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Copper-catalyzed asymmetric allylic alkylation and asymmetric conjugate addition in natural product synthesis Huang, Yange

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Copper-catalyzed Asymmetric Allylic Alkylation and Asymmetric Conjugate Addition in Natural Product Synthesis

Yange Huang

This Ph.D. thesis was carried out at the Stratingh Institute for Chemistry, University of Groningen, The Netherlands.

The authors of this thesis wish to thank the NRSC-Catalysis for scientific research funding.

Printed by: Ipskamp Drukkers B.V., Enschede, The Netherlands RIJKSUNIVERSITEIT GRONINGEN

Copper-catalyzed Asymmetric Allylic Alkylation and Asymmetric Conjugate Addition in Natural Product Synthesis

Proefschrift

ter verkrijging van het doctoraat in de Wiskunde en Natuurwetenschappen aan de Rijksuniversiteit Groningen op gezag van de Rector Magnificus, dr. E. Sterken, in het openbaar te verdedigen op vrijdag 20 september 2013 om 12.45 uur

door

Yange Huang

Geboren op 1 september 1984 te Taicang, China

Promotores: Prof. dr. B. L. Feringa

Prof. dr. ir. A. J. Minnaard

Copromotor: Dr. M. Fañanás-Mastral

Beoordelingscommissie: Prof. dr. F. J. Dekker Prof. dr. G. Roelfes Prof. dr. J. G. de Vries

ISBN: 978-90-367-6343-1 (book)

978-90-367-6342-4 (electronic)

Now this is not the end. It is not even the beginning of the end. But it is, perhaps, the end of the beginning.

-Sir Winston Churchill

To

Yang (my wife) & Xiyuan (my son)

Table of Contents

1. Copper-catalyzed Asymmetric Allylic Alkylation and Asymmetric Conjugate Addition in Natural Product Synthesis 1 1.1 Introduction 2 1.2 Copper-catalyzed ACA 2 1.2.1 ACA with organozincs 3 1.2.2 ACA with organoaluminum 9 1.2.3 ACA with Grignard reagents 13 1.2.4 ACA with organosilicon reagent 18 1.3 Copper-catalyzed AAA 18 1.3.1 AAA with organozinc reagents 18 1.3.2 AAA with organoaluminum reagents 19 1.3.3 AAA with Grignard reagents 20 1.3.4 AAA with organoboranes 22 1.3.5 AAA with organolithiums 23 1.4 Conclusion 24 1.5 Outline of this thesis 24 1.6 References 25

2. Formal synthesis of (R)-(+)-Lasiodiplodin 29 2.1 Introduction 30 2.2 Biosynthesis of Lasiodiplodin 30 2.3 Previous catalytic asymmetric syntheses 31 2.4 Formal synthesis of (R)-(+)-Lasiodiplodin 33 2.4.1 First retrosynthetic analysis 33 2.4.2 Results and discussion 34 2.4.3 Second retrosynthetic analysis 34 2.4.4 Results and discussion 35 2.5 Conclusion 36 2.6 Experimental Section 36 2.7 References and notes 40

3. A Concise Asymmetric Synthesis of (-)-Rasfonin 43 3.1 Introduction 44 3.2 Previous total syntheses of Rasfonin 44 3.3 Total synthesis of Rasfonin 46 3.3.1 Retrosynthetic analysis 46 3.3.2 Synthesis of upper half of Rasfonin 47 3.3.3 Synthesis of lower half of Rasfonin 48 3.4 Conclusion 53 3.5 Experimental Section 53 3.6 References 63

4. A Novel Catalytic Asymmetric Route towards Skipped Dienes with a Methyl-Substituted Central Stereogenic Carbon 65 4.1 Introduction 66 4.2 Previous methodologies 66 4.3 Synthesis of starting materials 68 4.4 Results of the Cu-catalyzed AAA and discussion 74 4.5 Conclusion 78 4.6 Experimental Section 78 4.7 References and notes 95

5. Towards a Total Synthesis and Structure Elucidation of Phorbasin B 99 5.1 Introduction 100 5.2 Previous synthesis of Phorbasins 100 5.3 First retrosynthetic analysis of Phorbasin B 101 5.4 Results and discussion 102 5.5 Second retrosynthesis of Phorbasin B 104 5.6 Results and discussion 105 5.7 Conclusion 108 5.8 Experimental Section 109 5.9 References 118

6. Total Synthesis of (S)-(–)-zearalenone 121 6.1 Introduction 122 6.2 Biosynthesis of zearalenone 122 6.3 Previous synthesis of zearalenone 123 6.4 Retrosynthetic analysis 125 6.5 Results and discussion 125 6.6 Biological studies 128 6.7 Conclusion 129 6.8 Experimental Section 129 6.9 References and notes 137

Summary Summary (English) 139 Samenvatting (Nederlands) 143

Acknowledgements 147

Chapter 1

Chapter 1

Copper‐Catalyzed Asymmetric Allylic Alkylation and Asymmetric Conjugate Addition in Natural Product Synthesis

This chapter gives an introduction on copper-catalyzed asymmetric allylic alkylation and asymmetric conjugate addition in natural product synthesis. Alternative catalytic asymmetric syntheses of the natural products prepared by Cu-catalyzed asymmetric allylic alkylation and asymmetric conjugate addition will also be presented.

1 Chapter 1

1.1 Introduction

Catalytic asymmetric C-C bond forming reactions using organometallic reagents are among the most important organic transformations.1 The asymmetric conjugate addition (ACA, Scheme 1, a) and asymmetric allylic alkylation (AAA, Scheme 1, b) are particular versatile in enantioselective C-C bond forming reactions.2 Especially applying ACA, the intermediate enolate formed could be further functionalized (Scheme 1, c) by reaction with other electrophiles (one-pot transformations) introducing to two vicinal stereocenters. These transformations are frequently applied in the synthesis of complex biologically active molecules.1 The major part of this chapter is concerned with the application in the total synthesis of natural products using copper-complex catalyzed AAA and ACA as the key step.

Scheme 1. General scheme of AAA, ACA and enolate functionalization.

1.2 Copper-catalyzed ACA

Copper-catalyzed ACA of organometallics (Grignard reagents, organozinc reagents, organoaluminum compounds and organosilicon reagents) to Michael acceptors (cyclic enones, acyclic enones, nitro-olefins, unsaturated lactones, unsaturated lactams, dehydropiperidinones, α,β-unsaturated esters, thioesters, amides and imides) has been a highly active research field in recent decades.3 Since the first discovery of ferrocenyl ligands such as TaniaPhos and JosiPhos as efficient ligands in the copper-catalyzed ACA of Grignard reagents, several other chiral ligands were discovered for the copper-catalyzed ACA including phosphines, phosphoramidites, phosphonites, peptides, NHC ligands, phosphine-phosphites and aminophosphine ligands.

2 Chapter 1

1.2.1 ACA with organozincs The copper-catalyzed ACA of organozinc reagents has been a longstanding objective in the field of ACA. A major breakthrough was achieved by the group of Feringa in 1996 based on the development of a BINOL-derived monodentate phosphoramide L1.4 Employing this ligand in the copper-catalyzed ACA of cyclic enones using dialkylzinc reagents for C-C bond formation was achieved in high yield, chemoselectivity, efficiency and enantioselectivity. An important feature is that phosphoramide ligands are readily accessible and due to their modular structure can be readily tuned for a specific 5 application. This methodology was applied in the synthesis of PGE1 methyl ester 6 (Scheme 2) using copper-catalyzed ACA of dialkylzinc 3 to cyclopentenone 1 followed by trapping with aldehyde 2 as the key step.6 The diastereoselectivity of the aldehyde trapping was only moderate (threo/erythro=83/17), however, reduction using Zn(BH4)2 followed by separation of the diastereomers gave advanced intermediate 5 as a single diastereomer with 94% ee. After another 5 steps PGE1 methyl ester 6 was obtained in high yield. In this way PGE1 methyl ester 6 was obtained in 7% overall yield with 94% optical purity in 7 steps from 1.

Scheme 2. Total synthesis of PGE1methyl ester 6.

Recently the group of Aggarwal reported a stereocontrolled organocatalytic synthesis of 7 PGF2α 13 in only 7 steps (Scheme 3). Intermolecular aldol reaction catalyzed by 2 mol% of proline 8 gave product 9 which underwent hemiacetal formation to 10 and an intramolecular aldol condensation to form α,β-unsaturated aldehyde 11. After acetal formation product 12 was obtained in 14% overall yield with 98% ee. PGF2α 13 was prepared in 5 steps from aldehyde 12. In this way PGE2α 13 was obtained in 4% overall yield with 98% optical purity in 7 steps from 7. 3 Chapter 1

OH OH OH COOH 8 O O N 2mol% O Intramolecular O H aldol condensation O then O O [Bn2NH2][OCOCF3] O O O 7 9 10 11

HO O O MeOH HO

13

O HO COOH 12, 14% based on 7 98% ee

Scheme 3. Total synthesis of PGF2α 13.

