82% OBs

22 17 16a: Ar = indole, 90% 16b: Ar = 4-(OMe)-indole, 81% ORTEP representation of 16b 16c: Ar = 6-(OMe)-indole, 87%

Figure 2 | Retrosynthetic plan and synthesis of the aminonitrile Pictet-Spengler substrates. a,Avarietyofyohimbinoidnaturalproductscanbeaccessed from the common hydrindanone precursor 18. b,TheDiels–Aldercyclizationbetweendiene19 and dienophile 20 provides hydrindanone 18,settingfour contiguous stereocentres. ConversionIndole of hydrindanone eﬀect in 18 into aldehyde 17 setsstereoselecvity the stage for a rapid formation of aminonitriles 16a–16c with varying substitution on the indole moiety. Bs benzenesulfonyl, TBS tert-butyldimethylsilyl, PhMe toluene, pyr pyridine, TBSOTf tert-butyldimethylsilyl ¼ ¼ ¼ ¼ ¼ triﬂate, oxone potassium monopersulfate. ¼

Table 1 | Role of the indole nucleophile in the stereoselectivity of the Pictet–Spengler cyclization.

BnO BnO BnO CO2Me CO2Me CO2Me

CO2allyl CO2allyl CO2allyl H H H A: 10% 1 N HCl/THF H CN H H H H + H H H N B: 160 oC, MW, MeCN N N N N N

R R R (β)(α) 16a: Ar = indole 16b: Ar = 4-(OMe)-indole 23a–23c 24a–24c 16c: Ar = 6-(OMe)-indole

Entry Aryl amine Method* Diastereomeric ratio (b:a)†

1 16a (R H) A 1:0 ¼ 2 16b (R 4-OMe) A 1:0 ¼ 3 16c (R 6-OMe) A 1:0 ¼ 4 16a (R H) B 1:3 ¼ 5 16b (R 4-OMe) B 1:1.8 ¼ 6 16c (R 6-OMe) B 1:8 ¼ ORTEP representation of 23b·HCl

*Method A: 10% 1 N HCl/THF, room temperature (r.t.). Method B: MeCN, 160 8C, microwave. †Ratio determined by NMR analysis of the crude reaction mixture.

128 NATURE CHEMISTRY | VOL 5 | FEBRUARY 2013 | www.nature.com/naturechemistry ARTICLES Solvents eﬀect in cyclizaon NATURE CHEMISTRY DOI: 10.1038/NCHEM.1528 Table 2 | Solvent effects in the Pictet–Spengler cyclization.

BnO BnO BnO CO2Me CO2Me CO2Me

CO2allyl CO2allyl CO2allyl H H H Conditions H CN H H H H + H H H N N N N N N

(β)(α) MeO MeO MeO 23b 24b 16b Entry Solvent Te m p e rat u re ( 88888 C)* Additive Diastereomeric ratio (b:a)‡ 1THFr.t.§ HCl† 1:0 2MeCN82! –n/r 3MeCN160– 1:1.8 4 Acetone 160 – 2.5:1 5 i-PrOH 160 – 1:1.3 6 i-PrCN 160 – 1.4:1 7PhMe160– 1.8:1 8 PhMe 160 Imidazole 1:1.6 9 PhMe 160 DMAP 1:2.0 10 MeCN 160 DMAP 1:4 11 MeCN 160 NaI 1:10

*Reaction carried out in a microwave reactor in a sealed vial unless otherwise noted. †When HCl was an additive more than 2.8 equivalents were employed. The conditions were 10% 1 N HCl/THF. ‡ § Ratio determined by NMR analysis of the crude reaction mixture. Reaction carried out in a sealed vial under a N2 atmosphere. !Reaction carried out in a sealed vial in an oil bath.

Same key reacons a Same solvent system BnO surely produced similar yield! (65% BnO vs 72%) nOe HO CO2Me H CO2Me CO2Me very similar substrate CO2allyl CO2Me H H H H Pd(OAc)2, PPh3/PS H H EtOH/H O, 70 °C BBr , CH Cl , –78 °C H H H 2 H H H H H H 3 2 2 H N N + N N N 41% (29) N N 60% N 19% (30)

MeO MeO MeO MeO 23b 29 30 9: Venenatine b BnO BnO HO CO2Me CO2Me CO2Me CO2allyl H H H Pd2dba3, pyrrolidine, MeCN, 70 °C BBr3, CH2Cl2, –78 °C H H H H H H H H H N N N N 65% N 69% N

MeO MeO MeO 24b 31 12: Alstovenine

Figure 4 | Synthesis of venenatine and alstovenine. Pd-catalysed deallylation/decarboxylation allows for removal of the allyl ester in 23b and 24b under mild conditions, which minimizes the b-hydroxy elimination. Treatment of 29 and 31 with BBr3 leads to the selective removal of the benzyl protecting group and access to venenatine and alstovenine, respectively. PS polymer supported, nOe nuclear Overhauser effect. ¼ ¼ complete computational details see the Supplementary Information) including alstovenine, 16-epialstovenine, 16-epivenenatine and 9- were found to be in better agreement with our synthetic data (mean methoxy-3-epi-a-yohimbine, which are all in line with our synthetic absolute deviation (MAD) of 1.37 ppm) than with the isolation data data. Although no conclusive statement can be made regarding the (MAD of 1.85 ppm). Furthermore, the computational predictions origin of this discrepancy, it is possible that the reported data for iso- identified two 13C resonances (at 104.3 ppm and 31.0 ppm) from lated venenatine may have been mistabulated. the isolation data as outliers, both deviating by 5.0 ppm or more In summary, we identified an effective, general path to the synth- from the computational data (a complete line-listing comparison eses of a subset of yohimbinoid alkaloids that are epimeric at C(3). is given in the Supplementary Information). Importantly, the This work, which has resulted in the first total syntheses of the outlier at 104.3 ppm reported in the 13C data for isolated venenatine, alkaloids venenatine and alstovenine in racemic form, builds on which was one of the resonances that differed markedly from our important observations by Stork pertaining to aminonitrile synthetic 13C data, also varies significantly from that of the corre- Pictet–Spengler cyclizations. Key to the success of our synthetic sponding 13C resonances reported for closely related congeners, studies was the identification of conditions to control effectively

130 NATURE CHEMISTRY | VOL 5 | FEBRUARY 2013 | www.nature.com/naturechemistry ARTICLES Trioxacarcin A-Myers is Myers NATURE CHEMISTRY DOI: 10.1038/NCHEM.1746

CN OPMB CH3 O + Modular components of similar synthetic complexity: 5 CH3 OTBS Convergent coupling reactions: 4 O MOMO O Linear steps: 11 17 18

tBuOLi,

(CH3O)2SO2 70% CH3O OPMB O O CH3O OPMB CH3 i K2OsO4, NaIO4, 67% CH3O N2 CH3 O + OTBS ii BrBcat CH3O O H H CH3 OTBS t O O O iii Bu2SiCl2, Et3N, HOBt Si 21 MOMO OH O 55% (two steps) tBu tBu 19 20 i Rh2(OAc)4, 82%

Graphical schematic ii Et3N·3HF, 32%

CH3O H O CH CH O O 3 CH3O OPMB CH3 OAc 3 CH + H H 3 O OAc O H O O O OH H OH H O OTBS 5 1

i TMSNTf2, 58–78% ii DDQ, 95%

CH3O O OCH O CH3 CH O OH H3C 3 CH3O 3 CH3 H H HO H H O + O O O H H O OTBS H O SPh CH 3 OAc 22 23 CH 3 O H O O OH O CH3 AgPF6 CH3 74% H3C O H3C O CH3 CH3 HO HO H H CH3O O CH3O O O O O CH3 CH O O O CH3 CH O O CH3O 3 H CH3O 3 H i K2CO3, CH3OH H H H H O H O H O ii Et3N·3HF O O O H O H O OH 55% (two steps) H O H O OTBS CH3 OH CH3 OAc CH CH 3 O H 3 O H O Trioxacarcin A O OH GI50 (H460) 0.85 ± 0.36 nM OH 24

Figure 4 | Modular synthesis of trioxacarcin A. An 11-step synthesis of trioxacarcin A was achieved by the convergent assembly of ﬁve modular components of similar structural complexity, each denoted with a coloured circle. This strategy provides a general route to analogues with deep-seated structural variations. MOM, methoxymethyl; BrBcat, B-bromocatecholborane.

NMR, FTIR, circular dichroism and mass spectroscopy) were found sequence used to prepare each analogue was otherwise the same as to be identical with data we obtained from an authentic sample of the route to trioxacarcin A illustrated in Fig. 4 (experimental details natural trioxacarcin A4,20. We observed that trioxacarcin A, like its are provided in the Supplementary Information). Figure 5 depicts 12 precursor 24, is unstable towards a base in alcoholic solvents; compounds representative of more than 30 analogues that we have decomposition is evidenced by the formation of a deep burgundy- prepared to date by component variation. The analogues are red solution (solutions of pure trioxacarcin A in ethanol and chloro- depicted in a manner that highlights their structural differences rela- form are typically orange). tive to trioxacarcin A, using colour to trace the origins of the vari- Having established a modular 11-step sequence for the assembly ations to the components that were modified (identified in Fig. 4). of trioxacarcin A from five components of similar synthetic com- Below each structure in Fig. 5 are listed average 50% growth 21 plexity (identified with circles in Fig. 4) we next sought to demon- inhibition (GI50) values we obtained in growth-inhibitory assays strate the versatility of the sequence for the synthesis of analogues using cultured H460 cells, a human lung cancer cell line that is by component variation. Deep-seated structural modifications known to be especially sensitive to trioxacarcin A (we confirmed were introduced by variation of four of the five key building this, obtaining a GI50 value of 0.85+0.36 nM for trioxacarcin A; blocks (the cyanophthalide 17, the cyclohexenone 18, the trioxacar- DC-45-A1 was found to have a GI50 value of 37+7 nM (we are cinose B donor 5 and the trioxacarcinose A donor 23); the synthetic unaware of any previous measurements of growth-inhibitory

890 NATURE CHEMISTRY | VOL 5 | OCTOBER 2013 | www.nature.com/naturechemistry ARTICLES NATURE CHEMISTRY DOI: 10.1038/NCHEM.1694

