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Enantioselective Total Synthesis of (-)-Strychnine1 Scheme1

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Enantioselective Total Synthesis of (-)-Strychnine1 Scheme1 J. Am. Chem. SOC.1993,115, 9293-9294 9293 Enantioselective Total Synthesis of (-)-Strychnine1 Scheme1 Steven D. Knight, Larry E. Overman,' and Garry Pairaudeau Department of Chemistry University of California Imine, California 9271 7-2025 Received July 8, 1993 OTlPS Strychnine (1) has played a vital role in the development of d h,i natural productschemistry. First isolated in 1818 fromstrychnos ignatii by Pelletier and Caventou, strychnine was among the first plant alkaloids obtained in pure form. After decades of inves- OBU' (-)-Strychnine(1) tigation, the structural elucidation of strychnine in 1946 repre- sented one of the crowning accomplishments of classical structural chemistry.394 Its total synthesis by Woodward only 8 years later was an achievement of even greater significance, since prior to this feat no compound approaching the complexity of strychnine ,oms had been prepared by chemical synthesis.* That strychnine's P' seven rings displayed on only 24 skeletal atoms still represents a formidable challenge for total synthesis is apparent in the fact that only last year was a second total synthesis of strychnine published by Magnus and co-workers.6 This synthesis, like the pioneering Woodward synthesis, involved intersection with an intermediate available by degradation of stry~hnine.~.~Most recently, two syntheses of (*)-strychnine have been communicated by the groups of Stork' and Kuehne.* Herein we report the first asymmetric total synthesis of (-)-strychnine. This highly efficient total synthesis features the use of the cationic aza-Cope-Mannich reaction to assemble the pentacyclic strychnan c~re.~J~ The preparation in enantiopure form of the unsaturated azabicyclo[3.2. lloctane 13, the key aza-Cope-Mannich rear- rangement substrate, is summarized in Scheme I. The sequence begins with (1R,4S)-(+)-4-hydroxy-2-cyclopentenyl acetate (2), which is available in high enantiomeric purity on a large scale from the hydrolysis of cis-1,4-diacetoxycyclopent-2-ene with 13 (1) Publication 26 in the series Synthesis Applications of Cationic Aza- Reaction conditions: (a) MeOCOCl, pyridine, CH2C12,23 "C, 97%; Cope Rearrangements. For part 25, see: Overman, L. E.; Shim, J. J. Org. (b) Bu'OCHzCOCH2C02Et, NaH, 1% Pdz(dba)3, 15% PPh3, THF, 23 Chem. 1993, 58, in press. OC, 91%; (c) NaCNBH3, Tic&,THF,-78 OC; (d) DCC, CuCl, benzene, (2) Pelletier, P. J.; Caventou, J. B. Ann. Chim. Phys. 1818, 8, 323. OC; (e) DIBAL, CH&, -78 OC, 98%; (f) TIPSCl, tetrameth- (3) The correct structure strychnine was first proposed in 1946: Briggs, 80 of ylguanidine, NMP, -10 "C; (9) Jones oxidation, acetone, -5 (h) L. H.; Openshaw, H. T.; Robinson, R. J. Chem. SOC.1946,903. Robinson, "C; R. Experientia 1946, 2, 28. L-Selectride, PhNTfZ, THF, -78 - 0 OC, 88%; (i) Me&n2, 10% (4) For an excellent historical survey, see: Smith, G. F. Alkaloids 1965, Pd(PPh3)4, LiC1, THF, 60 "C, 81%; u) t-BuOzH, Triton-B, THF, -15 591. OC, 91%, (k) PhoP--CHz, THF, 0-23 "C, 924%; TBAF, THF, -15 OC, (5) Woodward, R. B.; Cava, M. P.; Ollis, W. D.; Hunger, A.; Daeniker, 100%; (1) MsC1, i-PrzNEt, CHzC12, -23 "C; LiCI, DMF, 23 OC; H. U.; Schenker, K. J. Am. Chem. SOC.1954,76,4749. Woodward, R. B.; NHzCOCF3, NaH, DMF, 