In 2001 Hoveyda et al. reported a tandem reaction using the copper-catalyzed ACA of organozinc to cyclic enones with peptide-based phosphine ligand L2 which is easily accessible from commercially available components (Scheme 4).8 This methodology was applied to the synthesis of the natural product, Clavularin B (17). The Zn-enolate formed by copper-catalyzed ACA using dimethyl zinc was trapped with 4-iodo-1-butene resulted in cyclic ketone 15 in high yield, ee and chemoselectivity. Silyl enol ether formation followed by palladium-mediated oxidation (Saegusa–Ito oxidation) of 15 gave α,β-unsaturated ketone 16. Wacker oxidation of ketone 16 gave the anti-cancer agent, Clavularin B (17), in only four steps with 42% overall yield. Similar methodology was applied by the group of Minnaard in the total synthesis of Triglycosyl Phenolic Glycolipid PGL-tb1199 and PDIM_A 2010 (Scheme 5).

Scheme 4. Total synthesis of Clavularin B (17).

4 Chapter 1

O

O 18 O O 1. Cu(OTf)2 (0.5 mol%) O O L1 (1 mol%), Me2Zn O OMe Toluene, -25°C, on 18 2. EtI, HMPA 15 14 18 O 19 83%, >20:1, 95% ee HO O O Ph OMe O HO O P N O O OH Ph L1 O OMe MeO OMe

19 19

20

O O O O O

22 Scheme 5. Synthesis of Triglycosyl Phenolic Glycolipid PGL‐tb119 and PDIM_A 20.

Alexakis and coworkers reported a tandem enantioselective conjugate addition-cyclopropanation to cyclic and linear enones (Scheme 6).11 Copper-catalyzed ACA using dimethyl zinc with phosphoramidite ligand L3 afforded the Zn-enolate 22 which was following by trapping with TMSOTf. Cyclopropanation of the resulting silyl enolate gave 23 in excellent yield, moderate dr and with 97% ee. Ring expansion of 23 using FeCl3 resulted in 7-membered ring 24 in 90% yield which was a key intermediate for the synthesis of (R,S)-Isoclavukerin 25 and (S,S)-Clavukerin 26.

5 Chapter 1

Scheme 6. Formal synthesis of Isoclavukerin and Clavukerin.

The enantioselective total synthesis of Erogorgiane 31 was achieved by the group of

Hoveyda employing a double ACA of Me2Zn to linear enones catalyzed by 12 (CuOTf)2•C6H6 and peptidic phosphine ligands (L4 and L5) (Scheme 7). The first ACA to enone 27 using only 1 mol% of catalyst derived from ligand L4 gave 28 in 94% yield and >98% ee, and the product was easily transformed to dienone 29 in a few steps. A second ACA using 5 mol% of the catalyst derived from ligand L5 provided product 30 in moderate yield, however, the regioselectivity (1,4/1,6=9/1) and ee (94%) were excellent. The natural product Erogorgiane 31 was easily prepared from 30. One year later 31 was also synthesized by Davies and Walji13 by a kinetic enantiodifferentiating step. Product

34 was formed by [Rh2(R-dosp)4]-catalyzed reaction of racemic 32 with diazo ester 33 involving a C-H activation/Cope rearrangement sequence.

6 Chapter 1

O O

(CuOTf)2-C6H6 (1 mol%) (CuOTf)2-C6H6 (5 mol%) L4 (2.4 mol%), Me2Zn L5 (12 mol%), Me2Zn Br Toluene, -15ºC, 48 h Br Toluene, 4ºC, 24 h 29 27 28, 94%, >98% ee O

O H N NHBu N NHBu N O O PPh2 PPh2 L5 O L4 O 30, 50 %, 97:3 dr Erogorgiane 31 (1,4:1,6 = 9:1)

Hoveyda's approach, key step: double ACA

CO2Me [Rh2(S-dosp)4] + + H 23oC N2 H MeO2C 32 33 MeO C 2 35 34 Erogorgiane 31 Walji's approach, key step: kinetic enantiodifferentiation Scheme 7. Total synthesis of Erogorgiane.

In 2004 Feringa et al. reported another example of a domino reaction for the total synthesis of Pumiliotoxin C (38) (Scheme 8).14 Copper-catalyzed ACA of cyclohexenone 36 using dimethyl zinc provided the zinc enolate which was followed by palladium-catalyzed allylic substitution with allyl acetate to form ketone 37 in high yield and dr (trans/cis=8/1) with 96% ee. Ketone 37 was easily transformed to Pumiliotoxin C (38) in 7 steps. The same natural product was also synthesized enantioselectively by the group of Helmchen in 2011 by copper-catalyzed ACA with trimethyl aluminum using ligand L1.15 Diastereomeric products 40 and 41 (40/41=5/1) were isolated in 85% combined yield. Reductive amination gave (+)-Pumiliotoxin C (38) in 67% yield and >99 % ee.

7 Chapter 1

O 1. Cu(OTf)2 (0.5 mol%) O H H Ph L6 (1 mol%), Me Zn, -30oC N 2 O P N O 2. Pd(PPh3)4 (2 mol%) H allyl acetate Ph 36 37 Pumiliotoxin C (38) trans/cis=8/1 L6 96% ee Feringa's approach

Cbz NH O

Cbz H NH O CuTC (2.2 mol%) 40 H H L1 (3.8 mol%), AlMe3 N Et2O, -30°C, 19 h H2 (30 atm.) + Pd(OH)2/Rh/C Ph MeOH, rt H 39 Cbz O NH O P N Pumiliotoxin C (38) O 67%, >99% ee Ph H L1 41 85% combined yield Helmchen's approach Scheme 8. Total synthesis of Pumiliotoxin C.

(R)-Muscone 43, the major odorous constituent of the male musk deer, was synthesized by the group of Rosini in 2003 (Scheme 9) using copper-catalyzed ACA of dimethyl zinc to macrocyclic enone 42.16 Employing 3 mol% of copper and 6 mol% of chiral phosphite ligand L7, (R)-muscone 43 was obtained in 68% yield and 78% ee.

Scheme 9. Total synthesis of Muscone.

Recently the group of Cordova reported a novel catalytic enantioselective β-alkylation of α,β-unsaturated aldehydes by a combination of transition metal catalysis and 17 aminocatalysis (Scheme 10). Employing copper-PPh3 complex and amine 45, product 46 was obtained in 65% yield with excellent regioselectivity (1,4/1,2=93/7) and ee (94%). Intermediate 46 was used for the synthesis of three natural products (Curcumene 47, Dehydrocurcumene 48 and Tumerone 49).

8 Chapter 1

Scheme 10. Total synthesis of Curcumene, Dehydrocurcumene and Tumerone.

A copper-catalyzed ACA of Me2Zn was reported by Carreira et al. in the synthesis of Daphmanidin E (52) in 2011 (Scheme 11).18 Employing 19 mol% of copper salt and 20 mol% of ligand L8, in a conjugate addition to the nitro olefin moiety in 50, product 51 was obtained in 90% yield with dr = 5:1. Further functionalization led to the natural product, Daphmanidin E (52).

Scheme 11. Total synthesis of Daphmanidin E.

1.2.2 ACA with organoaluminum Besides organozinc reagents, organoaluminum compounds are frequently used in natural product synthesis. In 2007 excellent methodology to construct chiral quaternary center was developed by the group of Alexakis using copper-catalyzed ACA of trialkylaluminum reagents to cyclic enones (Scheme 12).19 Employing 5 mol% of CuTC and 10 mol% of ligand L9, product 54 was isolated in 81% yield and 95% ee. HCl-promoted hydrolysis of the acetal and in situ intramolecular cyclization gave 55 which was used for the preparation of Axane core structure 56.

9 Chapter 1

Scheme 12. Synthesis of Axane core structure 56.

Recently Hoveyda et al. reported a NHC-copper catalyzed ACA to cyclic enones using Si-containing vinylaluminum reagent 61 to install a chiral quaternary center (Scheme 13).20 Subsequently this method was applied in the synthesis of Riccardiphenol B (59).

Employing 5 mol% of CuCl2 and 2.5 mol% of 60 (dimeric NHC-Ag complex), product 58, after trapping using acetic anhydride, was isolated in 67% yield and excellent 96% ee. Riccardiphenol B (59) was obtained in only two steps from 58.

Scheme 13. Total synthesis of Riccardiphenol B.

In 2008 Feringa and coworkers reported a total synthesis of Myrtine (Scheme 14) using the copper-catalyzed ACA of N-Boc-2,3-dehydro-4-piperidone 62 with trimethyl aluminum and ligand L10.21 Product 63 was obtained in high yield and 96% ee. Further transformation of 63, after 3 steps, resulted in Myrtine 64 in 14% overall yield. Recently the groups of Pozo22 and Hong23 have reported the synthesis of the same molecule by applying an organocatalytic aza-Michael reaction to install the piperidine ring 67 and 70, respectively, with excellent yield and ee.

10 Chapter 1

Scheme 14. Total synthesis of Myrtine.