36,39 CO2Me COARTICLES2Me difficult than that of the well-precedentedNATURE 5-exo-dig cases CHEMISTRY. DOI: 10.1038/NCHEM.1694 ARTICLES O Attempts to form the silylNATURE enol ether CHEMISTRY18 in the presenceDOI: of 10.1038/NCHEM.1694 triisopro- Me Me Me pylsilyl trifluoromethanesulfonate (TIPSOTf) and triethylamine CO Me O CO Me 36,39 CO Me CO2Me 2 36 difficult than that of the well-precedented36,39 5-exo-dig cases . Me 2Me 2 Me (Et3N) or diisopropylethylaminedifficult than thatO of theled to well-precedented a mixture of 18 5-exoand-dig19 cases . Me Me O Attempts to form the silyl enol ether 18 in the presence of triisopro- HN N N (Fig. 4) in favourAttempts of the to form latter. the To silyl avoid enol the ether undesired18 in the presenceg-deproto- of triisopro- Me Me Me Me Me Me pylsilyl trifluoromethanesulfonateO pylsilyl trifluoromethanesulfonate (TIPSOTf) and triethylamine (TIPSOTf) and triethylamine nated productO 19, the less basic 2,6-lutidine was employed in the 36 Me Me (Et N)36 or diisopropylethylamine led to a mixture of 18 and 19 Me Me MeMe (Et3N) or diisopropylethylamine3 led to a mixture of 18 and 19 MeMe above reactionMe to give 18 almost exclusively. Compound 18 was 1 Methyl homoseco- 2 Methyl homo- Me HN3 Daphnilactone A NN N (Fig. 4) in favour of the latter.g To avoid the undesired g-deproto- HN N subjected to(Fig. standard 4) in favour cationic of the latter. gold To cyclization avoid the undesired conditions-deproto- daphniphyllate daphniphyllate nated product 19, the lessnated basic product 2,6-lutidine19, the was less employed basic 2,6-lutidine in the was employed in the NATUREDOI: 10.1038/NCHEM.1694 CHEMISTRY DOI: 10.1038/NCHEM.1694(chloro(triphenylphosphine)gold(I) (Au(PPh )Cl, 0.2 equiv.), ARTICLESARTICLES NATURE CHEMISTRY above reaction to give 18abovealmost reaction exclusively.3 to give Compound18 almost18 exclusively.was Compound 18 was 1 Methyl homoseco- 1 Methyl2 Methyl homoseco- homo- 3 2Daphnilactone MethylAgSbF homo- A (0.3 equiv.)3 Daphnilactone and MeOH), A which rapidly generated the NATURECO CHEMISTRY2Me 6 DOI: 10.1038/NCHEM.1694subjected to standardsubjected cationic gold to standard cyclization cationic conditions gold cyclization conditions ARTICLES daphniphyllate CO2Me daphniphyllatedaphniphyllate daphniphyllate CO Me Daphenylline-classic total synthesis O N 36,39 desired bridged36,39 bicycle 20 (28%), and a significant amount of the 2 COCO2Me2Me CO2Me difficult than that ofdifficult the well-precedented than that of 5- theexo-dig well-precedented cases . 5-exo-dig cases(chloro(triphenylphosphine)gold(. (chloro(triphenylphosphine)gold(I) (Au(PPh3)Cl, 0.2 equiv.),I) (Au(PPh3)Cl, 0.2 equiv.), O O O Me HO direct desilylated product 15 was recovered. From a mechanistic Attempts to form the silyl enol ether 18 in the presence of triisopro- CO Me AgSbF (0.3 equiv.) andAgSbF MeOH),(0.3 which equiv.) rapidly and generated MeOH), the which rapidly generated the CO Me Me CO Me Attempts to form the silyl enol ether 18 in the presence of2 triisopro- 6 CO36,392Me 6 2 2 difficultMe than thatCO2Me of the well-precedentedCO Me 5-exo-dig cases . Me Me Me point of view,2 desired the formation bridged bicycleof 20 involved20 (28%), protonolysis and a significant of the amount cor- of the Me Me O Me pylsilyl trifluoromethanesulfonatepylsilyl trifluoromethanesulfonate (TIPSOTf)O O andN triethylamine (TIPSOTf)O N and triethylamine desired bridged bicycle 20 (28%), and a significant amount of the N O 36O Attempts to form the silyl enol ether 18 in the presence of triisopro- Me (Et N) or diisopropylethylamine led to aMe mixture of 18ON36and 19Me HOresponding alkenyldirect desilylated goldHO species product under15 stronglywas recovered. acidic conditions,From a mechanistic Me MeMe 3 Me Me Me (EtHON) or diisopropylethylamine ledOAc to a mixture of 18 and 19 direct desilylated product 15 was recovered. From a mechanistic Me Me Me Me Me 3 Mepylsilyl trifluoromethanesulfonateAcO Me (TIPSOTf) and triethylamine HN Me N Me(Fig. 4) in favour of the latter. ToO avoid the undesired g-deproto- which may alsoMepoint lead of view,to rapid the formation desilylation. of 20 Thus,involved we protonolysisexamined the of the cor- HN N N (Fig. 4) in favourOAc of the latter. To avoid the undesired36 g-deproto- point of view, the formation of 20 involved protonolysis of the cor- N Me Me NMe (Et3N) or diisopropylethylamine led to a mixture of 18 and 19 nated product 19, theMe less basic 2,6-lutidine was employedN in the NeffectOAc of differentresponding silyl groups alkenyl in gold tuning species the under ratio strongly of cyclization acidic conditions,/ Me natedN product 19, theHO less basic 2,6-lutidine was employed in the Ng OAc responding alkenyl gold species under strongly acidic conditions, HN 4N Bukittinggine 5 Daphmanidin E (Fig. 4) in favourYuzurimine of theHO latter.AcO To avoid the undesiredwhich may-deproto- also lead to rapid desilylation. Thus, we examined the above reaction to give 18 almost exclusively. Compound OAc18 was desilylation products.AcO As we expected,which the maytert also-butyldimethylsilyl lead to rapid desilylation. Thus, we examined the 1 Methyl homoseco- 2 Methyl homo- 3 Daphnilactone A above reaction tonated give 18 productalmost19, exclusively. the less basic Compound 2,6-lutidineOAc 18 waseffectwas employed of different in the silyl groups in tuning the ratio of cyclization/ daphniphyllate 1 Methyldaphniphyllate homoseco- 2 Methyl homo- subjected3 Daphnilactone to A standard cationic gold cyclization conditions (TBS) enol ether gave a poorer ratio (14% of 20, 74% of 15), and subjected4 BukittinggineO to standardabove5 Daphmanidin reaction cationic toE give gold18 cyclizationalmostYuzurimine exclusively. conditions Compound 18 was effect of different silyl groups in tuning the ratio of cyclization/ daphniphyllate 1 Methyldaphniphyllate homoseco- (chloro(triphenylphosphine)gold(2 Methyl homo- 3 DaphnilactoneI )A (Au(PPh 4)Cl, Bukittinggine 0.2 equiv.), 5 Daphmanidin E desilylationYuzurimine products. As we expected, the tert-butyldimethylsilyl CO2Me 3 CO2Me the tert-butyldiphenylsilyl enol etherdesilylation improved products. the As reaction we expected, the tert-butyldimethylsilyl daphniphyllate daphniphyllate (chloro(triphenylphosphine)gold(subjected to standardI) (Au(PPh cationic3)Cl, gold 0.2 cyclizationequiv.), conditions CO Me AgSbF (0.3 equiv.) and MeOH),O which rapidlyO generated the (TBS) enol ether gave a poorer ratio (14% of 20, 74% of 15), and 2 6 Me O H outputI (60% of 20, 20% of 15). As(TBS) the enol counterion ether gave of the a poorer gold ratio (14% of 20, 74% of 15), and CO2Me Me (chloro(triphenylphosphine)gold(O ) (Au(PPh3)Cl, 0.2 equiv.), CO Me H AgSbF6 (0.3 equiv.)CO Me and MeOH), which rapidly generatedCO2Me the the tert-butyldiphenylsilyl enol ether improved the reaction O N desired bridgedOAc2 bicycle 20 (28%), and a2 significant amount of the CO2Me AgSbFMe (0.3 equiv.)CO Me and MeOH),complex which (initially rapidly from generated theCO corresponding the2Me the tert silver-butyldiphenylsilyl salt) may have enol a ether improved the reaction desired bridgedCO2Me bicycle 206 O(28%), and2 a significant amountH of the O Me O N HO HOdirect desilylatedCO Me product 15 was recovered. From aMe mechanistic Me O output (60% of 20, 20% of 15). As the counterion of the gold Me 2 H H H H O H 40 O MeMe O N HO OAc desired bridged bicycle 20 (28%),subtleMe and a influence significant on amountMe theO reactivity of the ofoutput the active (60% gold of 20 species, 20% of as15). As the counterion of the gold point of view, the formationdirect of desilylated20 involved product protonolysis15N was of the recovered.H cor-Me From a mechanisticcomplex (initially from the corresponding silver salt) may have a Me O Me N HO HO N OAc Me directH desilylated productH 15 waswellH recovered. as onMe theFrom Bro¨nsted a mechanistic acidity ofcomplex the reaction (initially environment, from the corresponding40 silver salt) may have a N Me N responding alkenyl goldpoint species of view, underH the stronglyHO formation acidic of conditions,20 involved protonolysis of thesubtle cor- influence on the reactivity of the active gold species as HO OAc Me Me HN H 40 AcO N point ofNO view, the formation of 20variousinvolved silver protonolysis salts (AgBF ofH the, cor- AgSbF subtle, silver influence trifluoromethanesulfo- on the reactivity of the active gold species as N whichN may also leadO toresponding rapid desilylation. alkenyl Thus, gold we species examined under the strongly acidic conditions,well as on the4 Bro¨nsted6 acidity of the reaction environment, OAc HO N OAc Me respondingN H alkenyl gold speciesN under strongly acidicN conditions, HOAcO MeN OAc nate (AgOTf), AgOCOCF and others)well wereas on also the tested. Bro¨nsted To acidityour of the reaction environment, effect of different silylwhich groups may in tuning alsoO lead the to ratio rapid of cyclization desilylation./ O Thus, weH examinedvarious the silver3 salts (AgBF4, AgSbF6, silver trifluoromethanesulfo- OAc Daphmanidin C AcO Calyciphylline B whichMe may6 Calyciphylline also lead to A rapid desilylation. Thus, we examined the 4 Bukittinggine 5 Daphmanidin E Yuzurimine desilylationOAc products. As we expected, the tertMe -butyldimethylsilyl delight, AgOTfnateO proved (AgOTf), to be AgOCOCF the mostvariousand effective others) silver additive salts were (AgBF also and tested.4, ren- AgSbF To6 our, silver trifluoromethanesulfo- effect of differenteffect silyl groups of different in tuningO silyl groups the ratio in tuning of cyclization the ratio/ of cyclization/ 3 4 Bukittinggine 5 Daphmanidin E Yuzurimine Daphmanidin C Calyciphylline B Me6 Calyciphylline A nate (AgOTf), AgOCOCF and others) were also tested. To our O 4 Bukittinggine (TBS)5 Daphmanidin enol E ether gave adesilylationYuzurimine poorer ratio products. (14% of 20 As, 74% weexpected, of 15), and the tert-butyldimethylsilyldered 20 indelight, 72% yield AgOTf from proved15 to(with be the 11% most of effective15 recovered). additive3 and ren- O O desilylationDaphmanidin products. C As weCalyciphylline expected, B the tert-butyldimethylsilyl6 Calyciphylline A CO Me CO2Me the tert-butyldiphenylsilyl enol ether improved the reaction dered 20 in 72% yielddelight, from 15 AgOTf(with proved 11% of to15 berecovered). the most effective additive and ren- 2 O (TBS) enol ether gave a poorer ratio (14% of 20Replacing, 74% of 15 MeOH), and with other protic solvents, such as isopropyl O O HO O O (TBS) enol ether gave a poorer ratio (14% of 20, 74% of 15), and Me O H output (60% of 20, 20% of 15). As the counterionH of the gold Replacing MeOH with otherdered protic20 in solvents, 72% yield such from as isopropyl15 (with 11% of 15 recovered). Me CO Me CO2Me the tert-butyldiphenylsilylMe enol etherm improvedalcohol, the did reaction not improve the efficiency, and decreasing the catalyst H 2 CO2Me HO CO2Me the tert-butyldiphenylsilylO O enol ether improved the reaction OAc Me O complex (initially from the corresponding silver salt)Me may haveH a alcohol, did not improveReplacing the efficiency, MeOH and decreasing with other the protic catalyst solvents, such as isopropyl HO O Me H Me H loadingm to 0.1 equiv. led to 70% yield of 20 with 29% of 15 recov- H Me MeMeO outputMe O (60% of 20output, 20%HO (60% of 15 of).20 As40, 20%the counterion of 15). As the of the counterion gold of the gold H H H H Mesubtle influence on the reactivityMe of theMe active gold species as Me H loading to 0.1 equiv. ledalcohol, to 70% yield did not of 20 improvewith 29% the of efficiency,15 recov- and decreasing the catalyst OAc OAc MeO complex (initiallyH from the corresponding silverered. salt) With may20 havein a hand, desulfonationm with p-thiocresol in the pres- N N N Me H Me O complex (initiallyHN from the corresponding silver salt) may have a HO HO well as on the Bro¨nsted acidityN of the reaction environment,Me N ered. With 20 in40 hand, desulfonationloading to 0.1 with equiv.p-thiocresol led to 70% in the yield pres- of 20 with 29% of 15 recov- H H H H N H H H H subtleN influenceMe on the reactivityence of of the K activeCO40 , gold followed species by condensationas with carboxylic acid 14 Me subtle influenceMe on the reactivitym = 1 or of 2 the active gold species2 3 as O N variousN silver salts (AgBFN 4N, AgSbF6, silver trifluoromethanesulfo-O H ence of K2CO3, followed by condensation with carboxylic acid 14 O N N N well as on theH Bro¨nsted aciditym = 1 or 2 of the reaction environment,N ered. With 20 in hand, desulfonation with p-thiocresol in the pres- Me H H well as on the Bro¨nsted acidity of the reactionN under environment, 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide hydrochlo- Me Me nate (AgOTf), AgOCOCF3 and others) were alsoN tested. To our under 1-ethyl-3-(3-dimethylaminopropyl)carbodiimideence of K CO , followed by condensationhydrochlo- with carboxylic acid 14 Daphmanidin C Calyciphylline B O 7 Daphnicyclidin A O 8 Daphenylline various6, silvern,5-bridged salts tricycle (AgBF4, AgSbFride6, silver (EDC trifluoromethanesulfo-.HCl)/1-hydroxybenzotriazolem = 1 or 2 (HOBt)2 3 conditions O 6 Calyciphylline OA delight, AgOTf provedvarious to7 Da bephnic the silveryclidin most A salts effective (AgBF8 additive Da4p,hen AgSbFylline and6, ren- silver6,n trifluoromethanesulfo-,5-bridged tricycle ride (EDC.HCl)/1-hydroxybenzotriazole (HOBt) conditions Me Me nate (AgOTf), AgOCOCF and others) were also tested. To our under 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide hydrochlo- nate (AgOTf), AgOCOCF3 and others) were3 alsoafforded tested. amide Toafforded our21 in amide 72% overall21 in 72% yield. overall Compound yield. Compound21 underwent21 underwent Daphmanidin C deredCalyciphylline20 inB 72% yield6 Calyciphylline from 15 A (with 11%7 Da ofphnic15yclidinrecovered). A 8 Daphenylline 6,n,5-bridged tricycle ride (EDC.HCl)/1-hydroxybenzotriazole (HOBt) conditions O O Daphmanidin C Calyciphylline B Zhang, Q. et al. Figure6 Calyciphylline 1 | SelectedOrg. A naturalLeFigure. 2009 products 1 | , Selected11 that, 2357. natural representdelight, products major AgOTf that subfamilies represent proved major of to be subfamilies the most of effective additive and ren- 26,27 26,27 Replacing MeOH withdelight, other protic AgOTf solvents, proved such to be as the isopropyl most effectiveintramolecular additive and ren-intramolecular Michael addition Michael additionpromotedpromoted by K2CO by3 Kin2CO3 in HO Daphniphyllum alkaloids. The 6,n,5-bridged tricycledered (bottom20 in right) 72% indicates yield from 15 (with 11% of 15 recovered). afforded amide 21 in 72% overall yield. Compound 21 underwent H Daphniphyllumdered 20alkaloids.in 72%The yield 6,n,5-bridged from 15 tricycle(with (bottom 11% right) of indicates15 recovered). Me O Om alcohol, did not improve the efficiency, andFigure decreasing 1 | Selected the natural catalyst products thatCH3 representCN at 100 majorCH8C3CN subfamilies to at give 10098 ofinC to 86% give yield,9 in 86% together yield, with together 9% with of the 9% of26,27 the O O a common structural motifa common shared structural by 6, 7 and motif8Replacing shared,aswellastherelated by 6 MeOH, 7 and 8,aswellastherelated with other protic solvents, such as isopropyl intramolecular Michael addition promoted by K2CO3 in Me Me HO loading to 0.1 equiv. ledReplacing to 70% yield MeOH of 20 withwith 29%other of protic15 recov- solvents, suchchromatographically as isopropylchromatographically separable C6 separable diastereomer. C6 diastereomer. Interestingly, Interestingly, H Daphniphyllum alkaloids. The 6,n,5-bridged tricycle (bottom right) indicates Me HO Me Daphniphyllum subfamilym membersalcohol, (up did to not about improve 50). A versatile the efficiency, intermediate and decreasing the catalyst CH CN at 100 8C to give 9 in 86% yield, together with 9% of the O H Me H Daphniphyllum subfamily members (up to about 50). A versatile intermediate treatment of 21 with potassium3 hexamethyldisilazide (KHMDS) N ered. With 20m in hand,alcohol, desulfonation did not with improvea commonp-thiocresol the structural efficiency, in the motif pres- and shared decreasing bytreatment6, 7 and the8,aswellastherelated catalystof 21 with potassium hexamethyldisilazide (KHMDS) H N Me Me for the divergent synthesisloading of these toDaphniphyllum 0.1 equiv.alkaloids led to 70% could yield be devised of 20 with 29% of 15 recov- chromatographically separable C6 diastereomer. Interestingly, N MeMe forence the divergent of K CO synthesis, followed of by these condensationDaphniphyllum withalkaloids carboxylic could acid be devised14 Me m = 1 or 2 O H2 3 loading to 0.1 equiv.Daphniphyllum led to 70%subfamily yield members of 20 with (up 29% to about of 15 50).recov- A versatile intermediate Me based onN this structural motif.ered. With 20 in hand, desulfonation with p-thiocresol in the pres- treatment of 21 with potassium hexamethyldisilazide (KHMDS) O H basedunderN on 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide this structural motif. hydrochlo- N ered. With 20 infor hand, the divergent desulfonation synthesis with of thesep-thiocresolDaphniphyllum in thealkaloids pres- could be devised H NN ence of K2CO3, followed by condensation with carboxylic acid 14 O O 7 Daphnicyclidin A 8 Daphenylline 6,n,5-bridged tricycle ride (EDC.HCl)/1-hydroxybenzotriazolem = 1 or 2 (HOBt) conditions O KHMDS,O N ence of K2CO3, followedbased on this by structural condensation motif. with carboxylic acid 14 O KHMDS, m = 1 or 2 under 1-ethyl-3-(3-dimethylaminopropyl)carbodiimideO hydrochlo-i. LiHMDS; I2 18-crown-6, afforded amide 21 inaromatization 72% overall yield. strategy, Compound deconstruction21 underwent of the benzene ring of 11 i. LiHMDS; I 18-crown-6, 7 Daphnicyclidin A aromatization8 Daphenylline strategy,6,n,5-bridgedunder deconstruction 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide tricycle ofride the benzene(EDC.HCl) ring/1-hydroxybenzotriazole of 11 hydrochlo- (HOBt)2 conditionsii. BnNH 2 allylOTs O Figure 1 | Selected natural products that represent major subfamilies of 26,27 ii. BnNH Me O intramolecular Michaelwould addition give a cis-triene. promoted12, which by could K2CO be3 furtherin disconnected at 2 allylOTsO KHMDS, 7 Daphnicyclidin A 8 Daphenylline 6,n,5-bridged tricycle ride (EDC HCl)/afforded1-hydroxybenzotriazole amide 21 in 72% overall (HOBt) yield. conditions Compound 21 underwent i. LiHMDS; I Me Daphniphyllum alkaloids. The 6,n,5-bridged tricycle (bottom right) indicates would give a cis-triene 12, which couldaromatization be further disconnected strategy, deconstruction at of the benzene ringiii. Et N of 11 Me 78%2 18-crown-6, CH3CN at 100 8C tothe give C1–C139 in 86% bond yield, to give together the devised with 9% bridged of the tricyclic intermediate26,27 3 6 Figure 1 | Selected natural products that represent majorafforded subfamilies amide of 21intramolecularin 72% overall Michael yield. Compound addition 21 underwentpromotediii. EtMeN by KMeCO in 78% ii. BnNH 2 allylOTs a common structural motif shared by 6, 7 and 8,aswellastherelated the C1–C13 bond to give the devised bridged tricyclic intermediate 3 2 3 BnN OMe 6 BnN Me Figure 1 | Selected naturalDaphniphyllum products thatalkaloids. representThe major 6,chromatographicallyn,5-bridged subfamilies tricycle of (bottom9 and separable right) a suitable indicates C6 side-chain diastereomer.would give coupling a Interestingly,cis-triene partner.26,27 12 Compound, whichMe could9 was be further disconnectedO O at CO2Me intramolecular MichaelCH CN addition at 100 8C to givepromoted9 in 86% by yield, K CO togetherin with 9% ofBnN the OMe Me Daphniphyllum subfamily members (up to about 50). A versatile intermediate 9 and a suitable side-chain coupling partner.3 Compound 9 was 2 O3 O iii. Et BnNN CO2Me O 78% a common structural motif sharedtreatment by 6, 7 and of 821,aswellastherelatedwithtraced potassium back to hexamethyldisilazide bridged-bicyclicthe C1–C13 amine bond (KHMDS)13 toand give carboxylic the devised acid bridged14. tricyclic intermediate O O 3 6 Daphniphyllum alkaloids. The 6,n,5-bridged tricycle (bottom right) indicates chromatographically separable C6 diastereomer. Interestingly,Cl OMe Me O for the divergent synthesis of these Daphniphyllum alkaloids could be devised traced back to bridged-bicyclicCH3CN at amine 100 8C13 toand give carboxylic9 in 86% yield, acid 14 together. with 9% of the ABnN single OMe BnN Daphniphyllum subfamily members (up to about 50). AThe versatile recombination intermediate 9 ofand these a suitable two fragments side-chain could coupling be achieved partner. Compound 9 wasO O O O CO2Me based on this structurala common motif. structural motif shared by 6, 7 and 8,aswellastherelated chromatographicallytreatment separable of 21 C6with diastereomer. potassium hexamethyldisilazide Interestingly,Cl OMe (KHMDS) diastereomer for the divergent synthesis ofThe these recombinationDaphniphyllum alkaloidsthrough of could these an be twoamide-bond devised fragmentstraced formation back could to followed bridged-bicyclic be achieved by an intramolecular amine 13 and carboxylic acid 14. A single O Daphniphyllum subfamily members (up to about 50). A versatile intermediate O diastereomer O O through an amide-bond1,4-addition;treatment formation both of followed21 reactionswith potassiumby are an well-precedentedO intramolecular hexamethyldisilazide in Dixon’s syn- (KHMDS) Cl OMe A single for the divergent synthesisbased of these on thisDaphniphyllum structural motif.alkaloids couldO be devised KHMDS,The recombination of these two fragments could be achieved i. LiHMDS; I2 18-crown-6, 26,27 diastereomer aromatization strategy, deconstruction of the benzene ring of 11 1,4-addition; both reactionsthesis of theare calyciphylline well-precedentedthrough A-type an inamide-bond skeleton Dixon’s (Fig. syn- formationO 2) . A followed gold- by an intramolecularO 43−51% (overall) based on this structural motif. ii. BnNH 2 allylOTs KHMDS, would give a cis-triene 12, which could be further disconnected at catalysed alkyne-cyclization processO 26,27 (Toste–Conia-eneMe reaction)36 thesis of the calyciphylline A-type skeleton1,4-addition; (Fig.i. 2) bothLiHMDS;. reactions I A2 gold- are well-precedented18-crown-6,43−51% (overall) in Dixon’s syn- aromatization strategy, deconstruction of the benzeneMe ring of 11 O O O O O the C1–C13 bond to give the devised bridged tricyclic intermediate iii. Et3N could be exploited to construct78% 13ii.from BnNH 2 the silylKHMDS, enol36 ether gener-allylOTs 26,27 would give a cis-triene 12catalysed, whichMe could alkyne-cyclization be further disconnected processO at (Toste–Conia-enethesis of the calyciphylline6 reaction) A-type skeleton (Fig. 2) .Me A gold- 43−51% (overall) BnN i. LiHMDS;OMe I2 18-crown-6, o 9 and a suitable side-chainaromatization coupling strategy, partner. deconstruction Compound 9 ofwas the benzene ringO of 11O ated from ketone 15. This precursorBnN shouldCO2Me be readily accessibleO O 36 LiCl,O 170 C ii. BnNH iii. Et N Me 78% the C1–C13 bond to givecould the devised be exploited bridged tricyclic to construct intermediate13 fromcatalysed2 the silyl alkyne-cyclization enol3 ether gener-allylOTs process (Toste–Conia-eneMe6 reaction) Me Me traced back to bridged-bicyclic amine 13 and carboxylic acid 14. from the known hydroxyMe enone 16 andO sulfonamide 17 through aMe O O O would give a cis-triene 12, which could be further disconnected at O O BnN OMe BnN o 9 and a suitable side-chainated coupling fromCl ketone partner.15OMe Compound. This precursor9 was shouldcouldMe be be exploited readilyO O accessible to construct 13 from the silyl enol etherCO gener-2Me LiCl, 170 C Mitsunobu reaction.iii. Et N Notably, theA single C18 stereochemistry78% of 8 could 6 73% The recombinationthe of C1–C13 these two bond fragments to give the could devised be achieved bridged tricyclic intermediate 3 Me6 O Me Me o traced back to bridged-bicyclicfrom the amine known13 and hydroxy carboxylicMe enone acid1614and. ated sulfonamide from ketonediastereomer17 through15. This a precursorO O should beBnN readily accessible BnN H BnN CO Me LiCl, 170 C through an amide-bond formation followed by an intramolecular be established by facial selectiveBnN hydrogenationOMe of the correspondingBnN H 2 9 and a suitable side-chain coupling partner. Compound 9 was O O Cl OMe CO2Me A single Me Me Me The recombination of theseMitsunobu two fragments reaction. could Notably, be achieved the C18from stereochemistry the known hydroxy of 8 could enone 16 and sulfonamide O17 through6 a O 73% O 1,4-addition; both reactions are well-precedented in Dixon’s syn- methylene substrate at a suitable stage. O diastereomer traced back to bridged-bicyclic amine 13 and carboxylic acid 14. O O BnN BnN H BnN CO Me 6 73% through an26,27 amide-bondbe formation established followed by facial by an selective intramolecular hydrogenationMitsunobu of the reaction. corresponding Notably, the C18 stereochemistryH Tricyclic core ofof 8 could A 3.2:1 mixture of 2 thesis of the calyciphylline A-type skeleton (Fig. 2) . A gold- 43−51% (overall) Based on theCl above analysis,OMe we started our adventure withA single the The recombination of1,4-addition; these two both fragments reactions could are well-precedented be achieved in Dixon’s syn- O calyciphylline O diastereomersBnN atH C6 O BnN H BnN CO2Me 36 methylene substrate at a suitable stage. be established by facial selective hydrogenationdiastereomer of the corresponding catalysed alkyne-cyclization process (Toste–Conia-ene reaction) initial26,27 aim to reach the bridged tricycle 9, as shown in Fig. 4. A-type alkaloids O O O through an amide-bondthesis formation of the calyciphylline followed by an A-typeO intramolecular skeleton (Fig. 2) O . A gold- methylene substrate43O−51% (overall) at a suitableTricyclic stage. core of A 3.2:1 mixture of could be exploited to construct 13 from the silyl enol ether gener- Based on the aboveCoupling analysis, of we the started36 enantioenriched our adventure hydroxy with enone the 16 (six steps 1,4-addition; both reactionscatalysed are alkyne-cyclization well-precedented process in Dixon’s (Toste–Conia-ene syn- reaction) 37calyciphylline diastereomers at C6Tricyclic core of A 3.2:1 mixture of Basedo on the above analysis, we started ourFigure adventure 2 | An inspiring with the synthesis of the tricyclic core of calyciphylline ated from ketone 15. This precursor should be readily accessible initial aim26,27 to reachfrom them bridged-methylanisole, tricycleLiCl, 98% 1709O, enantiomeric asC shown in excess Fig. (e.e.)) 4.O A-tandype sulfo-alkaloids O calyciphylline diastereomers at C6 thesis of the calyciphyllinecould be A-type exploited skeleton to construct (Fig. 2)13 from. A the gold- silyl enol ether gener-43−51% (overall) from the known hydroxy enone 16 and sulfonamide 17 through a CouplingMe of the enantioenrichednamide 17 underMe hydroxy Mitsunobuinitial enone aim conditions to16 reachMe(six provided the steps bridged compound tricycle15 in9, asA-type shown alkaloids in Fig. by Dixon 4. andA-t co-workersype alkaloids26. Ahighlydiastereoselective catalysed alkyne-cyclizationated from process ketone (Toste–Conia-ene15. This precursor reaction) should36 be readily accessible LiCl, 170 oC 6 Coupling73% of the37 enantioenriched hydroxy enone 16 (six steps Mitsunobu reaction. Notably, the C18 stereochemistry of 8 could from m-methylanisole,86% 98% yield,O enantiomeric which retained excess highMe (e.e.))O enantiopurityand sulfo- (97% e.e.).FigureMe NotO 2 sur- | An inspiringintramolecular synthesisMe Michael of the addition tricyclic reaction core was of calyciphyllinedeveloped for the construction could be exploited to constructfrom the known13 from hydroxy the silyl enoneBnN enol16 etherand gener- sulfonamideBnN17 through a BnN 36,38 37 be established by facial selective hydrogenation of the corresponding H prisingly, theH followingfrom gold-catalysedm-methylanisole, 6-COexo2Me-dig 98% cyclization enantiomericA-type alkaloidsto excessof by the (e.e.)) Dixon 5,6-bicyclic andand co-workers sulfo- system. LiHMDS,Figure26. Ahighlydiastereoselective 2lithium | An inspiringhexamethyldisilazide; synthesis of the tricyclic core of calyciphylline Mitsunobu reaction. Notably,namide the17 C18under stereochemistry Mitsunobu of conditions8 could provided compound 15 in 6 o 73% methylene substrateated at a from suitable ketone stage.15. This precursor should be readilyO accessibleconstruct theO bridgednamide [3.3.1]bicyclic17 under systemO Mitsunobu turnedLiCl, 170 out conditions C to be more providedTs , p-toluenesulfonyl. compound 15 in A-type alkaloids by Dixon and co-workers26. Ahighlydiastereoselective be established by facial selective86% yield, hydrogenation which retained of the corresponding highMe enantiopurityBnN (97%H e.e.).Me Not sur-BnN intramolecularH Me MichaelBnN additionCO reaction2Me was developed for the construction Based on the abovefrom analysis, the known we started hydroxy our enone adventure16 and with sulfonamide the Tricyclic17 corethrough of a A 3.2:1 mixture of 86% yield,O which retained36,38 highO enantiopurity (97% e.e.).O Not sur- intramolecular Michael addition reaction was developed for the construction methylene substrate at a suitableprisingly,calyciphylline stage. the followingdiastereomers gold-catalysed at C6 6-exo-dig cyclization to of the 5,6-bicyclic system. LiHMDS, lithium hexamethyldisilazide; Mitsunobu reaction. Notably, the C18 stereochemistry of 8 could680 6 73% NATURE CHEMISTRY36,38 | VOL 5 | AUGUST 2013 | www.nature.com/naturechemistry initial aim to reach the bridged tricycle 9, as shown in Fig. 4. A-type alkaloids prisingly,Tricyclic core the of followingA gold-catalysed 3.2:1 mixture of 6-exo-dig cyclization to of the 5,6-bicyclic system. LiHMDS, lithium hexamethyldisilazide; Based on the above analysis,construct we thestarted bridged our adventure [3.3.1]bicyclicBnN with the system turnedBnN out to be more Ts , pBnN-toluenesulfonyl. Coupling of the enantioenrichedbe established by hydroxy facial selective enone hydrogenation16 (six steps of the corresponding H calyciphylline H diastereomers at C6 CO2Me initial aim to reach the bridged tricycle 9, as shown in Fig. 4. construct the bridged [3.3.1]bicyclic system turned out to be more Ts , p-toluenesulfonyl. from m-methylanisole,methylene 98% enantiomeric substrate at excess a suitable (e.e.)) stage.37 and sulfo- Figure 2 | An inspiring synthesis ofO the tricyclic coreA-type of alkaloids calyciphyllineO O Coupling of the enantioenriched680 hydroxy enone 16 (six steps NATURE CHEMISTRY | VOL 5 | AUGUST 2013 | www.nature.com/naturechemistry namide 17 under Mitsunobu conditions provided compound 15 in A-type alkaloids by Dixon andTricyclic co-workers core of 26. AhighlydiastereoselectiveA 3.2:1 mixture of Based on the abovefrom analysis,m-methylanisole, we started our 98% adventure enantiomeric with excess the (e.e.))37 and sulfo- 680Figure 2 | An inspiring synthesis of the tricyclic core of calyciphylline NATURE CHEMISTRY | VOL 5 | AUGUST 2013 | www.nature.com/naturechemistry 86% yield, which retained high enantiopurity (97% e.e.). Not sur- intramolecular Michael additioncalyciphylline reaction was developeddiastereomers for the construction at C6 initial aim to reach thenamide bridged17 under tricycle Mitsunobu9, as shown conditions in Fig. provided 4. compoundA-type alkaloids15 in A-type alkaloids by Dixon and co-workers26. Ahighlydiastereoselective prisingly, the following gold-catalysed 6-exo-dig cyclization36,38 to of the 5,6-bicyclic system. LiHMDS, lithium hexamethyldisilazide; Coupling of the enantioenriched86% yield, which hydroxy retained enone high enantiopurity16 (six steps (97% e.e.). Not sur- intramolecular Michael addition reaction was developed for the construction construct the bridged [3.3.1]bicyclic system turned out to be more Ts , p-toluenesulfonyl.37 from m-methylanisole,prisingly, 98% enantiomeric the following excess gold-catalysed (e.e.)) and 6- sulfo-exo-dig cyclizationFigure 2 | An36,38 inspiringto of synthesisthe 5,6-bicyclic of the system. tricyclic LiHMDS, core of lithium calyciphylline hexamethyldisilazide; namide 17 under Mitsunobu conditions provided compound 15 in A-type alkaloids by DixonTs , p-toluenesulfonyl. and co-workers26. Ahighlydiastereoselective 680 construct the bridged [3.3.1]bicyclicNATURE system CHEMISTRY turned| VOL out 5 | to AUGUST be more 2013 | www.nature.com/naturechemistry 86% yield, which retained high enantiopurity (97% e.e.). Not sur- intramolecular Michael addition reaction was developed for the construction prisingly, the following680 gold-catalysed 6-exo-dig cyclization36,38 to of the 5,6-bicyclic system.NATURE LiHMDS, CHEMISTRY lithium hexamethyldisilazide;| VOL 5 | AUGUST 2013 | www.nature.com/naturechemistry construct the bridged [3.3.1]bicyclic system turned out to be more Ts , p-toluenesulfonyl.