23 "C; (m) NaH, benzene, 100 "C;KOH, Cava, M. P.; Ollis, W. D.; Hunger, A,; Daeniker, H. U.; Schenker, K. EtOH-Hz0,60 OC. R2N = 1,3-dimethylhexahydre2-oxo-1,3,5-triazinyl. Tetrahedron 1963, 19, 247. (6) (a) Magnus,P.;Giles, M.;Bonnert,R.;Kim, C.S.;McQuire, L.; Merritt, A.; Vicker, N. J. Am. Chem. SOC.1992, 114, 4403. electric eel acetylcholinesterase.ll Reaction of 2 with methyl (7) Stork,G. Reportedat theIschia AdvancedSchoolofOrganicChemistry, chloroformate, followed by selective palladium-catalyzed dis- Ischia Porto, Italy, September 21, 1992. (8) Kuehne, M. E.; Xu, F. Abstracts of Papers, 205th National Meeting placement of the allylic carbonate derivative12J3 with sodium of the American Chemical Society, Denver, CO, March 28-April 2, 1993; ethyl a-tert-butoxya~etoacetatel~provided the cis-adduct 3 (a American Chemical Society: Washington, DC, 1993; ORGN 187. (9) For brief reviews of the am-Cope-Mannich reaction, see: Overman, (1 1) Deardorff, D. R.; Windham, C.Q.;Craney,C. L. Org.Synth.,accepted L. E. Acc. Chem.Res. 1992,25,352. Overman,L. E. Abstracts, 33rdNational for review. Deardorff, D. R.; Matthews, A. J.; McMeckin, D. S.; Craney, C. Organic Symposium, June 13-17, Bozeman, Montana, ACS Division of L. Tetrahedron Lett. 1986,27, 1255. Organic Chemistry, 1993; p 96. (12) Tsuji, J.; Shimizu, I.; Minami, I.; Ohashi, Y.Tetrahedron Lerr. 1982, (10) The evolution of the am-Cope-Mannich strategy for total synthesis 23, 4809. of Strychnos alkaloids was recently detailed: Angle, S. R.; Fevig, J. M.; (13) BBckvall, J. E.; Gatti, R.; Schink, H. E. Synthesis 1993, 343. Knight, S. D.; Marquis, R. W., Jr.; Overman, L. E. J. Am. Chem. SOC.1993, (14) Raimund, M.; Leander, T. Eur. Pat. Appl. EP 67,408, 1982; Chem. 115, 3966. Absrr. 98(21):179198u. OOO2-7863/93/1515-9293$04.00/0 0 1993 American Chemical Society 9294 J. Am. Chem. SOC.,Vol. 115, No. 20, 1993 Communications to the Editor 1: 1 mixture of diastereomers) in 88% yield.15 Stereocontrolled Scheme I1 a reduction of 3 with Felkin-Ahn selectivity was best realized by OBu' reaction in THFat -78 OCwith excess NaCNBH3in the presence of 1.1 equiv of TiC14.16 This treatment converted both diaster- eomers of 3 to the corresponding anti @-hydroxy esters 4 (stereoselectivity >20: 1) in nearly quantitative yield. Direct syn MeCN, 80 "C dehydration'' of this mixture afforded, after chromatographic (98%) removal of 2-3% of the unwanted (Z)stereoisomer, the (E)- butenoate 5 in 89% overall yield from 3.18J9Reduction of 5 with excess i-Bu2AlH provided the corresponding diol, which was 13 selectively protected by careful treatment with triisopropylsilyl u chloride and 1,1,3,3-tetramethylguanidineZoat -10 OC in N-meth- yl-2-pyrrolidone to give 6 in 65% yield. Jones oxidation of 6 at -5 OC then provided cyclopentenone 7 in 97% yield. Regioselective conversion of cyclopentenone 7 to the enol triflate derivative,Zl followed by palladium-catalyzed coupling of this intermediate with hexamethylditin,2zprovided vinylstannane 8 in 80% yield. Using conditions we had recently optimized for a related transformation,lO palladium-catalyzed carbonylative cou- 15 pling of 8 with the triazone-protected ortho-iodoaniline 923 was accomplished in 80% yield to afford enone At this stage, the enantiomeric purity of 10 was confirmed to be >95% ee by IH NMR analysis of the a-methoxyphenylacetic esters