In 2008 the first catalytic enantioselective total synthesis of Clavirolide C (77) was achieved by the group of Hoveyda (Scheme 15).24 For the preparation of the chiral building block 73, a copper-catalyzed ACA of dimethyl zinc to substrate 71 with only 1 mol% of copper and 2.5 mol% of ligand L11 was used, providing pyranone 72 in excellent yield and ee. For the synthesis of building block 75, NHC•Cu-catalyzed ACA to 74 using trimethyl aluminum was employed followed by silyl enol ether formation using

Et3SiOTf to provide 75 in 72% yield and 84% ee. Subsequent aldol reaction with aldehyde 73 gave 76 as a mixture of diastereomers which was transformed into Clavirolide C (77).

11 Chapter 1

Scheme 15. Total synthesis of Clavirolide C.

Recently the group of Endo and Shibata reported the first case of copper-catalyzed ACA of organoaluminum to linear α,β-unsaturated amides and applied this ACA in the 25 synthesis of several natural products (Scheme 16). ACA on substrate 79 using Cu(OTf)2 and ligand L11 gave 80 in moderate yield, however, with excellent 96% ee. Product 80 was readily transformed into (S)-Florhydral 81 in three steps. For the formal synthesis of

Tumerone 85 and Deoxyanisatin 84, substrate 82 was used with Cu(OTf)2 and ligand L12 and conjugate addition product 83 was obtained in excellent yield and ee. Similar results were achieved with substrate 86 using Cu(OAc)2 and ligand L13. Frondosin B (88) could be easily prepared from adduct 87 in 4 steps.

12 Chapter 1

O O O Cu(OTf)2 (5 mol%) N L11 (10 mol%) N Me3Al, THF, rt, 2 h 79 80, 54%, 96% ee (S)-Florhydral 81

O O O O Cu(OTf)2 (5 mol%) N L12 (10 mol%) N Me Al,THF,rt,2h OH Me 3 Me 82 HO HO 83, 87%, 97% ee O PAr 2 O 8-Deoxyanisatin 84 OH O OH

L11 Ar = Ph Me (+)-ar-Tumerone 85 L12 Ar = 3,5-(CF3)2C6H3

O O Cu(OAc)2 (10 mol%) O O HO N L13 (10 mol%) N Me3Al,THF,2h O 86 Ph2P 87, 82%, 94% ee Frondosin B 88 MeO MeO

OH OH

L13 Ph2P Scheme 16. Formal synthesis of Florhydral, Tumerone, Deoxyanisatin and Frondosin B.

An elegant application of copper-catalyzed ACA of an organoaluminum reagent was recently reported by Baran et al. in the synthesis of taxane 91 using Alexakis’ methodology (Scheme 17).26 Employing 2 mol% of CuTC and 4 mol% of ligand L14, intermediate 90 with the key quaternary stereocenter present in taxane was isolated in excellent yield and ee. Further functionalization of 90 gave taxane 91 in good yield.

Scheme 17. Synthesis of taxane 79.

1.2.3 ACA with Grignard reagents A significant breakthrough was reported by Feringa and coworkers with the development of a highly enantioselective copper-catalyzed ACA with Grignard reagents.27 This has 13 Chapter 1 been a longstanding objective in catalytic asymmetric C-C bond formation as the organomagnesium reagents are versatile organometallic reagents. Subsequently this methodology was applied in the synthesis of several natural products. In the total synthesis of Phaseolinic acid 95 (Scheme 18),28 copper-catalyzed ACA of ester 92 using methylmagnesium bromide and ligand L15 resulted in magnesium enolate 93 which was trapped by hexanal afforded 94 in high yield and excellent dr and ee. Phaseolinic acid 95 was conveniently prepared from 94 in only 2 steps.

Scheme 18. Total synthesis of Phaseolinic Acid.

Recently Feringa and Minnaard reported a total synthesis of Rasfonin 100 using iterative ACA (Scheme 19).29 Important features are the low catalyst loading (1 mol%), the catalytic reaction can be performed up to 40 g scale and the discovery of JosiPhos L15 as the most effective ligand which is commercial available. The first ACA product 97 was achieved in 95% yield and 96% ee. Excellent results were also obtained for the second addition product 99 (86% yield and >95/5 dr). The same methodology was applied by the groups of Minnaard and Feringa in the synthesis of several natural products including Borrelidin, Phthioceranic acid, Mycocerosic acid, Mycolipenic acid, β-D-Mannosyl Phosphomycoketide, PDIMA, apple leafminer pheromones, Lardolure and 30 sulfoglycolipid Ac2SGL. Several alternative asymmetric synthesis of Rasfonin 100 have reported. The synthesis by the group of Boeckman in 2006 is based on the use of camphor lactam chiral auxiliaries (106a, 106b, 106c) in order to allow the synthesis of different stereoisomers.31 The oxazaborolidine catalyst 107 was applied in the key assembly of butenolide 104 via an asymmetric vinylogous Mukaiyama aldol addition. Recently, the group of Nanda32 reported a chemoenzymatic asymmetric synthesis of Rasfonin 100. Enantioselective enzymatic desymmetrization (EED) and Gluconobacter oxydans mediated oxidative kinetic resolution (OKR) were used for the introduction of three stereocenters.

14 Chapter 1

CuBr-SMe2 (1 mol%) SEt JosiPhos L15 (1.1 mol%) SEt SEt TBDPSO TBDPSO TBDPSO o O TBME, -78 C, 16h O O 96 95%, 96%ee 97 98

PCy2 Fe PPh2 O O CuBr-SMe2 (1 mol%) JosiPhos L15 (1.1 mol%) SEt O (R,S) -Josiphos L15 o TBDPSO TBME, -78 C, 16h OH O O 86%, dr>95/5 99 100 OH

Feringa and Minnaard's route, key step: Cu-catalyzed iterative ACA

O TMSO O N 1. LiAlH N 4 O 2. NWO, TPAP O O O 107 101 102 103

O O O O Rasfonin 100

OH OH 104 105 81%, threo/erythro=20/1 dr 20/1 H Ar Ar O O TfO NH N N N B H H H Boeckman's route O o-tol key steps:camphor lactam chiral auxiliary 106a 106b 106c 107 asymmetric vinylogous Mukaiyama based alkylation

TBSO EED TBSO OTBS OH OH 99% ee HO HO AcO OTBS 108 109 O 110 91%, ee>99% TBSO OH EED OKR AcO OAc HO OAc 114 TBSO OH 45%, 96% ee 111 112 113 86%, ee>99% TBSO OH

115 O

Nanda's route, key elements: enantioselective enzymatic desymmetrization Rasfonin 100 oxidative kinetic resolution Scheme 19. Total synthesis of Rasfonin.

15 Chapter 1

OTBS [Rh(cod)2]OTf (5 mol%) L16 (10 mol%) TBSO R O TBSO R O Pr Me2PhSi-Bpin, Et3N Pr OMe 1,4-Dioxane/H O, 45ºC Pr SEt MeO O 2 117 (Mismatched product) 118 116 55%, anti,90%de MeO R=SiMe2Ph O HN PPh2 PPh2 OMe O CuBr-SMe2 (5 mol%) L17 (6 mol%) TBSO R O N MeMgBr L16 Pr SEt MTBE, -78ºC O O 119 (Matched product) Cy P 90%, anti,anti, 90% de O O O 2 Ph P Fe (+)-Neopeltolide 120 2 Oestreich's route, key steps: iterative catalytic asymmetric conjugate silyl transfer reaction Cu-catalyzed ACA L17

124 (S)-BINAP-Ru(II) Ipc B OEt OEt O 2 MeOH, 6 atm H2 OH O O 100oC, 92% OH O OTBS Et O, -90oC 2 OTBS 121 122 123 81% 125 Hantzch ester OMe CHCl3, 80% OMe OTBS O (+)-Neopeltolide 120 OTBS 126 O N -TFA 128 O N H 127 Paterson's route, key steps: catalytic asymmetric hydrogenation and organocatalytic asymmetric reduction

Scheme 20. Synthesis of Neopeltolide.

Several catalytic asymmetric syntheses of Neopeltolide 120 were described. The group of Oestreich33 reported an iterative catalytic asymmetric conjugate silyl transfer reaction for the synthesis of one important intermediate of Neopeltolide 120 (Scheme 20). Rhodium-catalyzed ACA of 116 using BINAP L16 as the ligand gave the mismatched product 117 in 55% yield and excellent 90% de. Subsequent copper-catalyzed ACA of 118 derived from 117 resulted in matched product 119 in high yield and ee which could be used for the synthesis of Neopeltolide 120. Catalytic asymmetric hydrogenation and organocatalytic asymmetric reduction (transition metal catalysis and organocatalysis)

16 Chapter 1 were applied for the synthesis of stereogenic centers in Paterson’s total synthesis.34 Asymmetric allylation using chiral allyl borane was used for the introduction of the stereogenic centers in the Sasaki’s route.35

Scheme 21. Total synthesis of natural sulfated alkene 137.

Besides the application of 1,4-ACA in natural product synthesis, Feringa et al. also applied the 1,6-ACA in the total synthesis of natural sulfated alkene 137 (Scheme 21).36 Employing 2 mol% of Cu-catalyst based on chiral ligand L18 and Grignard reagent 135, product 136 was formed in moderate yield, however, with excellent regioselectivity (1,6/1,4=94/6) and ee. The final natural product, sulfated alkene 137, could be easily obtained from intermediate 137 in 3 steps.