680 NATURE CHEMISTRY | VOL 5 | AUGUST 2013 | www.nature.com/naturechemistry NATURE CHEMISTRY DOI: 10.1038/NCHEM.1694 NATURE CHEMISTRY DOI: 10.1038/NCHEM.1694 ARTICLES NATURE CHEMISTRY DOI: 10.1038/NCHEM.1694 ARTICLESARTICLES DOI: 10.1038/NCHEM.1694 NATUREMe CHEMISTRYH Me H O reagents were also fruitless. To our delight,ARTICLES however, photo- Me H Me H Me H Me H OO reagentsreagents were were also fruitless. fruitless. To To our our delight, delight, however, however, photo- photo- Me 18 Me isomerization of the trans-triene afforded the desired cis-isomer. Me 18Me 18 MeMe isomerizationisomerization of ofH the transtrans-triene-triene afforded afforded the the desired desiredHciscis-isomer.-isomer. Me H Me H O reagentsIndeed, were treatment also fruitless. of ketone To9 with our KHMDS delight, however, and N-phenylbis(tri- photo- H H H H NATUREIndeed,Indeed, CHEMISTRYtreatment treatmentN of ketone ketoneDOI: 10.1038/NCHEM.169499withwith KHMDS KHMDS andN andNN-phenylbis(tri--phenylbis(tri- ARTICLES N N N N NATUREMe 18 CHEMISTRY DOI: 10.1038/NCHEM.1694Me isomerizationfluoromethanesulfonimide) of the trans-trieneARTICLES (PhNTf afforded2) furnished the desired thecis corresponding-isomer. fluoromethanesulfonimide)fluoromethanesulfonimide)H (PhNTf (PhNTf22)) furnished furnished the theO correspondingH corresponding Radical Indeed,triflate, treatment which further of ketone underwent9 with KHMDS Suzuki coupling and N-phenylbis(tri- with trans-boro- O O triflate,Me whichN8 DaphenyllineH further underwent SuzukiMe couplingN H with10 trans-boro- RadicalRadical triflate, which further underwent Suzuki couplingO with transcyclizationreagents-boro- were also fruitless. To our delight, however, photo- 8 Daphenylline8 Daphenylline 1010 Me H Me H fluoromethanesulfonimide)nate 22 to render triene 23 (PhNTfwith2 good) furnished overall the efficiency. corresponding Irradiation cyclization Me 18 Me O reagents were also fruitless. To our delight, however, photo- cyclization natenate2222toto render render triene 2323withwith good good overall overall efficiency. efficiency.O Irradiation Irradiationisomerization of the trans-triene afforded the desired cis-isomer. 42 Me 18 H Me H Radical 42 triflate,of 23 whichwith a further 125 W underwent Hg lamp for Suzuki five minutes coupling provided with trans12-boro-readily . 8 Daphenylline 10 Indeed,isomerization treatment42 of of the ketonetrans-triene9 with afforded KHMDS the and desiredN-phenylbis(tri-cis-isomer. ofof23N23withwith a a 125 125H W Hg lamp lamp for for five fiveN minutes minutesH provided provided12cyclization12readilyreadily. . NATURE CHEMISTRY DOI: 10.1038/NCHEM.1694 6π-electrocyclization/ Indeed, treatmentnateSurprisingly,22 ofto ketone render9 thiswith triene KHMDScis23-trienewith and wasgoodN-phenylbis(tri- found overall to efficiency. be reluctant Irradiation to enter N N ARTICLES fluoromethanesulfonimide) (PhNTf2) furnished the corresponding 6π-electrocyclization/6π-electrocyclization/ Surprisingly,Surprisingly, this this cis-triene-triene was wasaromatization found found to to be be reluctant reluctant to to enter enter 42 43 Pd-catalysed coupling fluoromethanesulfonimide)of the23 with 6p-electrocyclization a 125 (PhNTf W Hg2) furnished lamp for pathway the five corresponding minutes under provided thermal12 readily conditions. ; aromatization Me H Me H O triflate,43 which further underwent Suzuki coupling with trans-boro- Pd-catalysed coupling aromatization NATURE CHEMISTRYthe 6p-electrocyclizationO reagentsDOI: were 10.1038/NCHEM.1694 also pathway fruitless. To ourunder delight, however, thermalRadical photo- conditions ;43 Pd-catalysed coupling Me 18 theMe 8 6 Daphenyllinep-electrocyclization pathway under10O thermal conditions ; isomerization of the trans6-trieneπ-electrocyclization/ afforded the desired ciscyclization-isomer.Radical triflate, whichSurprisingly, furtherelevating underwent thisthecis Suzuki reaction-triene coupling was temperature with foundtransARTICLES to-boro- be merely reluctant led to to enter skeletal H 8 DaphenyllineH 13 Indeed, treatment of ketone 9 with KHMDS and N-phenylbis(tri-10 nate 22 to render triene 23 with good overall efficiency. Irradiation N elevatingN Me the reaction temperaturearomatization merely led to skeletal 43 13 elevatingPd-catalysed the coupling reactionfluoromethanesulfonimide)O temperature (PhNTf ) furnished the merely correspondingcyclizationMe led tonate skeletalO22 to renderthe 6 trienep-electrocyclization23 with good overall pathway efficiency. under Irradiation thermal42 conditions44,45; Me13 NATUREMe CHEMISTRYNATUREDOI: 10.1038/NCHEM.1694 CHEMISTRY DOI: 10.1038/NCHEM.16942 44,45of 23 with a 125 Wdecomposition. Hg lamp for five Inspired minutes provided by Trauner’s12 readily . seminal work ,we O O NATURE CHEMISTRYO 1 triflate, which furtherDOI: underwent 10.1038/NCHEM.1694 Suzuki coupling with trans-boro- ARTICLES ARTICLES42 Me Radical O Me 8 DaphenyllineO decomposition.10 Inspired by Trauner’s seminal work 44,45,we ARTICLES 1 DOI:Retrosynthesis 10.1038/NCHEM.1694 of 23 with a 125 W Hg lamp for five minutes provided 12 readily . NATURE CHEMISTRY cyclization nate 22 to render triene 23 with good overall efficiency. Irradiation elevating the reaction temperature merely led to skeletal decomposition.13 Inspired6π-electrocyclization/ by Trauner’s seminal workSurprisingly,,weARTICLES thisexaminedcis-triene 6 wasp-electrocyclization found to be reluctant promoted to enter by a Lewis acid to take 1 H H 42 Me examinedMe 6p-electrocyclizationof 23 with a 125O6 W-electrocyclization/ Hg lamp for promotedNATURE fiveMe minutes provided CHEMISTRYby12 readily aMe LewisO. acidDOI:reagents toSurprisingly,O 10.1038/NCHEM.1694 take were this alsocis-triene fruitless. was found To to our be reluctant delight, to however, enter43 photo-44,45 Pd-catalysed coupling aromatizationπ p decomposition. Inspired by Trauner’s seminal workARTICLES,we 6π-electrocyclization/examined 6p-electrocyclization1 COSurprisingly,2Me this cis-triene was promoted found to be reluctant by to aenter Lewis acidthe to 6 take-electrocyclization pathway under thermal conditions 43; CO Me Me H aromatization MeN Me H Haromatization Me N 43 H CO2Me advantage of the carbonyl functionality conjugated to the triene 2 COPd-catalysedMeMe 18 coupling advantagePd-catalysed coupling of thethe carbonyl 6p-electrocyclizationMeO functionality pathway underreagents thermal conjugated conditions were; also toO fruitless. theisomerizationthe trienereagents 6 Top-electrocyclization our were delight, of the alsotrans however, pathwayfruitless.-triene photo- under To afforded our thermal delight, the conditions desired however,; cis photo--isomer. N CO2Me N H2 Me H Helevating the reaction temperature merelyMe led to skeletal H O elevating theexamined reaction 6 temperaturep-electrocyclization merely promoted led to skeletal by a Lewis acid to take Me CO Me 13 advantage13Me of the carbonylO reagents functionality were also conjugated fruitless. To to the our triene delight,reagents however, were also photo- fruitless. To our delight, however, photo- N MeN18 2 Me O MeH 18 Me MeMe O decomposition. Inspired by Trauner’sMe Me seminalH work44,45,we elevating thesystem reaction of temperature12. Unfortunately, merely led a variety to44,45 skeletal of Lewis acids, such as 1 13 COO Me isomerizationMe H ofO the trans-trieneisomerization affordedMe the ofH the desiredO transcis-triene-isomer. afforded the desired cis-isomer. Me 18 system of 12. Unfortunately,2 Me a variety of Lewis acids,decomposition. suchIndeed, as treatmentadvantage Inspired of of ketone by the Trauner’s carbonylreagents9 with seminal were KHMDS functionality also work fruitless. and conjugated,weN To-phenylbis(tri- our delight, to the however, triene photo- Me 18 H N systemMe 1Me ofN O12H . Unfortunately,examinedOTBSHO 6p-electrocyclizationisomerizationN a promoted variety by aMe of Lewis of the acidN Lewis totransH takeO -trieneOCO acids,2MeI afforded suchisomerization the as desireddiethylaluminiumcis of-isomer. the trans-triene chloride, afforded boron the44,45 trifluoride desired cis-isomer. diethyl etherate, O O CO2Me Me 18 decomposition.Me Inspired by Trauner’s seminal work ,we OTBS N N CO2Me1 advantage of the carbonyl functionalityIndeed, conjugated treatment to the triene of ketone 9Indeed,with KHMDS treatment and ofN ketone-phenylbis(tri-isomerization9 with KHMDS of the trans and -trieneN-phenylbis(tri- afforded the desired cis-isomer. N H I diethylaluminiumN H NH chloride, boron trifluorideN H diethylexamined etherate, 6p-electrocyclization promoted by a Lewis acid to take O OTBS O 12 system of 12. Unfortunately,Indeed, a variety treatment of Lewis acids, ofH such ketone as 119 withfluoromethanesulfonimide) KHMDSIndeed, andsystem treatmentN-phenylbis(tri-H of 12 of. ketone Unfortunately, (PhNTf9 with) furnished KHMDS a variety and the of LewisN corresponding-phenylbis(tri- acids,35 such as N I O diethylaluminiumNO chloride, boron trifluoride35 diethyl etherate,examined 6p-electrocyclizationSnCl and copper( promotedI) trifluoromethanesulfonate2 by a Lewis acid to take , failed to effect 12 11 OTBS N COI 2OMe diethylaluminium chloride, boronfluoromethanesulfonimide) trifluorideN diethylCO Me etherate,O advantage (PhNTffluoromethanesulfonimide) of) the furnished carbonyl4 the functionality correspondingIndeed, (PhNTf conjugated treatment) furnished of to ketone the the triene9 correspondingwith KHMDS and N-phenylbis(tri- SnClN and copper(OTBSI) trifluoromethanesulfonateN N 35 2 , failedI to effect N2 2 12 4 11 CO2Me SnCl and copper(I)fluoromethanesulfonimide) trifluoromethanesulfonateO , failed to effect35 (PhNTf ) furnished thediethylaluminium corresponding chloride, boron trifluoride diethyl etherate, 12 11 N 4 N CO2Me triflate,2 advantagefluoromethanesulfonimide) which of the further carbonyl underwent functionalityfluoromethanesulfonimide) (PhNTf Suzuki conjugated) furnished coupling to the (PhNTf trienewith the correspondingtrans) furnished-boro- the corresponding SnCl4 and copper(O the desiredI) trifluoromethanesulfonate cyclization, and desilylationtriflate, followed byORadical whichdecomposition further, failed underwentsystem to effecttriflate, of Suzuki12. which Unfortunately,the coupling desired further with underwentcyclization, atrans variety-boro- of Suzuki and Lewis2 desilylation coupling acids, such with followed astrans352 -boro- by decomposition NATURE CHEMISTRY DOI: 10.1038/NCHEM.16948 Daphenyllinethe desiredO cyclization,12 Radical andtriflate, desilylation which10 followed further underwent byRadical11 decomposition Suzuki couplingSnCl with transand-boro- copper(I) trifluoromethanesulfonate , failed to effect 8 Daphenylline 8 DaphenyllineAu-catalysed Radical10gradually occurred under these conditions. The failure10 of these ARTICLESsystem of 12O . Unfortunately,4 atriflate, variety which of Lewis further acids, underwent such as Suzuki coupling with trans-boro- O OTBS O O cyclization natetriflate,22 to render whichRadical triene further23 underwentwith good Suzuki overall coupling efficiency. with Irradiationtrans-boro- 8 Daphenylline the desiredalkyne cyclization10 cyclization,efforts maycyclization be and attributable desilylation to the formidablenate8 steric Daphenylline22 followed hindrancetoI render gener-Radicalcyclization byAu-catalysed triene decompositiondiethylaluminium23 withnate good22 to overallgradually10 render chloride, efficiency. triene occurred boron23 Irradiationwith trifluoride under good overall these diethyl efficiency. conditions. etherate, Irradiation The failure of these Au-catalysed Michael addition gradually8 DaphenyllineO occurredOTBScyclization under thesenate 22 conditions.to renderO triene10 TheI 23 failurewith good ofdiethylaluminium overall these efficiency.the desired Irradiationcyclization chloride, cyclization, boronnate trifluoride22 andto desilylation render diethyl triene 23 followed etherate,with good by overall decomposition efficiency. Irradiation Me O 12 ated in the disrotatory cyclization process. The conrotatory11 pathwaycyclizationalkyne cyclization 42 35 42 42 alkyne cyclization OTBS O of 23 with a 125 W Hg lampSnClof 23 fornateofandwith23 five22 copper(with minutes ato 125efforts a renderI 125) W trifluoromethanesulfonate provided W mayHg triene Hg lamp42 be lamp12 attributable23readily for forwith five five good. minutes minutes to, the overall failed formidable provided provided to efficiency. effect12 steric12readily Irradiationreadily hindrance. . gener- 42 Au-catalysed graduallyefforts may12 occurred be attributableisMichael expected under to addition develop toof these less the23 steric formidablewith congestion conditions. a 125 and, encouragingly, W11 steric Hg The lamp we hindrance failure for five minutes gener-ofSnCl4 theseand provided copper(12I)readily trifluoromethanesulfonate. of 23 with a 12535 W, failed Hg lamp to foreffect five minutes provided 12 readily . Michael addition + occasionally detected a trace amount of cyclization product under Au-catalysed 4 gradually occurred under these conditions. The failure of42 these alkyne cyclizationCO2Me Me H MeN H 6π-electrocyclization/ O6 -electrocyclization/photoirradiation6π conditions-electrocyclization/ in the processSurprisingly, of triene isomerization. this cis-trienethe desired wasofSurprisingly,23 found cyclization,withated toa 125 this be in and the W reluctantcis desilylation Hg-triene disrotatory lamp to was for followed enter cyclization found five minutes by to decomposition be process. reluctant provided The to12 conrotatory enterreadily . pathway 6π-electrocyclization/Oefforts mayreagentsMe be attributableMeπ were also toSurprisingly, the fruitless. formidable this Tocis steric our-trienealkyne delight, hindrance was 6cyclizationπ-electrocyclization/ foundSurprisingly,the however, gener- desired to be reluctant cyclization, photo- this tocis and-triene enter desilylationSurprisingly, was followed found this by tocis decomposition-triene be reluctant was found to to enter be reluctant to enter Me MichaelO addition atedHO2C CO in2Me the disrotatoryNH cyclization process.OTBS The conrotatory pathway efforts may be attributable to the formidable steric hindrance gener- O aromatization UnderMichael optimized additionaromatization conditions (500 W Hg lamp, 15 minutes), trans- 43 43 43 43 OTBSPd-catalysed coupling OTBSaromatizationPd-catalysed couplingaromatization6 -electrocyclization/Pd-catalysedtheAu-catalysed 6p coupling-electrocyclization aromatizationgradually pathwaySurprisingly,the occurredO 6p under-electrocyclization under thermalthis thesecis-triene conditions conditions.the pathway 6wasp-electrocyclization; found Theunder failure to thermal be of pathway reluctantthese conditions under to thermal; enter43 conditions ; Me 18 O Pd-catalysedMe coupling Pd-catalysed93O couplingated14 in the disrotatory1 triene 23 πconverted cyclizationthe cleanly into 6p pentacyclicprocess.-electrocyclization compoundAu-catalysed The24 (as conrotatory a pathwaythe underpathwaygradually 6p thermal-electrocyclization occurredis conditionsexpected under these to; develop pathway conditions. less under The steric failure congestion thermal of these conditions and, encouragingly,; we Me is expectedisomerizationMe toO develop less of the sterictrans congestion-trienealkyne and, affordedcyclization encouragingly, the desired we cis-isomer.ated in the disrotatory cyclization process. The conrotatory pathway OTBS Amide formation single diastereomeraromatization in 71% yield) through a photoinducedalkyne isomer- cyclization efforts may be attributable to the formidableelevating steric the hindrance reaction temperature gener- merely43 led to skeletal H H OPd-catalysed couplingMichael addition elevatingelevating theOTBS13 reaction the reactiontemperatureefforts temperature merelytheelevating may 6p be-electrocyclization led attributable merelythe to reaction skeletal led to the to formidable temperature pathway skeletal steric under merely hindrance thermal led gener- to conditions skeletal ; 13 13 is expected13 toMichael developization addition/electrocyclization less steric cascade congestion with theMe intermediacy and, of cis- encouragingly,+ elevatingO we Me theoccasionally reaction detected temperature a trace amountmerely of led cyclization to skeletal product under44,45 CO Me Me Me + O O 13 occasionallyMeMeOIndeed, detectedMeCO treatment2MeOO a traceO of amount ketone of9 with cyclizationMe KHMDSO productO andated underN in-phenylbis(tri- the disrotatoryis expected cyclization44,45O to develop process.decomposition.44,45 less The steric conrotatory congestion Inspired pathway by and, Trauner’s encouragingly,44,45 seminal work we ,we N 2 N Me N decomposition.decomposition.1 Inspired byInspired Trauner’s bydecomposition. Trauner’s seminal work seminal Inspired,we work by,we Trauner’s seminal work ,we N 1 Me O Me OO OTBS ated in the disrotatory cyclization process. The conrotatory pathway 1 13 O1 Me O O elevatingphotoirradiation the reaction conditions temperature in the merely process led of trieneto44,45 skeletal isomerization. + occasionallyphotoirradiation detected conditions a trace in amount theOTBS processi. TBDPSOTf, of cyclization of triene+ isomerization. product underMe occasionally detectedexamined a trace 6p amount-electrocyclization of cyclization promoted product by a Lewis under acid to take CO Me O2N Me fluoromethanesulfonimide) (PhNTf ) furnishedO isdecomposition. the expectedNH corresponding to develop Inspired less steric congestion by Trauner’s and, encouragingly, seminal we work ,we 2 1 Me OCO2Me examinedexamined 6pHO-electrocyclizationMeo2C 62pCO-electrocyclization2Me O promotedexamined by promoted a Lewis 6p acid by-electrocyclization a to Lewis take acid to promoted take by a Lewis acid to44,45 take N HO2C CO2Me ONH O 2,6-lutidine, −78 C is expected to develop less steric congestion and, encouragingly, we N O CO2Me O decomposition.Under optimized Inspired conditions by Trauner’s (500 seminal W Hg lamp, work 15 minutes),,we trans- + 1 17, PPh , ii. Au(PPh 3)Cl, CO Me advantage of the carbonyl functionality conjugated to the triene CO2Me HN S photoirradiationUnder optimizedO Me conditions conditionsOTBS 3 in (500 the W processN Hg+ lamp, of triene15 minutes), isomerization.occasionallytrans- N detectedphotoirradiation2 a trace amount conditions of cyclization in the product process under of triene isomerization. O COO2Me Me COCOMeCO2MeMe o advantageAgOTf, of MeOH the carbonyl functionalityexamined conjugatedMe 6p-electrocyclization to the triene promoted by a Lewis acid to take OTBS N N Me N N triflate,2 N 2 whichCO2MeDIAD, further 0 C underwentadvantageN + Suzuki ofCO the2Me coupling carbonyloccasionally with functionalityadvantagetrans detected-boro- of conjugated the a trace carbonyl amount to the functionality oftriene cyclization conjugated product under to the triene HO2C CO2Me NHO Radical N CON 932Me HO2C 14CO2Me NH1 p system of 12. Unfortunately, a variety of Lewis acids, such as 9314 101 OH N o-Ns examinedtriene 6 -electrocyclization23 converted cleanly promoted into by pentacyclic a Lewis acid compound to take 24 (as a 8 Daphenylline COUndertrieneMe optimized23 converted conditions cleanly86% into (500 pentacyclic W Hg lamp, compoundMe 15 minutes),24photoirradiation(astrans a - Under conditions optimized in the conditions process of (500triene W isomerization. Hg lamp, 15 minutes), trans- O cyclization 2 O OTBSMe systemMe of70% (two12 steps,O. 29% Unfortunately,N Me a varietyphotoirradiation of LewisO acids, conditions such as in the process of triene isomerization. OTBS 17 N 16 COnateMe 15 22 to render trieneHO2NC 23ofCOsystem 15 withrecovered)2MeCO2 goodMe of OTBS12NH. overall Unfortunately,Me efficiency.advantagesystem a varietyIrradiation of of the12 ofI. Lewis Unfortunately,carbonyl acids, functionalitydiethylaluminium such a variety as of conjugated chloride, Lewis acids, boron to such the trifluoride triene as diethyl etherate, O O2 OH NHO C CO Me o-Ns NH OTBS N 93o-Ns2 N 2 14 CO2Me Under1 advantage optimizedsingle conditions of the diastereomer carbonyl (500 W Hg functionality in lamp, 71% 15 yield) minutes), conjugated throughtrans a- photoinduced to the triene isomer- Amide formation9314 O OTBS1 trienesingleOAmideO23 diastereomer formationconvertedOTBSOTBSOI inI cleanly 71%diethylaluminium yield) into through pentacyclic12O a photoinduced chloride, compoundI boron isomer-24Under trifluoride(as optimized a triene diethyl11 conditions4223 etherate,converted (500 W cleanly Hg lamp, into 15 minutes),pentacyclictrans compound- 2435 (as a Figure 3 | Retrosynthetic analysis of daphenylline. The keyO strategies OTBS 16 15 diethylaluminium20 chloride,diethylaluminium boron trifluoride chloride,diethylSnCl etherate, boronand copper( trifluorideI) trifluoromethanesulfonate diethyl etherate, , failed to effect 12 93of 2311 with a 125 W Hg lamp14 for five minutes1 providedtrienesystem2312 ofconvertedreadily3512. Unfortunately, cleanly. into pentacyclic a4 variety compound of Lewis24 (as acids, a such as 12 include (1) a radical cyclization for the assemblyization of the seven-membered/12electrocyclization9311 cascadeSnCl and with14 copper( theI11) intermediacy trifluoromethanesulfonate1 oftrienecissystem-23 convertedsingle, of failedization12 diastereomer. cleanly to Unfortunately,/35electrocyclization effect into pentacyclic in 71% a yield) variety cascade compound through of with35 Lewis24 a(as photoinduced the acids, a intermediacy such isomer- as of cis- O DOI:singleAmide 10.1038/NCHEM.1694 diastereomer formation in 71% yield)4 O throughi. K2CO3, a photoinducedI isomer- I the desired cyclization, and desilylation followed by decomposition Amide formation NATURE CHEMISTRYring, (2) a sequence of 6p-electrocyclization/aromatizationOTBS for the SnCl4 Iand copper( ) trifluoromethanesulfonateSnCl4 and copper(,) failed trifluoromethanesulfonate to effect , failed to effect 6π-electrocyclization/ AmideO formationSurprisingly,OTBS this cis-triene wasOp-thiocresol found72%I to besinglediethylaluminium reluctant diastereomer to enter in 71% chloride, yield)ARTICLES through boron a photoinduced trifluoride isomer- diethyl etherate, construction of the tetrasubstituted arene, (3)Amide an intramolecular formation Michael the desired cyclization,ii. 14, HOBt, (two and desilylationsingle followeddiethylaluminium diastereomer byizationAu-catalysed decomposition/ inelectrocyclization 71% chloride, yield)gradually through boron cascade a occurred photoinduced trifluoride with under the isomer- these diethyl intermediacy conditions. etherate, The of failurecis- of these addition to form the pyrrolidine12 ring withization the desired stereochemistry/electrocyclization and cascadethe with11 desiredEDC·HCl, the Et3N cyclization, intermediacysteps) and desilylation ofthecis desired- followed cyclization,43 by decomposition and desilylation followed by35 decomposition Pd-catalysed coupling aromatization O 12 p gradually occurred11 under these conditions.izationSnClO /electrocyclizationand The copper(alkyne failure cyclization ofI) cascade these trifluoromethanesulfonate with the intermediacy of,35 failedcis- to effect O (4) a gold(I)-catalysed 6-exo-dig cyclization to prepare thethe bridgedAu-catalysed 6,6- 6 -electrocyclization O pathwayMichael under addition thermalizationSnCl4 conditions/electrocyclizationand copper(; cascadeI) trifluoromethanesulfonateefforts with may the be intermediacy attributable to of the,cis formidablefailed- to steric effect hindrance gener- gradually occurredO under thesegradually conditions.4 occurred The under failure these of these conditions. The failure of these bicyclic building block (13). Inspired by Dixon’s work26,intermediatealkyne9 is cyclizationAu-catalysed Au-catalysed Me H Me H efforts may beMe attributableO to the formidableO steric hindrance generated in the disrotatory cyclizationi. TBDPSOTf, process. The conrotatory pathway O2N Michaeldevised addition on the basis of the common structural motif of daphenylline-relatedelevatingO2Nalkyne the cyclization reactionO O temperaturei. TBDPSOTf,reagentsalkyne were cyclization merely alsothe fruitless. led desiredthe to desired To skeletalcyclization, our cyclization, delight, and and however, desilylation desilylation photo- followed followed by by decomposition decomposition 13 O Me O efforts may beO attributable to theeffortsOTBS formidable may be steric attributable hindranceO to the gener- formidable steric hindrance gener-o O DaphniphyllumMichaelalkaloids. addition The 6p-electrocyclization/aromatization strategy,Michael addition O Meo O is expected to develop less2,6-lutidine, steric congestion −78 C and, encouragingly, we MeO Me O Me O Oated in the2,6-lutidine, disrotatoryo N −78 cyclization C process. The conrotatory44,45 pathway i. TBDPSOTf, O Me 18 O Me O2N O K2CO3, 100 C O which was applied to our previous synthesisOTBS ofO tubingensin A, plays a CO Me Au-catalysedisomerization of thegraduallytransgradually-triene occurred afforded occurredO under the under desired thesei. theseTBDPSOTf, conditions.cis-isomer. conditions. The The failure failure of of these these Me O MeOdecomposition.2NO O Inspired2 ated byi. TBDPSOTf, in Trauner’s theAu-catalysed disrotatoryCO 2Me seminal cyclizationated work process. in the,we disrotatory The conrotatory cyclization pathwayi. TBDPSOTf, process. The conrotatoryo pathway O O2N 1 determining role in the entire daphenylline plan. o-Ns, 2-nitrobenzenesulfonyl.O N N 6 ii. Au(PPhO )Cl, O + occasionally detected a traceii. Au(PPh amount)Cl, of cyclization product under + OTBS 217O, PPh , +is expectedOTBS to86% developCO3 Me less steric congestion and, encouragingly, we17, PPh3, o 2,6-lutidine, −78 3 C HN S H O HN S 3 H O Nalkyne cyclizationalkyne2 cyclizationo O Me 2,6-lutidine,O −78 Co O Me O 2,6-lutidine,Indeed,Me treatment−78 C of ketoneefforts9 with mayO KHMDS be attributableO and No to-phenylbis(tri-2,6-lutidine, the formidable −78 C steric hindrance gener- Me examinedMichael additiono 6p-electrocyclizationO OTBSisAgOTf, expected promotedMeOH to develop by less aefforts LewisO stericis expected acidmaycongestion be to attributable taketo and, develop encouragingly,O less to stericphotoirradiation the formidable congestion we conditions and, stericii. Au(PPh encouragingly,AgOTf, hindrance in)Cl, the MeOH process gener- we of triene isomerization.O N MichaelO addition+ DIAD,N 0 C + occasionallyO detected a trace amount ofO cyclization product under17, DIAD,PPhMe 3, 0 ii.C Au(PPh )Cl, 3 O CO2Me 2 HN OS + OTBS HO2C O CO17,2 PPhMe 3, NH ii. Au(PPh3 )Cl, O O + N in tetrahydrofuranN (THF) at 78 8CHN affordedSHN the undesiredS C6 ORTEP of+ 9a (desilylated 9) 9 ii. Au(PPh 213)Cl, Me N Me 17, PPh3, 3 CO2Me OH O 17, PPh+ 3, Me OHoccasionallyfluoromethanesulfonimide)MeO + detectedMe a trace amountatedoccasionally (PhNTf in the ofo cyclization disrotatory) detected furnished a product tracecyclizationo theUnder correspondingamount under optimized process. of cyclization conditions TheAgOTf, conrotatory MeOH product(500 W Hg under lamp, pathway 15 minutes), trans- HN S COepimer2Me asCO the majorMeMeMe productO ( 3:1Me diastereomericadvantage ratioCO2 (d.r.)).Me of the carbonylphotoirradiationMe functionality conditionsOTBS conjugated in the processated in to of theDIAD, trienethe disrotatory 0 trieneC isomerization.2o DIAD, cyclization 0 C AgOTf,AgOTf, process. MeOH MeOH The conrotatory pathway N Me N 2o-Ns ! O o Me OTBS o-Ns DIAD, 0 C N In contrast, Dixon and co-workersHO2C observedN COO the2Me desired stereo-86% NH OTBS AgOTf,93 MeOH ON Me 14 1 86%triene 23 converted cleanly into pentacyclic compound 24Me(as a MeO DIAD,O 0 C OH UnderMeOH optimizedphotoirradiation70%triflate, (two conditionssteps, which N29%N conditions further (500O N W Hgunderwent inisphotoirradiation lamp, the expected process 15 Suzukiminutes), to of develop triene coupling conditionsMetrans isomerization.