prepared by cleavage of the TIPS ether (n-Bu4NF) of 10 and subsequent acylation (DCC, DMAP) of the liberated primary alcohol with (R)-or (S)-a-methoxyphenylaceticacid.25 The azabicyclooctane ring system was next assembled from 10 by stereoselective epoxidation, followed by Wittig methylenation and desilylation to afford 11. Conversion of this intermediate to the allylic trifluoroacetamide 12, followed by cyclization with NaH in 18 I (-)-Strychnine(l) I benzene at 100 OC10,26 and final removal of the trifluoroacetyl Reaction conditions: (a) LDA, NCCOzMe, THF, -78 OC; (b) 5% group with KOH, provided azabicyclooctane 13 in 43% overall HCl-MeOH, reflux; (c) Zn dust, 10% H2SO4-MeOH, reflux; (d) yield from enone 10. NaOMe, MeOH, 23 OC; (e) i-BuzAIH, CHC12, -78 OC; (f) The crucial aza-Cope-Mannich reorganization was accom- CH2(C02H)zr AczO, NaOAc, HOAc, 110 0C.31 plished in essentially quantitative yield by heating 13 in acetonitrile with excess paraformaldehyde and anhydrous Naz- Gumlich aldehyde 18.29 Finally, reaction of 18 with malonic SO4 to provide the highly crystalline pentacyclic diamine 14 (98% acid and acetic anhycdride, as described earlier by Anet and yield from an 800-mg scale reaction) (Scheme 11). Acylation of Robinson,30 led to (-)-strychnine in 65% yield: mp 278-285 OC 14 with methyl cyanof~rmate,~~followed by treatment of the (EtOH), mixture mp 278-285 OC, lit.5 mp 275-285 OC; [C~]~'D @-ketoesterwith 5% HCl in refluxing methanol, provided 15 (1 8- -139' (C = 0.4, CHClj), lit.30 [aIZ5~-139' (C = 1.0, CHCl3). hydroxyakuammicine) in 70% yield. Reduction of this inter- The first asymmetric total synthesis of strychnine has been mediate with Zn dust in acidic MeOH resulted in saturation of accomplished in 20 steps and -3% overall yield from the readily the vinylogous carbamate functionality from the @-faceto afford available11 enantiopure hydroxy cyclopentenyl acetate 2. The 16.28Base-promoted epimerization of this intermediate provided efficiency of this total synthesis, which is several orders of the known pester 17,28which was reduced at -78 OC in CH2C12 magnitude more efficient than the two previously published with i-Bu2AlH to provide, in 52% overall yield from 15, Wieland- strychnine synthese~,~.~provides an important benchmark of the (1 5) Intermediates were fully characterized by IH and 13C NMR, IR, and power of the aza-Cope rearrangement-Mannich reaction to solve MS analyses. The elemental composition of analytical samples of new formidable problems in alkaloid construction. The latent sym- compounds was confirmed by combustion analysis or high-resolution mass metry of 2,13 moreover, should allow this total synthesis strategy spectrometry. Unless noted otherwise, yields refer to purified products. (16) (a) Heathcock, C. Aldrichim. Acta 1990,23,99. (b) Theseconditions to be directly extended to the preparation of ent-strychnine." were suggested to us by Professor M. Dimare, Department of Chemistry, University of California, Santa Barbara, CA. Acknowledgment. The support of PHS Grant NS-12389 and (17) Alexandre, C.; Rouessac, F. Bull. SOC.Chim. Fr. 1971, 1837. (1 8) The alternative sequence19 involving chelation-controlled reduction SmithKline Beecham Pharmaceuticals is gratefully acknowl- of 3 followed by antidehydrationwas less satisfactory, since low syn selectivity edged.
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