Scheme 22. Formal synthesis of Spirovibsanin A.

For the formation of chiral quaternary centers Alexakis and coworkers developed a novel NHC-copper catalyzed ACA of cyclic enones using Grignard reagents in 2010 which was a major improvement compared to their earlier discovery of the ACA of trimethylaluminum (Scheme 22).37 Subsequently this methodology was employed in the formal synthesis of Spirovibsanin A (140). Employing 3 mol% of Cu(OTf)2 and 4 mol% of ligand L19, chiral cyclohexanone 139 was isolated in good yield and with high ee.

17 Chapter 1

1.2.4 ACA with organosilicon reagent Copper-catalyzed ACA of organosilicon reagents was used in natural product synthesis in a few cases. In 2012 Hoveyda and co-workers reported a NHC-Cu catalyzed enantioselective conjugate silyl addition to cyclic and acyclic enones with excellent yield (up to 97%) and ee (up to 98%) (Scheme 23).38 Subsequently they applied the methodology in the synthesis of an important intermediate 142 of Erysotramidine 143 using a one-pot procedure (conjugate addition-enolate alkylation). Trans-α,β-disubstituted cyclohexanone 142 was obtained in 92% yield, >98/2 dr and 97.5/2.5 er. From this intermediate Erysotramidine 143 could be prepared according to Stalke’s route.39

Scheme 23. Synthesis of intermediate 142 for Erysotramidine.

1.3 Copper catalyzed AAA

The copper-catalyzed AAA constitutes an attractive and versatile approach to a variety of chiral building blocks especially when using organometallic reagents to form product with allylic stereogenic centers. 40 Since the first copper-catalyzed AAA of Grignard reagents with allylic acetates reported by Bäckvall and van Koten,41 other organometallic reagents (organozinc reagents, organoaluminum reagents, organoboranes and organolithium reagents) have been applied successfully for this C-C bond formation. Besides acetates, chlorides, bromides and phosphates are frequently used leaving groups. The use of organolithium reagents allowed the use of allylic ethers as efficient substrates for copper-catalyzed AAA as reported by the group of Feringa recently.56 Chiral ligands frequently used are phosphoramidites, peptides, NHC ligands and ferrocene ligands.

1.3.1 AAA with organozinc reagents Since the breakthrough achieved by Knochel in 1999 who applied bulky organozinc as 42 the nucleophile in the SN2’ displacement of allylic chloride, organozinc reagents are frequently applied in copper-catalyzed AAA. As early as 2001 Hoveyda et al. reported a copper-catalyzed AAA using dialkylzinc reagents to form quaternary stereogenic centers.43 This methodology was applied in the synthesis of natural product (R)-Sporochnol 146 (Scheme 24). Employing 10 mol% of CuCN and 10 mol% of ligand 18 Chapter 1

L21 using organozinc reagent 145, the allylic alkylation was followed by hydrolysis of the tosyl group to provide Sporochnol 146 in 82% overall yield and 82% ee. A phosphate was applied as leaving group in this case.

Scheme 24. Synthesis of Sporochnol.

Subsequently a Cu-catalyzed AAA to prepare α-alkyl-β,γ-unsaturated esters was developed (Scheme 25).44 Employing 5 mol% of copper and 10 mol% of ligand L22, product 148 was prepared in 80% yield with excellent regioselectivity and ee. Elenic acid 151 was easily prepared from product 148 in two steps by alkene metathesis using 149 followed by ester hydrolysis and ether cleavage.

Scheme 25. Synthesis of Elenic Acid 151.

1.3.2 AAA with organoaluminum reagents Besides organozinc reagents, organoaluminum compounds are frequently applied in natural product synthesis involving asymmetric allylic alkylation as key step. In 2007 Hoveyda et al. reported a total synthesis of (+)-Baconipyrone C (157) using NHC-Cu catalyzed double-asymmetric allylic alkylation of substrate 153 (Scheme 26).45 Employing 15 mol% of copper and 7.5 mol% of ligand L23, product 156 was isolated in 61% yield with excellent >98% ee.

19 Chapter 1

Scheme 26. Total synthesis of Baconipyrone C.

The same group also achieved the synthesis of Nyasol 161 (Scheme 27) using a copper-catalyzed AAA with trisubstituted vinylaluminum reagent 159.46 Employing 2 mol% of CuCl2 and 1 mol% of ligand L24 with aluminum reagent 159, chiral 1,4-diene

160 (>98% E) was formed in 76% yield with excellent regioselectivity (>98 SN2’) and ee (97%). Further conversion of 160 which includes proto-desilylation with trifluoroacetic acid led to Nyasol 161 in high yield.

Scheme 27. Total synthesis of Nyasol 161.

1.3.3 AAA with Grignard reagents Copper-catalyzed AAA of Grignard reagents are frequently used in natural product synthesis. Recently the group of Feringa reported a formal synthesis of Lasiodiplodin (Scheme 28) using Cu-catalyzed AAA. 47 Employing 1 mol% of copper and 1.1 mol% of ligand L25, allylester 163 was obtained in excellent yield and 99% ee. The Lasiodiplodin precursor 164 could be synthesized in several steps from 163. An alternative catalytic 20 Chapter 1 asymmetric synthesis of Lasiodiplodin was reported by Jones and Huber using chromium-catalyzed enantioselective addition of dimethyl zinc to an aldehyde (86% ee) as a key step.48

Scheme 28. Formal synthesis of Lasiodiplodin.

Feringa and co-workers also reported a catalytic asymmetric synthesis of naturally occurring butenolides via hetero-AAA followed by ring closing metathesis (Scheme 29).49 Employing 3 mol% of copper bromide dimethyl sulfide complex and 3.6 mol% of ligand L26, Cu-catalyzed hetero-AAA of 126 with different Grignard reagents afforded allyl ester 166 in excellent yield and high ee. These chiral intermediates could be readily transformed into different natural butenolides (Whiskey Lactone 169, Cognac Lactone 170, Nephrosteranic Acid 167 and Roccellaric Acid 168). Similar methodology was 30h applied by the group of Minnaard for the total synthesis of Ac2SGL. Previously catalytic asymmetric synthesis of Whiskey Lactone 169 was reported by Bruckner using asymmetric Sharpless dihydroxylation as the key step.50

Scheme 29. Synthesis of natural occurring butenolides.

Recently the Feringa group developed a copper-catalyzed AAA with allyl Grignard reagents (Scheme 30).51 Starting from allyl bromides 171 and 174 and allyl Grignard 21 Chapter 1

using 5 mol% of (CuOTf)•C6H6 and 6 mol% of ligand L1, bisallyl compound 172 (precursor for Sabinene 173) and 175 were formed with high yield, regioselectivity and ee. The Martinelline alkaloids chromene derivative core 176 was prepared from 175 with 92% ee.52

Scheme 30. Cu‐catalyzed allyl‐allyl cross coupling.

1.3.4 AAA with organoboranes Copper-catalyzed AAA of vinylboron reagents were recently reported by the group of Hoveyda and applied in the synthesis of Pummerer ketones 180 and 181 (Scheme 31).53 Starting from allyl phosphates 177 and vinyl borane 178, α,β-unsaturated aldehyde 179, after hydrolysis of the acetal group, was obtained in 77% yield, >98% SN2’ and 96% ee. Pummerer ketone 180 and anti- Pummerer ketone 181 were prepared from 179 in 4 steps.

Scheme 31. Synthesis of Pummerer ketone and anti‐ Pummerer ketone.

In 2012 the Hoveyda group achieved the formal synthesis of Cuparenone by NHC-Cu catalyzed AAA of alleneboron reagent 183 (Scheme 32). Product 184 was obtained in 83% yield and 84% ee.54 Copper-catalyzed hydroboration using ligand L29 followed by oxidation afforded 186 in good yield. α-Cuparenone 187 could be easily prepared from 22 Chapter 1

186 in 3 steps. In the same year the group of Minnaard reported a synthesis of α-Cuparenone in only 2 steps via a palladium catalyzed asymmetric conjugate reaction of cyclopentenone 188 with p-tolyl boronic acid.55

Scheme 32. Formal synthesis of Cuparenone.

1.3.5 AAA with organolithiums In 2012 the group of Feringa developed a copper-catalyzed AAA of acyclic allylic ethers with organolithium reagents (Scheme 33).56 This methodology was applied in the shortest enantioselective synthesis of (S)-Arundic acid 193. Employing 5 mol% of CuTC and 11 mol% of ligand L30 provided chiral olefin 192. Ozonolysis and subsequently aldehyde oxidation gave (S)-Arundic acid 193 in 61% overall yield and >98% ee over 5 steps sequence from benzylether 191. (S)-Arundic acid 193 was also synthesized by the group of the Cozzi57 by enantioselective α-alkylation of aldehyde 195 with 1,3-benzodithiolylium tetrafluoroborate 194 using MacMillan catalyst 197.

23 Chapter 1

Scheme 33. Synthesis of Arundic acid 193.