- less with in steric thetrans congestion process-boro- ofMe triene and,70% encouragingly,isomerization.(two steps, 29% weN chemistry at C6 when synthesizing the 5,6-bicyclic system O OH OSiR2R' OSiR2R' o-Ns O 17 16 O OTBS N15 HO CsystemCO17 Me of 12.Radical Unfortunately,NH MeHO16C CO aMe varietyo-Ns ofoNH-Ns LewisMeis15 expected acids, such to develop as less steric congestion and, encouragingly, we OH 8 Daphenyllinethrough an intramolecular Michael addition2 under KHMDS2 10 Amide2 of formation 15 recovered)2 86%86% 86%Me single70% (two diastereomer steps, 29%Me in70% 71% (twoof 15 yield) steps, recovered)MeMe through29% a photoinducedN Me isomer- 93o-Ns 14 1 cyclizationRN2R'SiOTf, base Under optimized conditionso-Ns (500UnderMe W optimized Hg lamp,Me 15 conditions minutes),Me (50070%trans (two W -steps, HgN 29% lamp, 15N minutes), trans- o-Ns O O OconditionsOTBS (Fig. 2)26. The structure of 9OHwas17O confirmed by17OTBS X-ray 16 triene 1623 convertednate+22 cleanlyto15 render into triene pentacyclic15 23occasionallywithMe compound goodOH detected overall24 (as efficiency. a a trace amount Irradiation of cyclizationN product under OTBS I COdiethylaluminium217Me 86% 16 chloride,o-NsMe +70% boron (two steps, trifluoride 29%15 occasionally diethylMe etherate, detected a traceizationof amount of15 15 recovered) /recovered)electrocyclization of cyclizationo-Nsof 15 recovered) cascade product with the under intermediacy of cis- crystallographic analysis of itsCON desilylatedMe derivativeMe (Fig. 4; Tm Me Me N N N 42 o-Ns o-Ns 17 16 Amide formation 9315 N 2 9314 1 single diastereomertriene14of 2323with inconverted 71% a 125 yield)1 W cleanly through HgOH lampOH intotriene a photoinduced forpentacyclic23 fiveOHconverted minutes compound isomer- cleanlyN provided24 into(as12 pentacyclicreadily a . compoundo-Ns 24 (as a 12 144–147118C, ethyl acetate (EtOAc)/n-hexane16 1:1). All the above N ofN 15 recovered)N 35 photoirradiation conditionso-Nso-Ns in the processo-Ns of triene isomerization. Figure 3 | Retrosynthetic analysis of daphenylline. The key strategies Figure 3 | Retrosynthetico-Ns analysis15I of daphenylline.o-Ns Theo-Ns key strategiesMe20photoirradiationo-Ns 16 conditions in the process15 of triene isomerization. 20 transformations were performed readilyOH on a multigramSnCl scale4 withand copper(izationN)HO trifluoromethanesulfonate2C/electrocyclizationCO2Me cascadeNHMe with, failed the intermediacy to effect O of cis- Amide formation FigureAmide 3 |formationRetrosynthetic analysis15 of daphenylline.singleThe18 key diastereomer strategies19 in 71%O 16 yield)16single through diastereomer a photoinduced15 in 71% yield) isomer- through a photoinduced20 isomer- consistent efficiency. 6π-electrocyclization/FigureO Figure 3 | Retrosynthetic 3 | Retrosynthetic analysisHO analysis2C of daphenylline.o-CO ofNs2 daphenylline.Me Surprisingly,The key strategiesNHThe key this strategiescis-trieneUnder was optimized16 found to conditions be15 reluctant (50015 to W enter Hg lamp,20 15 minutes), trans20 - include (1) a radical cyclization for the assembly of the seven-memberedWith the tricyclic intermediate 9 in hand,include we focusedOTBS (1) our a atten- radical cyclization for the assembly of the seven-memberedUnder optimized conditions (500 W Hg lamp, 15 minutes),i. TBDPSOTf,trans - O aromatizationinclude (1) a radicalthe desired cyclization cyclization, for the assembly and ofization the desilylation seven-memberedO/2electrocyclizationN followed by cascade decompositionization with/electrocyclization the intermediacy cascade of cis with- the43 intermediacy of cis- Pd-catalysed coupling includeOTBS16 93 (1) a radical cyclizationFigure 4 | Construction for the of15 the assembly bridged tricycle14 of9. theAgold( seven-memberedI)-catalysedp alkynei. K2CO1 3, 20 i. K CO , o Figure 3 | Retrosynthetic analysis of daphenylline. The key strategiestion on the construction of the challenginginclude tetrasubstituted (1) a arene radicalO cyclization for the assemblytheO 6 of-electrocyclization the seven-memberedtriene pathway23 converted under thermal cleanly conditions into pentacyclici. K CO ;, compound2,6-lutidine,242 −(as378 C a ring, (2) a sequence of 6p-electrocyclization/aromatization formotif, the as shown in Fig.O 5a.93cis-Triene 12ring,was considered (2) a a sequence suitable cyclization of was 6 exploitedp-electrocyclization/aromatization to14 construct the bridged 6,6-bicycle 20.Silylenol1 p-thiocresol for the72% O i. K2 2CO3 3, O Au-catalysed ring, (2)ring, a (2) sequence agradually sequence of 6p of-electrocyclization/aromatization 6 occurredp-electrocyclization/aromatization under these for conditions. forthe the Thetriene failure23 converted of these cleanly into pentacyclic compoundi. K2ii.CO Au(PPhp24-thiocresol3, (as)Cl, a 72% O precursor to explore the feasibility of thering, 6p-electrocyclization (2) a sequence/ ether 18 of,thesubstrateforthisreaction,waspreparedfromenone 6p-electrocyclization/aromatizationHNelevatingS 15 (inset); the+ for reaction the temperature merely led to skeletal17p, -thiocresolPPhp-thiocresol3, 72%72% 3 include (1) a radical cyclization for the assembly of theO seven-membered2N13 Amide formation O ii. 14, HOBt,O i. (twoTBDPSOTf,single diastereomerMe in 71% yield) through a photoinducedp-thiocresol isomer-72% (two construction of the tetrasubstituted arene, (3) an intramoleculararomatizationalkyne Michael strategy. cyclization However, well-recognizedO construction approaches, such of the2,6-lutidine tetrasubstituted as a base wasO found to suppress arene, the formation (3) of an the undesired intramolecular MichaelMe ii. 14,o HOBt, (two(two AgOTf,ii. 14 MeOH, HOBt, Me O constructionconstruction of the of tetrasubstituted theMe tetrasubstituted arene, arene,O (3) an(3) intramolecular an intramolecular Michael Michaeli. K2CO3, single2,6-lutidine, diastereomer −78 oC in 71% yield) through44,45DIAD,ii. 014 ,C HOBt, a photoinduced isomer- Michael addition O as partialAmide hydrogenation formation41 of the correspondingconstruction ynediene,efforts proved of may theregioisomeric tetrasubstituted be attributable silyl enol ether 19.AnintramolecularMichaeladdition arene, to the (3)O an formidable intramolecularEDC·HCl, stericEt3N Michaelsteps) hindrance gener- N ii. 14, HOBt,EDC·HCl, Et(twoN steps) ring, (2) a sequence of 6p-electrocyclization/aromatization for the O decomposition.O InspiredOH ization byi. Trauner’s/ TBDPSOTf,electrocyclization seminal cascade work i. TBDPSOTf, withEDC·HCl,,weEDC·HCl, the Et Et 3N3N intermediacysteps)steps) of cis3- addition to form the pyrrolidine ring with the desired stereochemistry1 O2Nto be unsatisfactory and becauseaddition of theaddition pooraddition to positional form toO form the2 toN selectivity; formpyrrolidine the pyrrolidine theassembled pyrrolidine ring9 in a ring with diastereoselective with the ring desiredthe manner. desiredwith DIAD, stereochemistry the diisopropyl stereochemistry desiredp stereochemistry-thiocresol and and ii. Au(PPh72% and )Cl, o-NsO steps) + 17, PPh3, 3 o 86% o EDC·HCl, Et3N Me O HN S attempts to prepare seemingly simple cis-boronateated/stannane in theazodicarboxylate; disrotatoryMe TBDPSOTf, tert-butyldiphenylsilyl cyclization trifluoromethanesulfonate. process. The conrotatoryization/electrocyclization pathway cascadeMe with the intermediacyMe 70% (two of steps,cis 29%- N construction of theMe tetrasubstituted arene, (3) an intramolecularO Michael Me additionO to form the pyrrolidine ring with theexamined desiredo stereochemistryii. 6 p14-electrocyclization, HOBt, and(two 2,6-lutidine, promoted −78 byC a Lewis acid2,6-lutidine, to take −78 C (4) a gold(I)-catalysed 6-exo-dig cyclization to prepareOTBS the bridged 6,6- (4) a(4) gold((4) a gold(I)-catalysed a gold(I)-catalysedI)-catalysed 6-exo 6--digexo-dig cyclization 6-exo cyclization-dig to cyclization prepare to prepareDIAD,O the17 to the0 bridged C prepare bridged 6,6- 6,6- the bridged16 AgOTf, 6,6-O MeOHO 15 O of 15 recovered) O O EDC·HCl, Et N Osteps) ii. Au(PPh )Cl, OH O ii. Au(PPh )Cl, N O o-Ns 26HN SCO2NATUREMe CHEMISTRY | VOL+ 5 | AUGUST 2013 | www.nature.com/naturechemistry(4)HN ais gold(S expectedI)-catalysedN to+ 6- developexo-dig cyclization less steric26 26 to prepare congestion17, 681PPh theO3, bridged and,3 6,6- encouragingly,17 we, 3PPh3, 3 OO O additionbicyclic to form building the block pyrrolidine (13). Inspired ring with by the Dixon’s desired workN stereochemistry,intermediate and9 is OH bicyclicbicyclic building building blockN block (13). ( Inspired13).oCO-Ns Inspired2Me byO Dixon’s byMe Dixon’s work work,intermediateadvantage,intermediate269 ofis9 theis carbonylMe functionality conjugated to the triene o-Ns bicyclicMe building block (13). InspiredMe by Dixon’s workDIAD,,intermediate 0 oC 9 is AgOTf,DIAD, MeOH 0 oC Me AgOTf, MeOH O bicyclic building block (13). InspiredFigure byMe Dixon’s 3 |86%Retrosynthetic workO26,intermediate analysisMe of daphenylline.70%9 is (two steps,The 29% key strategiesN 16 i. TBDPSOTf, 15 20 I 17 O + 16 deviseddevised onO the2N onoccasionallyO basis the basis of the of15O common the common detectedN structural structural a motif trace motif ofsystem amount daphenylline-related of daphenylline-related of of12 cyclization.N Unfortunately, product a variety under of Lewis acids, such as (4) adevised gold( )-catalysed on the basisCO 6-2 ofexoMe the-dig common cyclization structural to prepare motif the of daphenylline-related bridged 6,6- OH devised on the basis of the commonMeOH O structural motif of daphenylline-relatedof 15 recovered) MeMe O O o N O o-NsOH include (1) a radical cyclizationN foro-Ns the assemblyO of the seven-memberedo-Ns Me2,6-lutidine,O −78 C 26O OTBS O2N Daphniphyllumdevised onalkaloids. theO basis The of 6 thep-electrocyclization/aromatization common structural motif of86% strategy, daphenylline-related 86% O Me i. TBDPSOTf, Me Me Daphniphyllum photoirradiationalkaloids. The 6p-electrocyclization/aromatizationI conditions indiethylaluminium theMe process strategy, ofo-Ns triene chloride, isomerization.OMe boron70%Me (two steps, trifluoride 29% diethylN Me Me etherate,70% (twoO steps, 29% Me N i. K2CO3, bicyclicDaphniphyllum building blockalkaloids. (13). Inspired The 6p-electrocyclization/aromatization by Dixon’s work ,intermediate strategy,9 is MeDaphniphyllum alkaloids. The 6p-electrocyclization/aromatization strategy,Me o o NN o O Me 17 16 17 + 15 16 ring, (2) a sequence of 6op-electrocyclization/aromatization15 N for17 the, PPh , K2KCO2CO3,2,6-lutidine, 3100, 100ii. CAu(PPhC −378)Cl, C p-thiocresolN 72% FigureHO 3 |2RetrosyntheticC 12 CO2Me analysis of daphenylline.NHOwhichHNwhich wasDaphniphyllumS applied wasThe applied key to strategies our toalkaloids. previous our11 previous synthesisThe synthesis 6p16-electrocyclization/aromatization of tubingensin of tubingensinK2 A,CO plays3 A,, 100 plays a C a15 Me strategy, of 15 recovered)20 3 35O of 15 recovered) o Me SnCl and copper(I) trifluoromethanesulfonateON COCO,MeN2Me failedo-Ns to effect COK2MeCO3, o-100Ns CN devisedwhich on was the appliedbasis of to the our common previous structural synthesis motifof tubingensin of daphenylline-related A, plays a whichUnder was applied optimized to our previous conditionsMe construction synthesisOH (500 of4 the tubingensin W tetrasubstituted Hg lamp, A, plays arene, 15 minutes), a (3) anOH intramoleculartransDIAD, Michael- 0 oC 2 AgOTf, MeOHK COCO22Me, 100 oC ii.O 14, HOBt, (two O OTBS + Me COO 2Me CO2Meo-Ns 17, PPh , N 6 6 o-Ns ii. Au(PPhOCOO3)Cl,Me 2 3 include (1) a radical cyclization forHN theS assemblydeterminingdeterminingwhichO of the role seven-memberedwas in role the applied in entire the entire to daphenylline our daphenylline previous plan. plan.synthesiso-Ns,o-Ns, 2-nitrobenzenesulfonyl. 2-nitrobenzenesulfonyl. of tubingensin A, plays a 3 N 86% 2 EDC·HCl, EtCO3N 2Mesteps) N 6 additionthe to form desired the pyrrolidine cyclization,MeN O ring with the and desired desilylation stereochemistry followed and by decomposition86% CO2Me CO2Me Daphniphyllumdeterminingalkaloids. role93 in the The entire 6p-electrocyclization/aromatization daphenylline plan. o-Ns,14 2-nitrobenzenesulfonyl. strategy, 1 triene 23 convertedMeOH cleanly16 into pentacyclic compound15 Me16 i.24 K2CO(as3, o a 1520 AgOTf,6 MeOH 20 O Figure 3 | Retrosynthetic analysisFigure of daphenylline.determining 3 | RetrosyntheticThe role key in strategies analysisthe entire of daphenylline daphenylline. plan.The86% keyo-Ns, strategies 2-nitrobenzenesulfonyl.o-NsN DIAD, 0 C N6 86% O ring, (2) a sequence of 6p-electrocyclization/aromatizationdetermining for role the in the entire daphenylline(4) a gold( plan.I)-catalysedK oCO-Ns,, 100 2-nitrobenzenesulfonyl. 6- exooC-dig cyclization to preparep-thiocresol the bridged86% 6,6-72% N Me O Au-catalysed gradually2 3N occurred under theseMe conditions.O O The failureOTBSOTBS Me of70% these (two steps, 29%86% N which was applied to our previous synthesis of tubingensininclude (1) a A, radical plays cyclization a forinclude the assembly17 (1) asingle radical of the diastereomer cyclization seven-memberedOH for16 the in assembly 71% yield) of the seven-membered through15 a photoinducedii.26 14 isomer-, HOBt, (two O Amide formation construction of the tetrasubstituted arene, (3) an intramolecular Michael O bicyclicCO2MeOTBS building block (13o-Ns). Inspired by Dixon’sCO2 workMe ,intermediate9 is of 15 recovered)OTBS in tetrahydrofuranalkyne (THF) cyclization2 at 278 8C afforded the undesired C6 ORTEP of 9aEDC·HCl, (desilylated Et 9)N i. steps)K2CO9 3, O i. 21KOTBS2COOTBS3, o-Ns determining role in the entire daphenylline plan.additionoring,-Ns, to (2) form2-nitrobenzenesulfonyl. a sequence the pyrrolidineMichael of addition 6p ring-electrocyclization/aromatization within thering, tetrahydrofuran desired (2) a sequencestereochemistry (THF) of 6p for and-electrocyclization/aromatization at the 78N 8C6 affordedefforts the undesired may for the be attributableC6O ORTEP of to OH9aMe the(desilylated formidable86% 9) 3 9 steric hindranceN Me O70% gener- (two steps,OTBS21 29% N Me 2 ization/electrocyclizationdevised cascade on the basis86% with of the the common intermediacy structuralOTBS motif of ofdaphenylline-relatedcis- p-thiocresol 72%o-Ns p-thiocresolMe O 72% in tetrahydrofuran (THF) at 78 8C afforded the undesired17 C6epimerepimerORTEP as the of as 9a the major (desilylated major product16 9) product ( 3:19 ( 3:1 diastereomeric2 diastereomeric ratio15 ratio (d.r.)). (d.r.)).21 of 15 recovered) OTBS OTBS constructionI Me O of the tetrasubstitutedconstruction arene,inin (3) tetrahydrofuran tetrahydrofuran an of intramolecular the tetrasubstituted (THF) Michael(THF) arene, at at 2 (3)7878 anated8C intramolecularC afforded afforded in the disrotatory the Michael the undesired undesired cyclization C6 C6 process.ORTEP ofii. The 149a, HOBt,(desilylated conrotatory (two9) pathway9 ii. 14, HOBt, (two 21 (4) a gold( )-catalysed 6-exo-dig cyclization to prepare the bridged 6,6- ! Daphniphyllum8 alkaloids. The 6p-electrocyclization/aromatizationOH 16 ORTEP of strategy, 9a (desilylatedN 9) 15 9 o-20Ns21 N Me epimer as the major product ( 3:1 diastereomeric ratio (d.r.)).Figure 3In | Retrosynthetic contrast,OTBS Dixon analysis and co-workers of! daphenylline. observedThe the key desired strategies stereo- EDC·HCl,O Et N steps) EDC·HCl, Et N steps)K CO , 100 oC bicyclicaddition building to block form ( the13). pyrrolidine Inspired by ring Dixon’sInaddition contrast,withepimer work the to26 desired form,intermediate Dixon as the stereochemistry the and pyrrolidine major9 co-workersis ring product and withO observed thewhichO ( desiredOTBS was3:1is the appliedexpected stereochemistrydiastereomeric desired to our stereo- to previous develop and synthesis ratio less (d.r.)). of steric tubingensin congestion A, plays a and, encouragingly,3 o-Ns we 3 2 3 ! O epimer as the major product ( 3:1 diastereomeric ratio (d.r.)). CO2Me CO2Me O include (1)chemistry a radical atcyclization C6 when for the synthesizing assembly! of the the 5,6-bicyclic seven-membered system O OSiR2R' OSiR2R' In contrast, Dixon and co-workers observed the desired stereo-chemistry at C6 when synthesizingdetermining the! 5,6-bicyclic role in the entire system daphenylline16 plan.OTBSoO-Ns, 2-nitrobenzenesulfonyl. 15 OSiR2R' NOSiR6 2R' 20 O in tetrahydrofuran (THF) at 278 8Cdevised afforded(4) on a gold( the the basisI)-catalysed undesired ofFigure the common 6-exo 3 | C6-digRetrosynthetic structural cyclization(4)ORTEP a motifIn gold(In contrast,to contrast,of ofI)-catalysed prepareanalysis9a daphenylline-related (desilylated the Dixon of+ 6- bridgedexo daphenylline. 9)-dig and and 6,6- cyclization co-workers co-workers9 The to keyprepareoccasionally observed strategies observed the bridged the the detectedMe 6,6- desired desiredO 21 a stereo- trace stereo- amount of cyclization product under i. K2CO3, 86% CO2Me through an intramolecular Michael addition underi. TBDPSOTf, KHMDS O O chemistry atO C62N when synthesizing theN 5,6-bicyclic systemring,through (2) a sequence an intramolecularO of26 6p-electrocyclization/aromatization Michael addition26 underOSiR2 forR' KHMDS the OSiR2R' R R'SiOTf, base p-thiocresol 72% Daphniphyllumbicyclic buildingalkaloids. block The ( 613p-electrocyclization/aromatization). Inspiredbicyclic by Dixon’schemistry building work block,intermediate at strategy, (2613 C6). Inspired when9 is by Dixon’ssynthesizing work ,intermediate the 5,6-bicyclic9 is o system R R'SiOTf,2 MeO base OSiR R' OSiR R' epimer as the major product ( 3:1 diastereomeric ratioinclude (d.r.)). (1) a radicalconditions cyclizationchemistry (Fig. for26 at 2) the C6. The assembly when structure of synthesizing the of 9 seven-memberedwasphotoirradiation confirmed the2,6-lutidine, 5,6-bicyclic by X-ray conditions −78 C system in theo processN2 O of triene isomerization. OSiRO 2R' 2OTBS OSiR2R' 2 through anO intramolecular Michael addition under KHMDSconstructionconditions of the (Fig. tetrasubstituted 2) . The structure arene, of (3)9Mewas an intramolecularO confirmed by Michael X-ray K2CO3, 100 C ii. 14, HOBt, (two ! whichdevised was applied on the to basis our previous of the common synthesisdevised structural of tubingensinHO on2C the motifO basis A,CO of plays2Me of daphenylline-related the a commonNH structural motif of daphenylline-related2 i. K2CO3, OTBS In contrast, Dixon and co-workers observed the desired stereo- crystallographicthroughthrough an analysis intramolecularR2R'SiOTf, of its desilylatedbase Michaelin Michael tetrahydrofuran derivative addition addition (Fig. (THF) under 4; under atTCO2Me78 KHMDS KHMDSMe8C affordedO the undesiredO CO2Me C6 MeORTEPO of 9a (desilylated 9) EDC·HCl,9 Et N steps) 21 26 + O ring, (2) a sequencecrystallographic of 6p-electrocyclization/aromatization analysis of17, itsPPh desilylated, derivativeUnder for the (Fig. optimizedii. Au(PPh 4; T 3)Cl, conditionsm (500MeMe W Hg lamp, 15 minutes), Metrans- p-thiocresol 3 72% conditionsHN (Fig.S 2) . The structuredetermining of 9 was role confirmed in theOTBS entire by daphenylline X-rayaddition plan.Me too form-Ns, 2-nitrobenzenesulfonyl. the pyrrolidine ring2626 with3 theepimer desired as stereochemistry the majorN product6 m and ( 3:1 diastereomericO ratio (d.r.)). R2R'SiOTf,R2R'SiOTf, baseMe base DaphniphyllumMe 93alkaloids. The 6p-electrocyclization/aromatizationDaphniphyllum144–147conditionsconditions8C,14alkaloids. ethyl (Fig. acetate The 2) 2) 6p. strategy,. (EtOAc)-electrocyclization/aromatization The Theo1 structure structure/n-hexane of of 1:1).99waswas AllAgOTf, confirmed the confirmed strategy, MeOH above by by X-ray X-ray86% Me Me chemistry at C6 when synthesizing the 5,6-bicyclicconstruction system of144–147 the tetrasubstituted8C, ethylO acetate arene,DIAD, (EtOAc) (3) 0 C an/n intramolecular-hexanetriene 1:1).OSiR All Michael232 theR' converted above ! cleanlyOSiR2R'N intoN pentacyclico compoundN NN 24 (as a ii.oN 14N , HOBt,N (two In contrast, Dixon and co-workers observedo- theNs K desired2CO3, 100 stereo- C o-Ns K2CO3, 100 C o-Ns crystallographicO analysis of its desilylatedwhich derivative was applied to(Fig. our previous 4;(4)TNm a gold(which synthesistransformationsI)-catalysed was of applied tubingensin 6- toMe wereexo our-dig A, previous performed plays cyclization a synthesis readily to prepare of on tubingensin a multigram the bridgedMe A, plays scale 6,6- awith o-Ns o-Ns EDC·HCl,o-Ns Et N steps) through an intramolecular MichaelOH addition under KHMDS transformationscrystallographiccrystallographic were performed analysis analysis readily of of its its on desilylated adesilylated multigramsingle diastereomer derivative scale derivative withO (Fig. (Fig. inOTBS 71% 4; 4;TCOm yield)T2mMe through a photoinducedMe Me COCO2Me2Me isomer- Me 3MeCO2Me O Amide formation addition to formo-Ns the pyrrolidine ring with the desiredchemistry stereochemistry26 at C6 andwhen synthesizing the15 5,6-bicyclic system 18O 19 OSiR2R' OSiR2R' 144–147 8C, ethyl acetate (EtOAc)/n-hexanedetermining 1:1). role All in the the entire abovebicyclic daphenyllinedetermining buildingconsistent plan. block roleoN efficiency.