1.4 Conclusion

In this chapter a brief overview of copper-catalyzed AAA and ACA in natural product synthesis is given. These two catalytic C-C bond forming reactions have shown to be highly versatile and selective steps for the enantioselective construction of complex natural molecules. Due to the broad substrate scope, variety of chiral ligands, readily available organometallic reagents and excellent enantioselectivity of these two reactions, it can be concluded that these methodologies are highly versatile and it is expected that these methods will become more popular in natural product synthesis in the future.

1.5 Outline of this thesis

In this thesis, copper-catalyzed asymmetric allylic alkylation and asymmetric conjugate addition are described in the total synthesis of several biologically active molecules (Lasiodiplodin, Rasfonin and Zearalenone). A second part of this thesis aims at the development of a novel catalytic asymmetric route towards skipped dienes with a methyl substituted central stereogenic carbon using copper-catalyzed AAA and its application in the total synthesis of natural product, Phorbasin B.

In chapter 2, the formal catalytic asymmetric synthesis of Lasiodiplodin is described. The 24 Chapter 1 copper-catalyzed AAA of Grignard reagents is applied for the introduction of the stereogenic center. Subsequently RCM and sp3-sp2 Suzuki coupling are used for the synthesis of the macrocyclic structure of the natural product.

In chapter 3, the catalytic asymmetric synthesis of Rasfonin is presented. An iterative copper-catalyzed ACA of Grignard reagent and stereospecific Achmatowicz rearrangement are the key strategic steps in this synthesis.

In chapter 4, a novel catalytic asymmetric route towards skipped dienes with a methyl substituted central stereogenic carbon by a copper-catalyzed AAA is shown. This transformation leads to important chiral 1,4-diene building blocks with excellent regio- and enantioselectivity (ee values up to 99%; SN2’/SN2 ratio up to 97:3) in nearly all cases.

In chapter 5, the asymmetric total synthesis of Phorbasin B is presented. A copper-catalyzed AAA of Grignard reagents, as described in chapter 4, is applied for the introduction of the side chain. Evans aldol reaction is used for the construction of the cyclohexenone ring.

Finally in chapter 6, the catalytic asymmetric synthesis of Zearalenone is described. The copper-catalyzed AAA of a Grignard reagent is the key strategic step in this synthesis. Biological studies of Zearalenone led to the identification of a novel lipoxygenases inhibitor.

1.6 Reference

1. (a) Jacobsen, E. N.; Pfaltz, A.; Yamamoto, H. Comprehensive Asymmetric Catalysis; Springer-Verlag: Berlin, 1999. (b) Noyori, R. Asymmetric Catalysis in Organic Synthesis; John Wiley and Sons; New York, 1994. (c) Blaser, H.-U.; Schmidt, E. Asymmetric Catalysis on Industrial Scale: Challenges, Approaches and Solutions; Wiley-VCH; Weinheim, 2004. 2. (a) Perlmutter, P. Conjugate Addition Reactions in Organic Synthesis; Pergamon; Oxford, 1992. (b) Krause, N. Modern Organocopper Chemistry; Wiley-VCH Verlag GmbH; Weinheim, 2002. (c) Pfaltz, A.; Lautens, M. Allylic Substitution Reactions. In Comprehensive Asymmetric Catalysis; Jacobsen, E. N., Pfaltz, A., Yamamoto, H., Eds.; Springer-Verlag: Berlin, 1999; p 833. (d) Tomioka, K.; Nagaoka, Y. Conjugate Addition of Organometallic Reagents. In Comprehensive Asymmetric Catalysis; Jacobsen, E. N.; Pfaltz, A.; Yamamoto, H., Eds.; Springer-Verlag: Berlin, 1999; p 1105. (e) Paquin, J.-F.; Lautens, M. Allylic Substitution Reactions. In Comprehensive Asymmetric Catalysis, Supplement 2; Jacobsen, E. N., Pfaltz, A.,

25 Chapter 1

Yamamoto, H., Eds.; Springer-Verlag: Berlin Heidelberg, 2004; p 73. (f) Tomioka K. Conjugate Addition of Organometals to Activated Olefinics. In Comprehensive Asymmetric Catalysis, Supplement 2; Jacobsen, E. N., Pfaltz, A., Yamamoto, H., Eds.; Springer-Verlag: Berlin, Heidelberg, 2004; p 109. 3. For reviews on conjugate additions see: (a) Rossiter, B. E.; Swingle, N. M. Chem. Rev. 1992, 92, 771; (b) Feringa, B. L.; de Vries, A. H. M. Advances in Catalytic Processes, 1995, 1, 151; (c) Alexakis, A.; Benhaim, C. Eur. J. Org. Chem. 2002, 19, 3221; (d) Alexakis, A.; Bäckvall, J. A.; Krause, N.; Pàmies, O.; Diéguez, M. Chem. Rev. 2008, 108, 2796; (e) Jerphagnon, T.; Pizzuti, M. G.; Minnaard, A. J.; Feringa, B. L. Chem. Soc. Rev. 2009, 38, 1039; (f) Endo, K.; Shibata, T. Synthesis, 2012, 44, 1427; (g) Beller, M.; Bolm, C. Transition metals for organic synthesis: building blocks and fine chemicals, 2nd ed, vol.1, Wiley-VHC, Weinheim, 2004. (f) Galestokova, Z.; Sebesta, R. Eur. J. Org. Chem. 2012, 6688–6695. 4. (a) de Vries, A. H. M.; Meetsma, A.; Feringa, B. L. Angew. Chem. Int. Ed. 1996, 35, 2374–2376. (b) Feringa, B. L.; Pineschi, M.; Arnold, L. A. Imbos, R.; de Vries, A. H. M. Angew. Chem. Int. Ed. 1997, 36, 2620–2623. 5. Teichert, J. F.; Feringa, B. L. Angew. Chem. Int. Ed. 2010, 49, 2486-2528. 6. Arnold, L. A.; Naasz, R.; Minnaard, A. J.; Feringa, B. L. J. Am. Chem. Soc. 2001, 123, 5841. 7. Coulthard, G.; Erb, W.; Aggarwal, V. K. Nature, 2012, 489, 278. 8. Degrado, S. J.; Mizutani, H.; Hoveyda, A. H. J. Am. Chem. Soc. 2001, 123, 755–756. 9. Barroso, S.; Castelli, R; Baggelaar, M. P.; Geerdink, D.; ter Horst, B.; Casas-Arce, E.; Overkleeft, H. S.; van der Marel, G. A.; Codée, J. D. C.; Minnaard, A. J. Angew. Chem. Int. Ed. 2012, 51, 11774. 10. Casas-Arce, E.; ter Horst, B.; Feringa, B. L.; Minnaard, A. J. Chem. Eur. J. 2008, 14, 4157. 11. Alexakis, A.; March, S. J. Org. Chem. 2002, 67, 8753. 12. Cesati, R. R.; de Armas J.; Hoveyda, A. H. J. Am. Chem. Soc. 2004, 126, 96. 13. Davies, H. M. L.; Walji, A. M. Angew. Chem. Int. Ed. 2005, 44, 1733 –1735. 14. Dijk, E. W.; Panella, L.; Pinho, P.; Naasz, R.; Meetsma, A.; Minnaard, A. J.; Feringa, B. L. Tetrahedron, 2004, 60, 9687. 15. Gärtner, M.; Qu, J.; Helmchen, G. J. Org. Chem. 2012, 77, 1186. 16. Scafato, P.; Labano, S.; Cunsolo G.; Rosini, C. Tetrahedron: Asymmetry, 2003, 14, 3873–3877. 17. Afewerki, S,; Breistein, P.; Pirttila, K.; Deiana, L.; Dziedzic, P.; Ibrahem, I.; Cordova, A. Chem. Eur. J. 2011, 17, 8784–8788. 18. Weiss, M. E.; Carreira, E. M. Angew. Chem. Int. Ed. 2011, 50, 11501. 19. Alexakis, A.; Vuagnoux-d´Augustin M. Chem. Eur. J. 2007, 13, 9647. 20. May, T. L.; Dabrowski J. A.; Hoveyda, A. H. J. Am. Chem. Soc. 2011, 133, 736. 21. Pizzuti, M. G.; Minnaard, A. J.; Feringa, B. L. Org. Biomol. Chem. 2008, 6, 3464.

26 Chapter 1

22. Fustero, S.; Moscardo, J.; Sanchez-Rosello, M.; Flores, S.; Guerola, M.; del Pozo, C. Tetrahedron, 2011, 67, 7412–7417. 23. Ying, Y.; Kim, H.; Hong, J. Org. Lett. 2011, 13, 796–799. 24. Brown, M. K.; Hoveyda, A. H. J. Am. Chem. Soc. 2008, 130, 12904. 25. Endo, K.; Hamada, D.; Yakeishi, S.; Shibata, T. Angew. Chem. Int. Ed. 2013, 52, 606. 26. Mendoza, A.; Ishihara, Y.; Baran, P. S. Nature Chem. 2012, 4, 21. 27. Feringa, B. L.; Badorrey, R.; Pena, D.; Harutyunyan, S. R.; Minnaard, A. J. Proc. Natl. Acad. Sci. USA, 2004, 101, 5834–5838. 28. Howell, G. P.; Fletcher, S. P.; Geurts, K.; ter Horst, B.; Minnaard, A.J.; Feringa B. L. J. Am. Chem. Soc. 2006, 128, 14977. 29. Huang, Y.; Minnaard, A. J.; Feringa, B. L. Org. Biomol. Chem. 2012, 10, 29. 30. For Borrelidin see: (a) Madduri, A. V. R.; Minnaard, A.J. Chem. Eur. J. 2010, 16, 11726. For Phthioceranic Acid see: (b) ten Horst, B.; Feringa, B. L.; Minnaard, A. J. Org. Lett. 2007, 9, 3013. For Mycocerosic acid see: (c) ten Horst, B.; Feringa, B. L.; Minnaard, A. J. Chem. Commun. 2007, 489. For Mycolipenic acid see: (d) ten Horst, B.; van Wermeskerken, J.; Feringa, B. L.; Minnaard, A. J. Eur. J. Org. Chem. 2010, 38. For β-D-Mannosyl Phosphomycoketide see: (e) van Summeren, R. P.; Moody, D. B.; Feringa, B. L.; Minnaard, A. J. J. Am. Chem. Soc. 2006, 128, 4546. For PDIM A see reference 10. For apple leafminer pheromones see: (f) van Summeren, R. P.; Reijmer, S. J. W.; Feringa, B. L.; Minnaard, A. J. Chem. Commun. 2005, 1387. For Lardolure see: (g) Des Mazery, R.; Pullez, M.; López, F.; Harutyunyan, S. R.; Minnaard, A. J.; Feringa, B. L. J. Am. Chem. Soc. 2005, 127, 9966. For

sulfoglycolipid Ac2SGL see: (h) Geerdink, D.; ter Horst, B.; Lepore, M.; Mori, L.; Puzo, G.; Hirsch, A. K. H.; Gilleron, M.; de Libero, G.; Minnaard, A. J. Chem. Sci. 2013, 4, 709. 31. Boeckman, R. K. Jr.; Pero, J. E.; Boehmler, D. J. J. Am. Chem. Soc., 2006, 128, 11032. 32. Bhuniya, R.; Nanda, S. Tetrahedron, 2013, 69, 1153-1165. 33. Hartmann, E.; Oestreich, M. Angew. Chem. Int. Ed. 2010, 49, 6195. 34. Paterson, I.; Miller, N. A. Chem. Commun. 2008, 4708–4710. 35. Fuwa, H.; Saito, A.; Naito, S.; Konoki, K.; Yotsu-Yamashita, M.; Sasaki, M. Chem. Eur. J. 2009, 15, 12807 – 12818. 36. den Hartog, T.; Harutyunyan, S. R.; Font, D.; Minnaard, A. J.; Feringa, B. L. Angew. Chem. Int. Ed. 2008, 47, 398. 37. Kehrli, S.; Martin, D.; Rix, D.; Mauduit, M.; Alexakis, A. Chem. Eur. J. 2010, 16, 9890. 38. Lee, K.; Hoveyda, A. H. J. Am. Chen. Soc. 2010, 132, 2898–2900. 39. Tietze, L. F.; Tolle, N.; Kratzert, D.; Stalke, D. Org. Lett. 2009, 11, 5230–5233. 40. For reviews in allylic alkylation, see: (a) Endo, K.; Shibata, T. Synthesis, 2012, 44, 1427; (b) Harutyunyan, S. R.; den Hartog, T.; Geurts, K.; Minnaard, A. J.; Feringa, B. L. Chem. Rev. 2008, 108, 2824; (c) Alexakis, A.; Malan, C.; Lea, L.; Tissot-Croset, K.; Polet, A.; Falciola, C.

27 Chapter 1

Chimia, 2006, 60, 124. 41. (a) Van Klaveren, M.; Persson, E. S. M.; del Villar, A.; Grove, D. M.; Bäckvall, J. E.; van Koten, G. Tetrahedron Lett. 1995, 36, 3059. (b) Karlstrom, A. S. E.; Huerta, F. F.; Meuzelaar, G. J.; Bäckvall, J. E. Synlett 2001, 923. (c) Cotton, H. K.; Norinder, J.; Bäckvall, J. E. Tetrahedron 2006, 62, 5632. 42. Dubner, F.; Knochel, P. Angew. Chem. Int. Ed. 1999, 38, 379–381. 43. Luchaco-Cullis, C. A.; Mizutani, K. E. M.; Hoveyda, A. H. Angew. Chem., Int. Ed. 2001, 40, 1456. 44. Murphy, K. E.; Hoveyda, A. H. J. Am. Chem. Soc. 2003, 125, 4690. 45. Gillingham, D.; Hoveyda, A. H. Angew. Chem. Int. Ed. 2007, 46, 3860. 46. Akiyama, K.; Gao, F.; Hoveyda, A. H. Angew. Chem. Int. Ed. 2010, 49, 419. 47. Huang, Y.; Minnaard, A. J.; Feringa, B. L. Synthesis, 2011, 1055–1058. 48. Jones, G. B.; Huber, R. S. Synlett, 1993, 367. 49. Mao, B.; Geurts, K.; Fañanás-Mastral, M.; van Zijl, A.W.; Fletcher, S.P.; Minnaard, A. J.; Feringa, B. L. Org. Lett. 2011, 13, 948. 50. Harcken, C.; Bruckner, R. Angew. Chem., Int. Ed. Engl. 1997, 36, 2750. 51. Hornillos, V.; Pérez, M.; Fañanás-Mastral, M.; Feringa, B. L. J. Am. Chem. Soc. 2013, 135, 2140–2143. 52. Urabe, H.; Suzuki, K.; Sato, F. J. Am. Chem. Soc. 1997, 119, 10014. 53. Gao, F.; Carr, J. L.; Hoveyda, A. H. Angew. Chem., Int. Ed. 2012, 51, 1–6. 54. Jung, B.; Hoveyda, A. H. J. Am. Chem. Soc. 2012, 134, 1490. 55. Gottumukkala, A. L.; Matcha, K.; Lutz, M.; de Vries, J. G.; Minnaard, A. J.; Chem. Eur. J. 2012, 18, 6907 – 6914. 56. Pérez, M.; Fañanás-Mastral, M.; Hornillos, V.; Rudolph, A.; Bos, P. H.; Harutyunyan, S. R.; Feringa, B. L. Chem. Eur. J. 2012, 18, 11880 – 11883. 57. Gualandi, A.; Emer, E.; Capdevila, M. G.; Cozzi, P. G. Angew. Chem., Int. Ed. 2011, 50, 7842–7846.

28 Chapter 2

Chapter 2

Formal Synthesis of (R)‐(+)‐Lasiodiplodin

In this chapter the catalytic asymmetric formal synthesis of (R)-(+)-Lasiodiplodin is described. Copper-catalyzed asymmetric allylic alkylation is the key strategic element in this synthesis.

Parts of this chapter have been published: Y. Huang, A. J. Minnaard, B. L. Feringa, Synthesis, 2011, 1055-1058.

29 Chapter 2

2.1 Introduction

Important bioactive compounds, such as resorcylic acid lactones (RALs) which are closely related to salicylic acid derivatives, are popular synthetic targets since several natural products of this type were identified as pharmacophores (Figure 1). For example, (S)-Zearalenone exhibits antibacterial, uterotropic and anabolic activity,1 and recently we found it’s a moderately active lipoxygenase inhibitor.2 Zeranol, a nonsteroidal livestock, including beef cattle, growth-promoting antagonist, is in clinical trials for the treatment of (post)menopausal syndrome.3 Pochonin C has shown inhibition in the herpes simplex virus (HSV) replication.4 Aigialomycin D was discovered to possess antimalarial activity.5 Lasiodiplodin 1, which was isolated from the fungus Botrysdiplodia theobromae and the wood of Euphorbia splendens and E. fidjiana, displays antileukemia activity.6 And its demethylated congener 2, a secondary metabolite found in Chinese traditional medicine, efficiently inhibits prostaglandin biosynthesis.7

Figure 1. Bioactive resorcylic acid lactones.

2.2 Biosynthesis of Lasiodiplodin

The primary metabolites are a kind of metabolite including nucleic acids, proteins, carbohydrates and fats which are directly involved in living, growth and reproduction of all organisms. The secondary metabolites, on the other hand, are not involved in these processes and their presence is restricted in nature. The natural functions of most secondary metabolites are not clear yet, however, they are often involved in defending against predators. And most biologically active molecules are secondary metabolites.8

30 Chapter 2

NR-PKS PKS-R OH S O O S HO Malonyl-CoA x 3 cyclization HO 5x SCoA O Aldol Condensation HO

3 4 5

OH O O O o-methylation O O

HO HO

2 1 Figure 2. Proposed biosynthesis pathway to 1.9

According to recent work from the group of Nabeta,9 Lasiodiplodin 1 and its demethylated congener 2 are synthesized via a polyketide biosynthetic pathway in L. Theobromae, similar to other resorcylic acid lactones (Figure 2). Condensation of 5 acetyl-CoA catalyzed by R-PKS gave the intermediate 4. The acyl chain was transferred to NR-PKS which was followed by condensation with 3 malonyl-CoA molecules and one aldol condensation to give a resorcylyl intermediate 5. NR-PKS then catalyzed the cyclization to form the macrolactone. Finally, O-methylation of the hydroxyl group gave Lasiodiplodin 1.