-Ns, in (13 2-nitrobenzenesulfonyl. the). entireInspiredR286% daphenyllineR'SiOTf, by Dixon’s base plan. worko-Ns,,intermediateN 2-nitrobenzenesulfonyl.9 is N 6 15 Me OTBS O N 6 18 19 O 26 in tetrahydrofuran (THF) at 278consistent8C afforded144–147144–147 efficiency. the88C, undesiredMe ethyl acetate acetate C6 ORTEP (EtOAc) (EtOAc) of 9a (/desilylated/nn-hexane-hexaneMe 9) 70% 1:1). 1:1).(two9 Allsteps, All the 29% the above aboveN 86%21 N N 86% N N N N conditions (Fig.17 2) . The structure of169 was confirmed(4) by a gold( X-rayI15)-catalysed 6-exo-digo-Ns cyclization to preparethrough theization bridged ano- intramolecular/Nselectrocyclization 6,6- Michaelo-Ns cascadeaddition under with KHMDS the intermediacy of cis- transformations were performed readily on a multigram scale withdevised on theWith basis the oftricyclic the common intermediate structural9 in hand, motif we of daphenylline-related focusedof 2615 our recovered) atten- o-Nso-Ns R2R'SiOTf, baseo-Ns o-Ns o-Ns o-Ns epimer as the major product ( 3:1With diastereomerictransformationstransformations the tricyclic15 ratio intermediate were were (d.r.)). performed performed9 in hand,conditions readily readily weN focused18 (Fig. on on a 2) aour multigram multigram. atten- The structure scale19 scale of with9 withwas confirmedo-Ns by X-ray Me O O crystallographic analysis of its desilylated derivative (Fig. 4; Tm! tion on theOH constructionMe of the challenging26 tetrasubstitutedMe arene FigureO 4 | ConstructionOTBS of15 the bridgedO tricycle 9. Agold(OTBS I)-catalysed18 alkyne 19 consistent efficiency. In contrast, Dixonbicyclic and co-workers buildingDaphniphyllumtion blockobserved on the (alkaloids.13 construction). the Inspired desired The by 6 stereo-p of Dixon’s-electrocyclization/aromatization the challenging workcrystallographic,intermediate tetrasubstitutedo-Ns analysis9 is strategy, arene of itsFigure desilylated 4 | Construction derivative of (Fig. the 4;15 bridgedT tricycle 9. Agold(I)-catalysed alkyne18 Me 19 consistentconsistent efficiency. O m OTBS Me o NOTBS Me 144–147 8C, ethyl acetate (EtOAc)/n-hexanein tetrahydrofuran 1:1). All the (THF) above at in2O78motif, tetrahydrofuran8C as afforded shownN in the (THF) Fig. undesired 5a. atcis-Triene278 C68C12 affordedwasORTEP considered ofN 9a the (desilylated undesired a suitable 9) C6cyclizationN9 ORTEP was of exploited9a (desilylated to construct 9) the219 bridged 6,6-bicycleK220CO.Silylenol3, 100 C 21 With the tricyclic intermediate 9chemistryin hand, we at focused C6 when our synthesizing atten- the 5,6-bicyclic16 system 144–147O 8C, ethyl acetate (EtOAc)cyclization/n-hexaneOSiR was2R' exploited 1:1). All to theOSiR construct above2R' the bridged 6,6-bicycle 20.Silylenol Figure 3 | Retrosynthetic analysis of daphenylline. Thedevised key strategies onwhich themotif, basis was asapplied of shownWith the to common intheo- ourNs Fig. tricyclic previous 5a. structuralcis-Triene intermediatesynthesis motif12 ofwas of tubingensin daphenylline-related915 consideredin hand,o- A,Ns we a plays suitable focused a our atten-o-Ns 20 i. TBDPSOTf, Me O CON Me N N epimer asO2 theN major productepimer (precursor3:1With as diastereomeric the to the explore major tricyclic product the ratio feasibility intermediate (d.r.)). ( 3:1 of diastereomeric the9 6inp-electrocyclization hand, we ratio focused (d.r.))./ ourether atten-18,thesubstrateforthisreaction,waspreparedfromenone2o-Ns 15 (inset); o-CONs 2Me o-Ns transformations were performed readilythrough on a multigram an intramolecular scale with MichaelFigure addition 4 | Construction under KHMDS of the bridgedtransformations tricycle 9. Agold( wereI performed)-catalysed readily alkyne on a multigram scale with 6 tion on the construction of the challenging tetrasubstituted arenedeterminingprecursor! role to in explore the entire the daphenylline feasibility! of plan. theo-Ns, 6p-electrocyclization 2-nitrobenzenesulfonyl./ ether 18,thesubstrateforthisreaction,waspreparedfromenoneo N 15 (inset); O I include (1) a radical cyclization for the assemblyO of theDaphniphyllum seven-memberedalkaloids.tion The on15 6 thep-electrocyclization/aromatization construction of the challenging18 strategy,R tetrasubstituted2R'SiOTf, base 19 arene Figure2,6-lutidine, 4 | Construction −78 C of the15 bridged tricycle86%9. Agold()-catalysed18I Me alkyne 19 In contrast, Dixon26 and co-workersInaromatization contrast,tion observed on Dixon the strategy. the construction and desired However, co-workers stereo- of well-recognized theconsistent observed challengingO efficiency. the approaches, desired tetrasubstituted such stereo-2,6-lutidine areneO as a baseFigure was 4 found | Construction to suppress the of the formation bridged of the tricycle undesiredo 9. Agold(N )-catalysed alkyne consistentmotif, as efficiency. shown in Fig. 5a. cis-Trieneconditions12 was considered (Fig. 2) . The a suitable structurearomatization ofcyclization9 was confirmed strategy. was exploited byHowever, X-ray to construct well-recognized the bridged approaches, 6,6-bicycle such20.Silylenoli.2,6-lutidine K2CO3, as a base wasii. Au(PPh found)Cl, to suppress the formationK2O ofCO the3, 100 undesired C HN S which was+ applied tomotif, our previous as shown synthesis in41 Fig. of 5a. tubingensincis-Triene A,12 playswas considered a 17, PPh3, a suitableregioisomericcyclization silyl enol ether was193 .AnintramolecularMichaeladdition exploited to construct the bridged 6,6-bicycle 20.Silylenol ring, (2) a sequence of 6p-electrocyclization/aromatizationchemistry at C6 for when the synthesizingchemistryasmotif, partial at asthe hydrogenation shown C6 5,6-bicyclic when41 in Fig.of synthesizing systemthe 5a. correspondingcisMe-TrieneWith the the12 5,6-bicyclic tricyclic ynediene,Owas considered intermediate proved systemp-thiocresol a9 in suitable hand, we focusedO72%cyclizationOSiR 2R' our atten- was exploitedOSiRCO2 toR'Me constructOSiR2 theR' bridged 6,6-bicycleOSiR2R' 20.Silylenol Withprecursor the tricyclic to explore intermediate the feasibility9 incrystallographic hand, of the we 6p focused-electrocyclization analysis our of atten- its desilylated/as partialetherMe derivative18 hydrogenation,thesubstrateforthisreaction,waspreparedfromenone (Fig. 4;ofT them correspondingMe ynediene, provedo 15regioisomeric(inset); Me silyl enolAgOTf, ether 19MeOH.AnintramolecularMichaeladditionO 2 OTBS CO2Me precursor to explore the feasibility of the 6p-electrocyclizationDIAD, 0 C assembled/ 9 inether a diastereoselective18,thesubstrateforthisreaction,waspreparedfromenone manner.Figure6 DIAD, 4 | Construction diisopropyl of the bridgedO tricycle 9. Agold(15 (inset);I)-catalysed alkyne construction of the tetrasubstituted arene,144–147through (3)C, anO ethyl intramolecular an intramolecular acetatedetermining (EtOAc) Michael role/through Michaeln in-hexaneto theprecursor be entire unsatisfactory an addition1:1). intramolecular daphenylline All to the explore under because above plan. KHMDS theN Michael ofo feasibility-Ns, thetion 2-nitrobenzenesulfonyl. poor addition on positionalthe of construction the under 6p selectivity;-electrocyclization KHMDS of theii. challenging 14, HOBt, / tetrasubstituted(twoether 18,thesubstrateforthisreaction,waspreparedfromenone areneN OTBS 15 (inset); 8 in tetrahydrofurantoOHFigure be2,6-lutidine unsatisfactory 4 | Construction as (THF)a base because was at of found2 the of78 the bridged toC poorsuppress afforded tricycleN positional the the9 formation. Agold( selectivity;undesired ofI)-catalysed theR R'SiOTf, undesired C6assembled base alkyneN 9 in a diastereoselectiveN R R'SiOTf, manner. base DIAD, diisopropyl86% tionaromatization on the construction strategy. of However, the challenging well-recognized tetrasubstituted approaches,26 arene such attemptsaromatization to prepare26 strategy. seemingly However,8o-Ns simplemotif, well-recognized aso-cisNs shown-boronate in Fig./stannane approaches, 5a. cis2 -TrieneEDC·HCl,azodicarboxylate; such12ORTEP waso-EtNsN considered 2,6-lutidineof 9asteps) TBDPSOTf, (desilylated a suitable aso-tert aNs9)2-butyldiphenylsilyl base wascyclization9 found was trifluoromethanesulfonate. to exploited suppress to the construct formation the bridged of21 the 6,6-bicycle undesired20.Silylenol transformationsconditions (Fig. were performed 2) . The structurereadilyconditions onaromatization of a multigram9 (Fig.was confirmed 2) scale strategy.. The with structure by However, X-ray of 9 well-recognizedwas confirmed86% by approaches, X-rayazodicarboxylate; such3 TBDPSOTf,2,6-lutidinetert-butyldiphenylsilyl as a base was found trifluoromethanesulfonate. toMe suppress the formation of the undesired addition to form the pyrrolidine41 ring with the desired stereochemistryepimerattempts andregioisomeric as the to major prepare silyl product enol seemingly ether (1941.AnintramolecularMichaeladdition3:1 simple diastereomericcis-boronateMe/stannane ratio (d.r.)). Me 70% (two steps, 29%ether 18,thesubstrateforthisreaction,waspreparedfromenoneN 15 (inset); motif,as aspartial shown hydrogenation in Fig. 5a. cis-Trieneof the corresponding12 was considered17 ynediene, a suitable proved 16cyclizationas partial was hydrogenation exploited to construct1541 of theprecursor the15 corresponding bridged to explore 6,6-bicycle the ynediene, feasibility20.Silylenol proved of18 the 6pregioisomeric-electrocyclization19 silyl/ enolO ether 19.AnintramolecularMichaeladditionOTBS consistentcrystallographic efficiency. analysis of itscrystallographic desilylatedas partial derivative hydrogenation analysis (Fig. of its! 4; desilylatedTofm the corresponding derivativeMe (Fig. ynediene, 4; Tm proved regioisomericof 15Me Merecovered) silyl enol ether 19.AnintramolecularMichaeladditionMe (4) a gold(I)-catalysed 6-exo-dig cyclization to prepare the bridgedIn 6,6- contrast,assembledNATURE Dixon CHEMISTRY9 in and a| diastereoselective VOL co-workers 5 | AUGUST 2013 observed | manner.www.nature.com/naturechemistryaromatization DIAD,OH the strategy. diisopropyl desired However, stereo- well-recognizedN approaches, such 2,6-lutidine aso-Ns a base was found to suppress681 the formation of the undesired precursorto be tounsatisfactory explore the because feasibility of theof theWith poor 6 thep positional-electrocyclization tricyclic intermediate selectivity;/ 9NATUREinether hand, CHEMISTRYto18 we be,thesubstrateforthisreaction,waspreparedfromenone focused unsatisfactory| VOL 5 our | AUGUST atten-because 2013 | www.nature.com/naturechemistry of the poor positional selectivity;15 (inset); Nsassembled 9 in a diastereoselective manner. DIAD, diisopropyl681 OTBS 144–147 8C,26 ethylin tetrahydrofuran acetate (EtOAc)144–147to/n (THF)-hexane be8C, unsatisfactory ethyl at 1:1). acetate278 All8C the (EtOAc) because afforded above/asn-hexane of partial the the undesiredhydrogenation poor 1:1).N All positional the C641 of above the selectivity;ORTEP corresponding of 9ao- (desilylatedN ynediene,assembledNO 9) proved9 in a9 diastereoselectiveregioisomericN silyl enolN manner. ether 19.AnintramolecularMichaeladdition DIAD, diisopropylN21 attemptsbicyclic building to prepare block (13 seemingly). Inspired by simple Dixon’scis work-boronate,intermediate/stannanechemistry9 is2,6-lutidineazodicarboxylate; at C6 as a when base TBDPSOTf, was synthesizing foundtert-butyldiphenylsilyl to suppress the 5,6-bicyclicthe formation trifluoromethanesulfonate.o-Ns of system the undesired O o-Nso-Ns o-Ns o-OSiRNs 2R' o-NsOSiR2R' aromatization strategy. However, well-recognizedtionFiguretransformations on 3 the | Retrosynthetic construction approaches, were of analysis performed such the challenging oftransformations daphenylline. readilyattempts tetrasubstituted on aThe multigram to were key prepare performed strategiesarene scale seeminglyFigure with readilyto 4 | beConstruction on simple unsatisfactory a16 multigramcis of the-boronate because bridgedscale with/ tricycleofstannane the9 poor. 15Agold( positionalazodicarboxylate;I)-catalysed selectivity; alkyne TBDPSOTf,assembledtert920-butyldiphenylsilylin a diastereoselective trifluoromethanesulfonate. manner. DIAD, diisopropyl 41 epimer as the majorattempts product to prepare ( 3:1 diastereomeric seemingly simple ratio15 cis (d.r.)).-boronate/stannane 15azodicarboxylate;18 TBDPSOTf,19 tert-butyldiphenylsilyl18 trifluoromethanesulfonate.19 as partialdevised hydrogenation on the basis of theof the common correspondingmotif, structuralincludeconsistent as (1) shown motif a ynediene, radical efficiency. in of Fig. daphenylline-related cyclization 5a. provedcis-Trienethrough for theconsistentregioisomeric12 assemblywas an considered intramolecular efficiency. of silyl the seven-membered enol a suitable! ether Michael19cyclization.AnintramolecularMichaeladditionattempts addition wasMe exploited toO under prepare to construct KHMDS seemingly the bridged simple 6,6-bicyclecis-boronate20.Silylenol/stannane azodicarboxylate; TBDPSOTf, tert-butyldiphenylsilyl trifluoromethanesulfonate. NATURE CHEMISTRY | VOL 5 | AUGUST 2013 | www.nature.com/naturechemistryIn contrast, Dixon and co-workers26 observed the desired stereo- 681 R2R'SiOTf, base Daphniphyllum alkaloids. The 6 -electrocyclization/aromatizationprecursor to explore the feasibilityconditions strategy, of theNATURE (Fig. 6p-electrocyclization CHEMISTRY 2) . The| structure VOL/ 5 | AUGUSTether of189 2013,thesubstrateforthisreaction,waspreparedfromenonewas | www.nature.com/naturechemistry confirmed by X-ray 15 (inset); i. K2CO3, 681 to be unsatisfactory because ofp thering, poor (2)With positional a sequence the tricyclic selectivity; of 6p intermediate-electrocyclization/aromatizationassembledWith9NATUREin the hand, CHEMISTRY9 tricyclicin we a diastereoselectivefocused intermediate| for VOL the our 5 | AUGUST atten-9 manner.in 2013 hand, | www.nature.com/naturechemistry DIAD, we focused diisopropyl our atten- N Me 681 chemistry at C6 when synthesizing2,6-lutidine theNATURE 5,6-bicyclic as CHEMISTRY a base was| VOL found system 5K | AUGUSTCO to suppress, 100 2013 oC | thewww.nature.com/naturechemistry formationO of the undesired p-thiocresol 72%OSiR R' OSiR R' 681 aromatization strategy. However,crystallographic well-recognized analysisapproaches, of such its desilylatedFigure derivative 4 | Construction (Fig.2 4;3 ofT the bridgedFigure tricycle 4 | Construction9. Agold(Me I)-catalysed of the bridged alkyne tricycle 9. Agold(2 MeI)-catalysed alkyne 2 attemptswhich to was prepare applied to seemingly our previous simple synthesisconstructiontioncis of on-boronate tubingensin the of the construction tetrasubstituted/stannane41 A, plays of a thetion arene,azodicarboxylate; challenging on (3) the an construction intramolecular tetrasubstituted TBDPSOTf, of Michael thetert arene-butyldiphenylsilyl challenging tetrasubstituted trifluoromethanesulfonate. arenem ii. 14, HOBt, (two regioisomeric silylCO enol2Me ether 19.AnintramolecularMichaeladditionCO2Me as partial hydrogenationthroughof the an corresponding intramolecular ynediene, Michael proved additioncyclization under was KHMDS exploited to constructcyclization the bridged was 6,6-bicycleexploited to20EDC·HCl, construct.Silylenol Et theN bridgedsteps) 6,6-bicycle 20.Silylenol determining role in the entire daphenyllineadditionmotif, plan. to aso form-Ns, shown the 2-nitrobenzenesulfonyl. pyrrolidinein Fig. 5a.144–147 ringcis-Trienemotif, with8C, the as12 ethyl shown desiredwas acetate considered in stereochemistry Fig. (EtOAc) 5a. acis suitable-Triene and/n-hexane12N was6 1:1). considered All the a suitable above O N R R'SiOTf, base 3 N N to be unsatisfactory because of the poor26 positional selectivity; assembled 9 in a diastereoselective86% manner. DIAD, diisopropyl o-Ns 2 o-Ns o-Ns NATURE CHEMISTRY | VOL 5 | AUGUST 2013 | www.nature.com/naturechemistryprecursor to exploreconditions thetransformations feasibility (Fig.precursor 2) of. Thethe to were explore 6 structurep-electrocyclization performed the feasibility of readily9 was/ of onconfirmedether the a multigram 618p,thesubstrateforthisreaction,waspreparedfromenone-electrocyclization by X-ray scale with/ ether68118,thesubstrateforthisreaction,waspreparedfromenone15 (inset); 15 (inset); attempts(4) a gold( toI)-catalysed prepare seemingly 6-exo-dig cyclization simple cis to-boronate prepare the/stannane bridged 6,6-azodicarboxylate; TBDPSOTf, tert-butyldiphenylsilyl trifluoromethanesulfonate.15 18 19 aromatization strategy.crystallographic However,consistentaromatization well-recognized analysis efficiency. ofstrategy.26 its approaches, desilylated However, such well-recognized derivative2,6-lutidineO (Fig. approaches, asOTBS a 4; baseT wasm such found to2,6-lutidine suppress the asMe formation a base was of found the undesired to suppress theO formationMe of the undesired bicyclic building block (13). Inspired41 by Dixon’s work ,intermediate41 9 is as partial hydrogenation ofas the partial corresponding hydrogenation ynediene,of proved the correspondingregioisomeric ynediene, silyl enol proved ether 19.AnintramolecularMichaeladditionregioisomericOTBS silyl enol ether 19.AnintramolecularMichaeladdition in tetrahydrofuran (THF) at 2NATUREdevised78 8C CHEMISTRY on afforded the basis| VOL144–147 the 5 of | AUGUST the undesired common8 2013C,With | ethylwww.nature.com/naturechemistry structural C6 the acetate tricyclicORTEP motif (EtOAc) ofintermediate of daphenylline-related9a (desilylated/n-hexane9 9)in hand, 1:1).9 we All focused the above our atten- 21N 681 N N to be unsatisfactory becauseto of be the unsatisfactory poor positional because selectivity; of the poorassembled positional9 in a diastereoselective selectivity; assembled manner.Me Oo- DIAD,Ns9 in a diisopropyl diastereoselective manner. DIAD,o- diisopropylNs o-Ns epimer as the major product ( 3:1 diastereomerictransformations ratiotion (d.r.)). on the were construction performed of readily the challenging on a multigram tetrasubstituted scale with arene Figure 4 | Construction of the bridged tricycle 9. Agold(I)-catalysed alkyne Daphniphyllumattempts toalkaloids. prepare The seemingly 6p-electrocyclization/aromatizationattempts simple tocis prepare-boronate seemingly/stannane strategy, simpleazodicarboxylate;cis-boronate TBDPSOTf,/stannanetert-butyldiphenylsilylazodicarboxylate;15 trifluoromethanesulfonate. TBDPSOTf, tert-butyldiphenylsilylN Me18 trifluoromethanesulfonate.19 ! cyclization was exploitedK CO , 100 to o constructC the bridged 6,6-bicycle 20.Silylenol In contrast, Dixon and co-workerswhich observed was applied theconsistent to our desired previousmotif, efficiency. stereo- synthesis as shown of tubingensin in Fig. 5a. A,cis plays-Triene a 12 was considered a suitable 2 3 CO2Me CO2Me precursor to explore the feasibility of the 6p-electrocyclization/ ether6 18,thesubstrateforthisreaction,waspreparedfromenoneO 15 (inset); chemistry at C6 when synthesizingdeterminingNATURE CHEMISTRY the role in 5,6-bicyclic the| VOLWith entire 5 | AUGUST daphenylline the system tricyclicNATURE 2013 | www.nature.com/naturechemistry plan.CHEMISTRY intermediateo-Ns,| 2-nitrobenzenesulfonyl. VOLO 5 |9 AUGUSTin hand, 2013 | wewww.nature.com/naturechemistry focused our atten-OSiR2R' N OSiR2R' 681 681 2,6-lutidine as a base86% was found to suppress the formation of the undesired tion onaromatization the construction strategy. of the However, challenging well-recognized tetrasubstituted approaches, arene suchFigure 4 | Construction of the bridged tricycle 9. Agold(I)-catalysed alkyne through an intramolecular Michael addition under KHMDS 41 conditions (Fig. 2)26. The structure of 9 was confirmedmotif, asas shown by partial X-ray in hydrogenation Fig. 5a. cis-Trieneof the12 correspondingwasR2R'SiOTf, considered base ynediene, a suitable provedcyclizationregioisomericO was exploitedOTBS silyl enol to ether construct19.AnintramolecularMichaeladdition the bridged 6,6-bicycle 20.Silylenol in tetrahydrofuran (THF)to at be2 unsatisfactory78 8C afforded because the undesired of the C6 poor positional selectivity; assembled 9 in a diastereoselective manner.OTBS DIAD, diisopropyl crystallographic analysis of its desilylated derivativeprecursor (Fig. to 4; exploreT the feasibility of the 6p-electrocyclizationORTEP of 9a (desilylated/ 9)ether 189,thesubstrateforthisreaction,waspreparedfromenone21 15 (inset); epimer as the major productattempts ( m3:1 to diastereomeric prepare seeminglyMe ratio (d.r.)). simple cis-boronate/stannaneMe azodicarboxylate; TBDPSOTf, tert-butyldiphenylsilyl trifluoromethanesulfonate. 2,6-lutidine as a base was found to suppress the formation of the undesired 144–147 8C, ethyl acetate (EtOAc)In contrast,/n-hexane Dixon 1:1).aromatization and All co-workers the above strategy.! observed However, theN desiredwell-recognized stereo- approaches, suchN N as partial hydrogenation41 of theo- correspondingNs ynediene, provedo-Ns regioisomeric silylo-Ns enol ether 19.AnintramolecularMichaeladdition transformations were performedchemistry readily on at a multigram C6 whenNATURE scale synthesizing with CHEMISTRY the| VOL 5,6-bicyclic 5 | AUGUST 2013 system | www.nature.com/naturechemistryO OSiR R' OSiR R' 681 15 18 19 2 2 consistent efficiency. through an intramolecularto be unsatisfactory Michael because addition of under the poor KHMDS positional selectivity; assembled 9 in a diastereoselective manner. DIAD, diisopropyl With the tricyclic intermediateconditions9 in hand, (Fig. weattempts 2) focused26. The our structure to atten- prepare of 9 seeminglywas confirmed simple by X-raycis-boronate/stannane azodicarboxylate;R2R'SiOTf, base TBDPSOTf, tert-butyldiphenylsilyl trifluoromethanesulfonate. tion on the construction of thecrystallographic challenging tetrasubstituted analysis of its arene desilylatedFigure derivative 4 | Construction (Fig. 4; ofT them bridged tricycle 9Me. Agold(I)-catalysed alkyne Me NATURE CHEMISTRY | VOL 5 | AUGUST 2013 | www.nature.com/naturechemistry 681 motif, as shown in Fig. 5a. cis-Triene144–14712 8wasC, ethyl considered acetate a (EtOAc) suitable/n-hexanecyclization 1:1). was All exploited the above to construct the bridgedN 6,6-bicycle 20.Silylenol N N precursor to explore the feasibilitytransformations of the 6p-electrocyclization were performed readily/ ether on a18 multigram,thesubstrateforthisreaction,waspreparedfromenone scale with o-Ns 15 (inset); o-Ns o-Ns 15 18 19 aromatization strategy. However,consistent well-recognized efficiency. approaches, such 2,6-lutidine as a base was found to suppress the formation of the undesired as partial hydrogenation41 of the correspondingWith the tricyclic ynediene, intermediate proved9 in hand,regioisomeric we focused silyl enol our atten- ether 19.AnintramolecularMichaeladdition to be unsatisfactory because oftion the on poor the construction positional selectivity; of the challengingassembled tetrasubstituted9 in a diastereoselective arene Figure manner. 4 | Construction DIAD, diisopropyl of the bridged tricycle 9. Agold(I)-catalysed alkyne motif, as shown in Fig. 5a. cis-Triene 12 was considered a suitable cyclization was exploited to construct the bridged 6,6-bicycle 20.Silylenol attempts to prepare seemingly simple cis-boronate/stannane azodicarboxylate; TBDPSOTf, tert-butyldiphenylsilyl trifluoromethanesulfonate. precursor to explore the feasibility of the 6p-electrocyclization/ ether 18,thesubstrateforthisreaction,waspreparedfromenone15 (inset); aromatization strategy. However, well-recognized approaches, such 2,6-lutidine as a base was found to suppress the formation of the undesired NATURE CHEMISTRY | VOL 5 | AUGUST 2013 | www.nature.com/naturechemistry 681 as partial hydrogenation41 of the corresponding ynediene, proved regioisomeric silyl enol ether 19.AnintramolecularMichaeladdition to be unsatisfactory because of the poor positional selectivity; assembled 9 in a diastereoselective manner. DIAD, diisopropyl attempts to prepare seemingly simple cis-boronate/stannane azodicarboxylate; TBDPSOTf, tert-butyldiphenylsilyl trifluoromethanesulfonate.

NATURE CHEMISTRY | VOL 5 | AUGUST 2013 | www.nature.com/naturechemistry 681 NATURE CHEMISTRY DOI: 10.1038/NCHEM.1694 ARTICLES

Me H Me H O reagents were also fruitless. To our delight, however, photo- Me 18 Me isomerization of the trans-triene afforded the desired cis-isomer. H H Indeed, treatment of ketone 9 with KHMDS and N-phenylbis(tri- N N fluoromethanesulfonimide) (PhNTf2) furnished the corresponding O Radical triflate, which further underwent Suzuki coupling with trans-boro- 8 Daphenylline 10 cyclization nate 22 to render triene 23 with good overall efficiency. Irradiation of 23 with a 125 W Hg lamp for five minutes provided 12 readily42. 6π-electrocyclization/ Surprisingly, this cis-triene was found to be reluctant to enter 43 Pd-catalysed coupling aromatization the 6p-electrocyclization pathway under thermal conditions ;

13 elevating the reaction temperature merely led to skeletal Me O Me O 44,45 1 decomposition. Inspired by Trauner’s seminal work ,we examined 6p-electrocyclization promoted by a Lewis acid to take CO2Me N N CO2Me advantage of the carbonyl functionality conjugated to the triene system of 12. Unfortunately, a variety of Lewis acids, such as O OTBS O I diethylaluminium chloride, boron triﬂuoride diethyl etherate, 12 11 35 SnCl4 and copper(I) triﬂuoromethanesulfonate , failed to effect the desired cyclization, and desilylation followed by decomposition Au-catalysed gradually occurred under these conditions. The failure of these alkyne cyclization Michael addition efforts may be attributable to the formidable steric hindrance gener- Me O ated in the disrotatory cyclization process. The conrotatory pathway OTBS O is expected to develop less steric congestion and, encouragingly, we