2.3 Previous catalytic asymmetric syntheses

Many of the synthetic routes to Lasiodiplodin 1 either lead to the racemate10 or are based on chiral starting materials11 or on chiral auxiliaries.12 The first catalytic asymmetric synthesis of (R)-(+)-Lasiodiplodin was reported by Jones and Huber (Figure 3).13 In their synthesis the key step was an enantioselective addition of dimethyl zinc to aldehyde 7, affording the corresponding chiral alcohol 8 with 86% ee.

31 Chapter 2

Figure 3. Huber’s first catalytic asymmetric route to (R)‐(+)‐Lasiodiplodin.

Recently, the group of Faber14 reported a chemenzymatic asymmetric route towards (R)-(+)-lasiodiplodin (Figure 4). The stereogenic center was introduced by alkyl sulfatase Pisa1-catalyzed deracemization reaction. Racemic Sulfate ester 16 was hydrolyzed with inversion of the stereocenter using alkyl sulfatase Pisa1 to product 18 with >99% ee. The remaining enantiomer 17 was hydrolyzed using p-TSA with retention of configuration to give product 18 with 93% ee.

32 Chapter 2

Figure 4. Faber’s enzymatic route.

In order to develop a short catalytic asymmetric route with precise control over the absolute configuration in this important class of compounds, a catalytic approach is reported for the synthesis of (R)-(+)-lasiodiplodin methyl ether using highly enantioselective copper-catalyzed asymmetric hetero allylic alkylation as the key step.15

2.4 Formal synthesis of (R)-(+)-Lasiodiplodin 2.4.1 First retrosynthetic analysis

Figure 5. First retrosynthesis of (R)‐(+)‐Lasiodiplodin.

In our retrosynthetic analysis (Figure 5), the macrocycle 22 was planned to be formed not by macrolactonization, but by ring-closing metathesis to afford, after hydrogenation, the C13–C14 bond. In this way, the starting material for this ring-closing reaction, 23, can be prepared from allyl bromide 24 by copper-catalyzed asymmetric allylic alkylation

33 Chapter 2 recently developed in our group. Bromide 24 can be prepared from iodide 25.

2.4.2 Results and discussion The synthesis started with a Vilsmeier-Haack reaction16 of commercially available iodide 25 to afford aldehyde 26 in 64% yield (Figure 6). The initially attempted sp2-sp3 Suzuki coupling using PdCl2 and 1,1′-bis(diphenylphosphino)ferrocene (dppf), water and Cs2CO3 at room temperature17 led to isomerization of the terminal alkene of compound 27.

Fortunately, a catalytic amount of AsPh3 significantly improved the outcome and afforded compound 27 in 52% yield. This approach is reminiscent to the work of Bracher and

Schulte, who used a Pd(PPh3)4 catalyzed cross coupling between a related aryl triflate and a 9-BBN alkyl borane obtained via hydroboration.11a Subsequent Pinnick oxidation18 of aldehyde 27 gave acid 28 in 80% yield. The preparation of acid bromide 29 using oxalyl bromide gave full conversion, however, the synthesis of allyl bromide 24 using acid bromide 29 and acrolein gave a complex mixture of products.

Figure 6. Attempted synthesis of allyl bromide 24.

2.4.3 Second retrosynthetic analysis

Figure 7. Second retrosynthesis of (R)‐(+)‐Lasiodiplodin.

In our second retrosynthetic analysis (Figure 7), starting material 23 for the ring-closing reaction can be prepared from allylic alcohol 31, containing the stereogenic center, and tetrasubstituted benzene 30 (as its acyl fluoride, vide infra). Alcohol 31 is accessible in

34 Chapter 2 high yield and enantioselectivity by the above mentioned copper-catalyzed asymmetric hetero allylic alkylation reaction recently developed in our group (Figure 8).

Figure 8. Copper‐catalyzed asymmetric allylic alkylation.

2.4.4 Results and discussion

Figure 9. Synthesis of (R)‐(+)‐Lasiodiplodin.

The allylic alcohol 31 was formed by hydrolysis of ester 33 in turn obtained by catalytic asymmetric allylic alkylation (Figure 8). Attempted esterification of acid 28 and alcohol 31 using carbodiimide-based reagents or the Yamaguchi method met with failure. Fortunately, the method reported by Fürstner strongly improved the outcome.19 Switching to acid fluoride 34 prepared by treatment of 28 with cyanuric fluoride,20 afforded compound 23 in 90% yield. This was followed by alkene ring-closing metathesis21 using Hoveyda-Grubbs 2nd generation catalyst (Grubbs’ first and second generation catalysts gave only low yield) in refluxing toluene to afford a cis/trans mixture of alkene 35 in 80% yield (E/Z =1/2). A similar strategy was applied by the group of Fürstner for the construction of the macrocycle.10b,11c,11d Hydrogenation22 of alkene 35 afforded the final product 2223 in quant. yield and the optical rotation and spectroscopic data were in agreement with the reported data.11d (R)-(+)-lasiodiplodin (1) and its de-O-methyl congener 2 are readily prepared from 22 according to literature procedures, although with low yields.10a,10c,11a 35 Chapter 2

2.5 Conclusion

In summary, we have completed the formal total synthesis of (R)-(+)-lasiodiplodin using catalytic asymmetric allylic substitution, sp3-sp2 Suzuki coupling and RCM as key steps. Asymmetric allylic alkylation was the step in the synthesis used to obtain the chiral allylic alcohol building block. The new synthetic route is currently explored in the preparation of other biologically active resorcylic acid lactones.

2.6 Experimental section

Starting materials were purchased from Aldrich, Alpha Aesar or Acros and used as received unless stated otherwise. All solvents were reagent grade and, if necessary, dried and distilled prior to use. Column chromatography was performed on silica gel ® (SiliaFlash 60, 230-400 mesh). TLC was performed on silica gel 60/Kieselguhr F254. 1H and 13C NMR spectra were recorded on a Varian VXR300 (299.97 MHz for 1H, 75.48 MHz for 13C) or a Varian AMX400 (399.93 MHz for 1H, 100.59 MHz for 13C) spectrometer in CDCl3 unless stated otherwise. Chemical shifts are reported in  values 1 13 (ppm) relative to the residual solvent peak (CHCl3, H = 7.24, C = 77.0). Carbon assignments are based on 13C and APT 13C experiments. Splitting patterns are indicated as follows: s (singlet), d (doublet), t (triplet), q (quartet), m (multiplet), br (broad). High resolution mass spectra (HRMS) were recorded on an AEI-MS-902 and a FTMS orbitrap (Thermo Fisher Scientific) mass spectrometer. Optical rotations were measured on a Schmidt+ Haensch polarimeter (Polartronic MH8) with a 10 cm cell (c given in g/100 mL).

2-Iodo-4,6-dimethoxybenzaldehyde (26):16 To a stirred solution of iodide 25 (4.70 g,

O 17.8 mol) in 30 mL of DMF was carefully added POCl3 (7.25 g, 47.3 o o O mol) at 0 C. The resulting mixture was heated to 100 C for 4 h, then

O I poured onto ice and left overnight. The precipitate was filtered and washed with water. The aqueous solution was extracted with DCM (3x 30 mL). The combined organic layers were dried over Na2SO4, filtered, concentrated and purified by flash chromatography (eluent pentane/ether = 10/1) to give 26 as a yellow solid (3.33 g, 11.4 mol, 64%). The NMR data are in agreement with these reported.16 1H NMR (400

MHz, CDCl3) δ 10.14 (s, 1H), 7.13 (d, J = 2.2 Hz, 1H), 6.48 (d, J = 2.2 Hz, 1H), 3.89 (s, 3H), 3.86 (s, 3H).

36 Chapter 2

6-(Hept-6-en-1-yl)-2,4-dimethoxybenzaldehyde (27): To a stirred solution of O 7-bromohept-1-ene (651 mg, 0.52 mL, 3.68 mmol) in THF

O (15 mL) was added rapidly t-BuLi (4.60 mL, 1.6 M in

O heptane, 7.36 mmol) at –78 °C. After 30 min, 9-methoxy-9-borabicyclo[3.3.1]nonane (9.20 mL, 1 M in hexanes, 9.20 mmol) was added. The resulting solution was stirred for 10 min at –78 °C and then allowed to warm to room temperature for 1.5 h. Cs2CO3 (4.80 g, 14.7 mol)) was added, followed by the addition of iodide 26 (1.07 g, 3.68 mmol) in 20 mL of DMF. PdCl2-(dppf) (150 mg, 0.184 mmol, dppf = 1,1′-bis(diphenylphosphino)ferrocene) was added, followed by water (1.59 mL) and AsPh3 (169 mg, 0.55 mmol). The resulting solution was stirred for 16 h at room temperature. Et2O was added, and the organic solution was washed with H2O. The aqueous layer was extracted with Et2O (3x 30 mL), and the combined organic layers were dried over Na2SO4, filtered, concentrated and purified by flash chromatography (eluent pentane/ether = 10/1) to give 27 as a colorless oil with traces of 9-BBN residues visible in the 13C-NMR spectrum (500 mg, 1.91 mmol, 52%) that were removed in the next step. 1H

NMR (400 MHz, CDCl3) δ 10.45 (s, 1H), 6.31 (s, 2H), 5.80 (ddt, J = 16.9, 10.1, 6.7 Hz, 1H), 4.99 – 4.89 (m, 1H), 4.93 – 4.89 (m, 1H), 3.86 (s, 3H), 3.85 (s, 3H), 2.95 – 2.92 (m, 2H), 2.41 – 2.38 (m, 2H), 1.89 – 1.83 (m, 2H), 1.59 – 1.50 (m, 2H), 1.41– 1.38 (m 2H). 13 C NMR (100 MHz, CDCl3) δ 190.4, 165.6, 164.7, 149.8, 139.4, 117.0, 114.4, 108.2, 95.9, 56.0, 55.6, 34.8, 34.0, 31.3, 29.5, 29.0. HRMS (ESI+): m/z [M+H]+ calc. for

C16H23O3: 263.1642, found: 263.1643.