CO Me + occasionally detected a trace amount of cyclization product under N 2 Me photoirradiation conditions in the process of triene isomerization. HO2C CO2Me NH Under optimized conditions (500 W Hg lamp, 15 minutes), trans- ARTICLES NATURE CHEMISTRY DOI: 10.1038/NCHEM.1694 O OTBS Forward synthesis 9314 1 triene 23 converted cleanly into pentacyclic compound 24 (as a 46–48 Amide formation single diastereomer in 71% yield) through a photoinduced isomer- a O triene 12 . In a stepwise experiment, purified 12 was subjected to i. KHMDS, PhNTf 2, ization/electrocyclization cascade with the intermediacy of cis- −78 oC the same conditions to give 24 ( 50% overall yield from 23). The Me O ! ii. 22, Pd(PPh3)4, Me yield of 24 significantly decreased when O2 was not strictly excluded O o O K2CO3, 60 C (vide infra). This cyclohexadiene displayed unexpected reactivity in i. TBDPSOTf, CO2Me O2N CO Me the subsequent aromatization process; treatment with various oxi- N 2 O 2,6-lutidine, −78 oC 73% (two steps) N O O dants or dehydrogenation reagents, such as 2,3-dichloro-5,6- ii. Au(PPh )Cl, O Me + 17, PPh3, 3 HN S Me O OTBS Me O O dicyano-p-benzoquinone, 2-iodoxybenzoic acid, MnO , Me DIAD, 0 oC AgOTf, MeOH O OTBS 2 O 93B 2 OH N I2/MeOH, Pd/C and Pd(OCOCF3)2, failed to aromatize it, but o-Ns Me Me O Me 86% Me 70% (two steps, 29% N Me resulted in skeletal decomposition on prolonged reaction times. 17 16 15 of 15 recovered) 22 OH N o-Ns Finally, in the presence of 1,8-diazabicyclo[5.4.0]undec-7-ene o-Ns 49 hυ (Hg lamp, (DBU)/air at elevated temperature , 24 was converted into arene 16 Me Figure 3 | Retrosynthetic analysis of daphenylline. The key strategies 15 20 O 500 W), 0 oC 25 in 67% yield, taking advantage of the C8 proton acidity include (1) a radical cyclization for the assembly of the seven-membered induced by the carbonyl group conjugated to the cyclohexadiene i. K2CO3, ring, (2) a sequence of 6p-electrocyclization/aromatization for the CO2Me p-thiocresol 72% N system. Compound 25 retained high enantiopurity (96% e.e., construction of the tetrasubstituted arene, (3) an intramolecular Michael ii. 14, HOBt, (two measured by high-performance liquid chromatography using syn- EDC·HCl, Et3N steps) addition to form the pyrrolidine ring with the desired stereochemistry and 71% O OTBS thetic (+)-25 as a reference). (4) a gold(I)-catalysed 6-exo-dig cyclization to prepare the bridged 6,6- 12 Interestingly, a clean, but unexpected, transformation occurred O bicyclic building block (13). Inspired by Dixon’s work26,intermediate9 is to furnish hexacyclic lactone 26 when 23 was photoirradiated in devised on the basis of the common structural motif of daphenylline-related Me O the presence of air, as shown in Fig. 5b. Desilylation of 26 gave Daphniphyllum alkaloids. The 6p-electrocyclization/aromatization strategy, Me Me Me alcohol 27, the structure of which was determined by X-ray crystal- o N O O K2CO3, 100 C which was applied to our previous synthesis of tubingensin A, plays a 8 o lographic analysis of single crystals of (+)-27 (Tm 136–138 8C, CO2Me CO2Me DBU, air, 60 C determining role in the entire daphenylline plan. o-Ns, 2-nitrobenzenesulfonyl. N 6 O EtOAc/n-hexane 1:1). We postulate that cyclohexadiene 24 86% H H CO2Me N 67% N CO2Me reacted with singlet O2 to provide the Diels–Alder adduct 28, O OTBS which may further undergo Norrish type I fragmentation to O O OTBS in tetrahydrofuran (THF) at 278 C afforded the undesired C6 OTBS OTBS 8 ORTEP of 9a (desilylated 9) 9 21 24 25 generate the diradical species 29. The O–O bond could epimer as the major product ( 3:1 diastereomeric ratio (d.r.)). ! cleave homolytically and recombine with the diradical to In contrast, Dixon and co-workers observed the desired stereo- render 26. Thus, the stereochemical outcome of the conrotatory chemistry at C6 when synthesizing the 5,6-bicyclic system O OSiR2R' OSiR2R' 6p-electrocyclization was verified indirectly by the structural b O through an intramolecular Michael addition under KHMDS elucidation of 27. 26 R2R'SiOTf, base conditions (Fig. 2) . The structure of 9 was confirmed by X-ray Me O Me Having forged the pentacyclic scaffold of the natural product, we crystallographic analysis of its desilylated derivative (Fig. 4; Tm Me Lu, Z.; Li ,Y.; Deng, J.; Li, A. Me Nat. Chem. 2013, 5, 679. entered the final stage of the total synthesis, as shown in Fig. 6. hυ, 0 oC 144–147 8C, ethyl acetate (EtOAc)/n-hexane 1:1). All the above N N N CO Me H H According to our retrosynthetic analysis, a suitable precursor for o-Ns o-Ns o-Ns 2 N transformations were performed readily on a multigram scale with N CO2Me the 7-exo-trig cyclization needed to be prepared. Thus, ketone 25 15 18 19 consistent efficiency. O was converted into enone 30, in 81% overall yield, through a O OTBS OTBS 50 With the tricyclic intermediate 9 in hand, we focused our atten- 23 24 sequence of silyl enol ether formation and Saegusa–Ito oxidation . tion on the construction of the challenging tetrasubstituted arene Figure 4 | Construction of the bridged tricycle 9. Agold(I)-catalysed alkyne Desilylation followed by iodination furnished compound 11, which cyclization was exploited to construct the bridged 6,6-bicycle 20.Silylenol 1 set the stage for the intramolecular 1,4-addition. The radical motif, as shown in Fig. 5a. cis-Triene 12 was considered a suitable O2 precursor to explore the feasibility of the 6p-electrocyclization/ ether 18,thesubstrateforthisreaction,waspreparedfromenone15 (inset); cyclization reaction was initiated by 2,2′-azobisisobutyronitrile aromatization strategy. However, well-recognized approaches, such 2,6-lutidine as a base was found to suppress the formation of the undesired (AIBN) at 75 8C; tris(trimethylsilyl)silane ((TMS)3SiH) was found as partial hydrogenation41 of the corresponding ynediene, proved regioisomeric silyl enol ether 19.AnintramolecularMichaeladdition O O O O to be superior to tributyltin hydride as a hydride source. It was Me O Me O crucial to add CH Cl as cosolvent to obtain an excellent to be unsatisfactory because of the poor positional selectivity; assembled 9 in a diastereoselective manner. DIAD, diisopropyl Norrish type I 2 2 attempts to prepare seemingly simple cis-boronate/stannane azodicarboxylate; TBDPSOTf, tert-butyldiphenylsilyl trifluoromethanesulfonate. fragmentation and consistent yield, presumably because of the poor solubility of H H H H N N 11 in toluene. Under the above conditions, the primary radical CO2Me CO2Me NATURE CHEMISTRY | VOL 5 | AUGUST 2013 | www.nature.com/naturechemistry 681 attacked the enone moiety to afford compound 31 (98%) as a O O OTBS OTBS single diastereomer. The excellent facial selectivity is 29 28 attributable to the strong stereochemical bias of the substrate. With the hexacyclic core of daphenylline in hand, a series of O–O bond cleavage/ 71% recombination reductions was performed. Hydrogenation of the exocyclic C C bond with Crabtree’s catalyst provided the desired C18 diastereomer¼ Me predominantly (.30:1 d.r.), but reduction with Pd/C led to the O O opposite stereochemical output. This product was subjected to O Krapcho demethoxycarbonylation conditions (LiCl.H O, dimethyl H H 2 N CO2Me sulfoxide, 160 8C) employed by Dixon and co-workers in their synthesis of the tricyclic core of calyciphylline A-type O OR alkaloids (Fig. 2)26, in this case to give compound 10 as a single 26 R = TBS HF·py, 0 oC detectable diastereomer. The structure of 10 was unambiguously 27 R = H ORTEP of (±)-27 verified by X-ray crystallographic analysis (Fig. 6, Tm 165–167 8C, 92% EtOAc/n-hexane 1:1), which ensured the connectivity of Figure 5 | Assembly of the pentacyclic intermediate 25. a, Trans-triene 23 the hexacyclic scaffold and stereochemical output of the radical underwent a photoinduced C C bond isomerization/6p-electrocyclization cyclization, hydrogenation and decarboxylation reactions. ¼ cascade reaction to afford the compact pentacyclic product 24,which Finally, deoxygenation promoted by Pd/C under a H2 atmosphere, further aromatized to 25 under oxidative conditions. b, An unexpected followed by LiAlH4 reduction of the lactam moiety, rendered photoinduced cascade reaction occurred when 23 was photoirradiated in the synthetic daphenylline (8) in 66% yield over two steps. The presence of air. A postulated mechanism involved a Norrish type I daphenylline purified with silica gel displayed identical spectral fragmentation followed by O–O bond homolysis and radical recombination. and physical properties to those of an authentic sample provided HF.py, hydrogen fluoride pyridine. by Hao and co-workers34.

682 NATURE CHEMISTRY | VOL 5 | AUGUST 2013 | www.nature.com/naturechemistry ARTICLES NATURE CHEMISTRY DOI: 10.1038/NCHEM.1694

46–48 a O triene 12 . In a stepwise experiment, puriﬁed 12 was subjected to i. KHMDS, PhNTf 2, −78 oC the same conditions to give 24 ( 50% overall yield from 23). The Me O ! ii. 22, Pd(PPh3)4, Me yield of 24 signiﬁcantly decreased when O2 was not strictly excluded o K2CO3, 60 C (vide infra). This cyclohexadiene displayed unexpected reactivity in CO Me 2 CO Me the subsequent aromatization process; treatment with various oxi- N 2 73% (two steps) N dants or dehydrogenation reagents, such as 2,3-dichloro-5,6- Me O OTBS Me O O dicyano-p-benzoquinone, 2-iodoxybenzoic acid, MnO , O OTBS 2 B 932 I2/MeOH, Pd/C and Pd(OCOCF3)2, failed to aromatize it, but Me O Me resulted in skeletal decomposition on prolonged reaction times. 22 Finally, in the presence of 1,8-diazabicyclo[5.4.0]undec-7-ene 49 h (Hg lamp, (DBU)/air at elevated temperature , 24 was converted into arene Me υ O 500 W), 0 oC 25 in 67% yield, taking advantage of the C8 proton acidity induced by the carbonyl group conjugated to the cyclohexadiene CO2Me N system. Compound 25 retained high enantiopurity (96% e.e., measured by high-performance liquid chromatography using syn- 71% O OTBS thetic (+)-25 as a reference). 12 Interestingly, a clean, but unexpected, transformation occurred to furnish hexacyclic lactone 26 when 23 was photoirradiated in the presence of air, as shown in Fig. 5b. Desilylation of 26 gave

Me O Me O alcohol 27, the structure of which was determined by X-ray crystal- 8 DBU, air, 60 oC lographic analysis of single crystals of (+)-27 (Tm 136–138 8C, EtOAc/n-hexane 1:1). We postulate that cyclohexadiene 24 H H CO2Me N 67% N CO2Me reacted with singlet O2 to provide the Diels–Alder adduct 28, which may further undergo Norrish type I fragmentation to O O OTBS OTBS 24 25 generate the diradical species 29. The O–O bond could cleave homolytically and recombine with the diradical to NATURErender 26. CHEMISTRY Thus, the stereochemicalDOI: 10.1038/NCHEM.1694 outcome of the conrotatory ARTICLES Final game 6p-electrocyclization was verified indirectly by the structural b O elucidation of 27. i. TMSOTf, Et 3N, 7. Kobayashi, J. & Kubota, T. The Daphniphyllum alkaloids. Nat. Prod. Rep. Me O Me O o Me O Me Having forged the pentacyclic − 78 C scaffold of the natural product, we 26, 936–962 (2009). entered the final stageii. of Pd(OAc) the total2 synthesis, as shown in Fig. 6. 8. Piettre, S. & Heathcock, C. H. Biomimetic total synthesis of proto- o hυ, 0 C CO Me CO Me daphniphylline. Science 248, 1532–1534 (1990). H H AccordingN to2 our retrosynthetic analysis, a suitableN precursor2 for CO2Me N 81% (two steps) 9. Heathcock, C. H. The enchanting alkaloids of yuzuriha. Angew. Chem. Int. Ed. N CO2Me the 7-exo-trig cyclization needed to be prepared. Thus, ketone 25 31, 665–681 (1992). was convertedO OTBS into enone 30, in 81% overallO yield,OTBS through a O 10. Kobayashi, J., Inaba, Y., Shiro, M., Yoshida, N. & Morita, H. Daphnicyclidins O OTBS OTBS 25 30 50 23 24 sequence of silyl enol ether formation and Saegusa–Ito oxidation . A–H, novel hexa- or pentacyclic alkaloids from two species of Daphniphyllum. Desilylation followed by iodination furnished compound 11, which J. Am. Chem. Soc. 123, 11402–11408 (2001). o i. HF·py, 0 C 93% 1 set the stage for the intramolecular 1,4-addition. The radical 11. Mu, S-Z. et al. Secophnane-type alkaloids from Daphniphyllum oldhami. O2 ii. I , PPh , 2 3 (two steps) Chem. Biodivers. 4, 129–138 (2007). cyclization reaction was initiated by 2,2imidazole′-azobisisobutyronitrile (AIBN) at 75 8C; tris(trimethylsilyl)silane ((TMS) SiH) was found 12. Morita, H. et al. Daphmanidins C and D, novel pentacyclic alkaloids from 3 Daphniphyllum teijsmanii. Org. Lett. 7, 459–462 (2005). O O O O to beMe superior toH tributyltinO (TMS) hydride3SiH, AIBN, as a hydrideMe source. ItO was o 13. Matsuno, Y. et al. Pordamacrines A and B, alkaloids from Daphniphyllum Me O Me O crucial to add CH Cl 75as C cosolvent to obtain an excellent Norrish type I 2 2 macropodum. J. Nat. Prod. 70, 1516–1518 (2007). fragmentation and consistent yield, presumably because of the poorCO solubilityMe of 14. Ruggeri, R. B., Hansen, M. M. & Heathcock, C. H. Total synthesis of ( )-methyl N 98% N 2 + H H H H homosecodaphniphyllate: a remarkable new tetracyclization reaction. J. Am. N N 11 in toluene. Under the above conditions, the primary radical CO2Me CO2Me CO2Me attackedO the enone moiety to afford compoundO 31 (98%)I as a Chem. Soc. 110, 8734–8736 (1988). 11 15. Heathcock, C. H., Davidsen, S. K., Mills, S. & Sanner, M. A. Total synthesis of O O single diastereomer.31 The excellent facial selectivity is OTBS OTBS ( )-methyl homodaphniphyllate. J. Am. Chem. Soc. 108, 5650–5651 (1986). þ 29 28 attributable to the strong stereochemical bias of the substrate. 16. Ruggeri, R. B. & Heathcock, C. H. Biomimetic total synthesis of (+)-methyl With the hexacyclic core of daphenylline in hand, a series of homodaphniphyllate. J. Org. Chem. 55, 3714–3715 (1990). O–O bond cleavage/ i. H , Crabtree's ORTEP of 10 71% 86%reductions (two steps) was2 performed. Hydrogenation of the exocyclic C C 17. Heathcock, C. H., Ruggeri, R. B. & McClure, K. F. Daphniphyllum alkaloids. recombination catalyst ¼ 15. Total syntheses of (+)-methyl homodaphniphyllate and (+)-daphnilactone bond with Crabtree’sii. LiCl·H O, 160 catalyst oC provided the desired C18 diastereomer 2 A. J. Org. Chem. 57, 2585–2594 (1992). Me predominantly (.30:1 d.r.), but reduction with Pd/C led to the O 18. Heathcock, C. H., Stafford, J. A. & Clark, D. L. Daphniphyllum alkaloids. 14. O oppositeMe stereochemicalH output. This product was subjected to O O i. Pd/C, MeOH Me H Total synthesis of (+)-bukittinggine. J. Org. Chem. 57, 2575–2585 (1992). MeKrapcho demethoxycarbonylation, 40 o conditionsC (LiCl.H O, dimethyl H H ii. LiAlH4 Me 2 19. Stafford, J. A. & Heathcock, C. H. Asymmetric total synthesis of N H CO2Me sulfoxide, 160 8C) employed by Dixon and co-workersH in their (–)-secodaphniphylline. J. Org. Chem. 55, 5433–5434 (1990). N synthesis of the tricyclic66% (2 steps) core of calyciphyllineN A-type 20. Heathcock, C. H., Kath, J. C. & Ruggeri, R. B. Daphniphyllum alkaloids. 16. Total O OR 26 synthesis of ( )-codaphniphylline. J. Org. Chem. 60, 1120–1130 (1995). alkaloidsO (Fig. 2) , in this case to give compound 10 as a single þ 26 R = TBS 10 8 Daphenylline 21. Weiss, M. E. & Carreira, E. M. Total synthesis of ( )-daphmanidin E. Angew. o detectable diastereomer. The structure of 10 was unambiguously þ HF·py, 0 C Chem. Int. Ed. 50, 11501–11505 (2011). 27 R = H ORTEP of (±)-27 verified by X-ray crystallographic analysis (Fig. 6, T 165–167 C, Figure 6 | Completion of the total synthesis of daphenylline. Them last ring 8 22. Ikeda, S., Shibuya, M., Kanoh, N. & Iwabuchi, Y. Synthetic studies on 92% EtOAc/n-hexane 1:1), which ensured the connectivity of of the natural product was forged by a 7-exo-trig radical cyclization. Facial daphnicyclidin A: enantiocontrolled construction of the BCD ring system. Figure 5 | Assembly of the pentacyclic intermediate 25. a, Trans-triene 23 19 steps from the hexacyclic scaffold16, which takes 9 steps to make. and stereochemical output of the radical selective hydrogenation followed by Krapcho demethoxycarbonylation gave Org. Lett. 11, 1833–1836 (2009). underwent a photoinduced C C bond isomerization/6p-electrocyclization cyclization, hydrogenation and decarboxylation reactions. 23. Dunn, T. B., Ellis, J. M., Kofink, C. C., Manning, J. R. & Overman, L. E. ¼ an advanced intermediate 10,whichconvertedreadilyintodaphenylline cascade reaction to afford the compact pentacyclic product 24,which Finally, deoxygenation promoted by Pd/C under a H atmosphere, Asymmetric construction of rings A–D of daphnicyclidin-type alkaloids. through two reductions. TMSOTf, trimethylsilyl trifluoromethanesulfonate.2 further aromatized to 25 under oxidative conditions. b, An unexpected followed by LiAlH reduction of the lactam moiety, rendered Org. Lett. 11, 5658–5661 (2009). 4 24. Be´langer, G., Boudreault, J. & Le´vesque, F. Synthesis of the tetracyclic core photoinduced cascade reaction occurred when 23 was photoirradiated in the synthetic daphenylline (8) in 66% yield over two steps. The Conclusion of daphnilactone B-type and yuzurimine-type alkaloids. Org. Lett. 13, presence of air. A postulated mechanism involved a Norrish type I daphenylline purified with silica gel displayed identical spectral We accomplished the total synthesis of the Daphniphyllum alkaloid 6204–6207 (2011). fragmentation followed by O–O bond homolysis and radical recombination. and physical properties to those of an authentic sample provided 25. Coldham, I., Watson, L., Adams, H. & Martin, N. G. Synthesis of the core HF.py, hydrogen fluoride pyridine. daphenylline.by Hao and co-workersThe synthesis34. features a gold-catalysed 6-exo-dig ring system of the yuzurimine-type Daphniphyllum alkaloids by cascade cyclization reaction for the construction of a bridged bicyclic condensation, cyclization, cycloaddition chemistry. J. Org. Chem. 76, 682 motif,NATURE and a photoinduced CHEMISTRY | VOL olefin 5 | AUGUST isomerization 2013 | www.nature.com/naturechemistry/6p-electrocycliza- 2360–2366 (2011). tion/aromatization sequence to forge the sterically compact arene. 26. Sladojevich, F., Michaelides, I. N., Darses, B., Ward, J. W. & Dixon, D. J. The chemistry developed may find use in the synthesis of other Expedient route to the functionalized calyciphylline A-type skeleton via a polycyclic natural products and pharmaceutically interesting mol- Michael addition–RCM strategy. Org. Lett. 13, 5132–5135 (2011). 27. Kourra, C., Klotter, F., Sladojevich, F. & Dixon, D. J. Alkali base-initiated ecules. The above endeavour represents the first example of a chemi- Michael addition/alkyne carbocyclization cascades. Org. Lett. 14, cal synthesis of a member of the Daphniphyllum alkaloid 1016–1019 (2012). subfamilies that share a bridged 6,n,5-tricyclic motif. Taking advan- 28. Darses, B. et al. Expedient construction of the [7–5–5] all-carbon tricyclic core tage of the versatility of the tricyclic intermediate 9, studies towards of the Daphniphyllum alkaloids daphnilongeranin B and daphniyunnine D. the total synthesis of related Daphniphyllum alkaloids are currently Org. Lett. 14, 1684–1687 (2012). underway, which, together with this work, should accelerate further 29. Xu, C., Wang, L., Hao, X. & Wang, D. Z. Tackling reactivity and selectivity within a strained architecture: construction of the [6262527] tetracyclic biological and biosynthetic investigations of these fascinating core of calyciphylline alkaloids. J. Org. Chem. 77, 6307–6313 (2012). natural products. 30. Li, H. et al. Rapid construction of the [6262625] tetracyclic skeleton of the Daphniphyllum alkaloid daphenylline. Chem. Asian J. 7, 2519–2522 (2012). Received 31 December 2012; accepted 3 May 2013; 31. Fang, B. et al. Synthesis of the tetracyclic core (ABCE rings) of daphenylline. published online 30 June 2013 J. Org. Chem. 77, 8367–8373 (2012). 32. Yang, M. et al. Tandem semipinacol-type 1,2-carbon migration/aldol reaction References toward the construction of [5–6–7] all-carbon tricyclic core of calyciphylline 1. Balunas, M. J. & Kinghorn, A. D. Drug discovery from medicinal plants. A-type alkaloids. Org. Lett. 14, 5114–5117 (2012). Life Sci. 78, 431–441 (2005). 33. Yao, Y. & Liang, G. Rapid construction of the ABC ring system in the 2. Keasling, J. D., Mendoza, A. & Baran, P. S. Synthesis: a constructive debate. Daphniphyllum alkaloid daphniyunnine C. Org. Lett. 14, 5499–5501 (2012). Nature 492, 188–189 (2012). 34. Zhang, Q. et al. Daphenylline, a new alkaloid with an unusual skeleton, from 3. Tarselli, M. A. et al. Synthesis of conolidine, a potent non-opioid analgesic Daphniphyllum longeracemosum. Org. Lett. 11, 2357–2359 (2009). for tonic and persistent pain. Nature Chem. 3, 449–453 (2011). 35. Bian, M. et al. Total syntheses of anominine and tubingensin A. J. Am. Chem. 4. Wender, P. A. et al. Gateway synthesis of daphnane congeners and their protein Soc. 134, 8078–8081 (2012). kinase C affinities and cell-growth activities. Nature Chem. 3, 615–619 (2011). 36. Staben, S. T. et al. Gold(I)-catalyzed cyclizations of silyl enol ethers: 5. Mendoza, A., Ishihara, Y. & Baran, P. S. Scalable enantioselective total application to the synthesis of ( )-lycopladine A. Angew. Chem. Int. Ed. 45, þ synthesis of taxanes. Nature Chem. 4, 21–25 (2012). 5991–5994 (2006). 6. Editorial Committee of the Administration Bureau of Traditional Chinese 37. Piers, E. & Oballa, R. M. Concise total syntheses of the sesquiterpenoids Medicine in Chinese Materia Medica (Zhonghua Bencao), Vol. 4, 865–867 (2)-homalomenol A and (2)-homalomenol B. J. Org. Chem. 61, (Shanghai Science & Technology Press, 1999). 8439–8447 (1996).

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Answers to quesons Semi pinacol rearragement CN CN CN [1,2]-vinyl LA=TMSOTf shift OH O OH Base + TfOH O O OH LA LA

Me Me H O Me H H O Me H H OH H OH PMBN PMBN PMBN PMBN favored E-selective disfavored E-selective

OMe OPMB Me MeO O O OTBS MeO H O O O H Si tBu O tBu O