6-(Hept-6-en-1-yl)-2,4-dimethoxybenzoic acid (28): To a stirred solution of aldehyde

O OH 27 (420 mg, 1.61 mmol) in 30 mL t-BuOH/H2O (5/1) was

O added NaH2PO4 (0.340 g, 2.87 mol), NaClO2 (0.690 g,

O 7.64 mol) and 2-methyl-2-butene (806 mg, 1.21 mL, 11.5 mmol). The resulting solution was stirred for 4 h and the solvent was removed. The resulting mixture was extracted with DCM (3x 30 mL). The combined organic layers were dried over Na2SO4, filtered, concentrated and purified by flash chromatography (eluent heptane/EtOAc =1/2) to give 28 as a brown oil (360 mg, 1 1.29 mmol, 80%). H NMR (400 MHz, CDCl3) δ 10.03 (br, 1H), 6.38 (d, J = 2.2 Hz, 1H), 6.35 (d, J = 2.2 Hz, 1H), 5.79 (ddt, J = 16.9, 10.2, 6.7 Hz, 1H), 5.01– 4.90 (m, 2H), 3.87 (s, 3H), 3.82 (s, 3H), 2.79 – 2.75 (m, 2H), 2.06 – 2.01 (m, 2H), 1.66 – 1,58 (m, 2H), 1.44 13 – 1.36 (m, 4H). C NMR (100 MHz, CDCl3) δ 171.6, 162.2, 159.1, 146.1, 139.3, 114.5,

37 Chapter 2

114.0, 107.3, 96.6, 56.4, 55.6, 34.8, 33.9, 31.5, 29.3, 28.9. HRMS (ESI+): m/z [M+H]+ calc. for C16H23O4: 279.1591, found: 279.1594.

6-(Hept-6-en-1-yl)-2,4-dimethoxybenzoyl fluoride (34): To a stirred solution of acid 28

O F (100 mg, 0.36 mmol) in 4 mL of DCM was added

O cyanuric fluoride (72.9 mg, 46.0 μL, 0.54 mmol) and o O pyridine (85.4 mg, 87.0 μL, 1.08 mmol) at 0 C. The resulting mixture was stirred for 4 h and then quenched with water. The aqueous mixture was extracted with DCM (3x 30 mL). The combined organic layers were dried over Na2SO4, filtered, concentrated and purified by flash chromatography (eluent heptane/EtOAc =1/1) to give 34 as a colorless oil (77 mg, 0.27 1 mmol, 76%). H NMR (400 MHz, CDCl3) δ 6.37 (d, J = 2.2 Hz, 1H), 6.34 (d, J = 2.2 Hz, 1H), 5.80 (ddt, J = 17.0, 10.2, 6.7 Hz, 1H), 5.02 – 4.95 (m, 1H), 4.95 – 4.89 (m, 1H), 3.85 (s, 3H) , 3.84 (s, 3H), 2.76 – 2.64 (m, 2H), 2.09 – 1.98 (m, 2H), 1.66 – 1.51 (m, 2H), 1.46 13 – 1.29 (m, 4H). C NMR (100 MHz, CDCl3) δ 163.8, 161.2 (d, J = 2 Hz), 159.1, 155.6, 147.7 (d, J = 3 Hz), 139.2, 114.5, 107.2, 96.5, 56.2, 55.7, 34.8, 33.9, 31.6, 29.2, 28.9; 19F + + NMR (376 MHz, CDCl3) δ 51.49. HRMS (ESI ): m/z [M] calc. for C16H21FO3: 303.1367, found: 303.1364.

(-)-(R)-But-3-en-2-yl-6-(hept-6-en-1-yl)-2,4-dimethoxybenzoate (23): To a stirred

O O aqueous solution (4 mL) of ester 33 (138 mg, 0.78 mmol,

O 97% ee) was added KOH (439 mg, 7.8 mmol). The resulting mixture was stirred overnight, extracted with O Et2O (3x 5 mL), and the combined organic layers were dried over Na2SO4, and filtered. The crude alcohol 31 was used for the next step without complete removal of the solvent in order to avoid loss due to evaporation of the volatile alcohol.

1 H NMR (400 MHz, CDCl3) δ 5.92 (ddd, J = 17.2, 10.4, 5.8 Hz, 1H), 5.24 – 5.19 (m, 1H), 5.12 – 4.97 (m, 1H), 4.40 – 4.17 (m, 1H), 1.57 (br, 1H), 1.28 (d, J = 6.4 Hz, 3H). To a stirred solution of alcohol 31 (0.78 mmol) in 2 mL of THF was added NaHMDS (0.78 mL, 0.78 mmol, 1 M in THF) at 0 oC and the solution was stirred for 10 min. A THF solution (2 mL) of acid fluoride 34 (30.0 mg, 0.10 mmol) was added slowly; the mixture was warmed to room temperature and stirred overnight. The reaction was quenched with aq. sat. NH4Cl and extracted with Et2O (3x 10 mL), and the combined organic layers were dried over Na2SO4, filtered, concentrated and purified by flash

38 Chapter 2 chromatography (eluent pentane/ether =20/1) to give 23 as a colorless oil (30.2 mg, 0.09 mmol, 90%).

1 H NMR (400 MHz, CDCl3) δ 6.31 (s, 2H), 5.92 (ddd, J = 16.7, 10.6, 5.9 Hz, 1H), 5.85 – 5.72 (m, 1H), 5.63 – 5.59 (m, 1H), 5.36 (d, J = 17.3 Hz, 1H), 5.18 (d, J = 10.5 Hz, 1H), 5.04 – 4.88 (m, 2H), 3.79 (s, 3H), 3.78 (s, 3H), 2.58 – 2.53 (m, 2H), 2.04 – 2.02 (m, 2H), 1.63 – 1.56 (m, 2H), 1.42 (d, J = 6.5 Hz, 3H), 1.48 – 1.18 (m, 4H). 13C NMR (100 MHz,

CDCl3) δ 162.8, 156.5, 153.2, 137.9, 134.2, 132.9, 111.9, 111.3, 109.5, 101.0, 91.5, 66.9, 51.0, 50.6, 29.0, 28.9, 26.4, 24.3, 24.0, 15.2. HRMS (ESI+): m/z [M+Na]+ calc. for

C20H28NaO4: 355.1880, found: 355.1862. [α]D= –7.4 (c= 1.1, CHCl3).

(+)-(R)-12,14-Dimethoxy-3-methyl-3,4,5,6,7,8,9,10-octahydro-1H-benzo[c][1]oxacycl ododecin-1-one (22): To a stirred solution of compound 23 (13.0 mg, 0.04 mmol) in toluene (30 mL) was added Hoveyda-Grubbs 2nd generation OMe O catalyst ((1,3-bis-(2,4,6-trimethylphenyl)-2-imidazolidinylidene) O dichloro(o-isopropoxyphenylmethylene)ruthenium, 1.30 mg, 2.00 MeO mol, 5 mol%). The resulting solution was heated to reflux for 10

min under N2. The solvent was removed and the product was purified by flash chromatography (eluent pentane/ether) to give 35 as a green oil (9.50 mg, 32.0 mol, 80%, E/Z =1/2) which was difficult to purify, so they were used immediately for the next step. To a stirred solution of compound 35 (9.50 mg, 0.03 mmol) in 2 mL of EtOAc was added

Pd/C (10.0 mg) and the resulting solution was treated with 1 atm of H2. The mixture was stirred overnight and the solvent was removed, purified by flash chromatography (eluent pentane/ether =10/1) to give 22 as a colorless oil (9.6 mg, 0.03 mmol, 100%). The NMR data are in agreement with those reported previously.

1 H NMR (400 MHz, CDCl3) δ 6.30 (d, J = 2.2 Hz, 1H), 6.32 (d, J = 2.2 Hz, 1H), 5.31 – 5.25 (m, 1H), 3.80 (s, 3H), 3.78 (s, 3H), 2.77 – 2.67 (m, 1H), 2.58 – 2.49 (m, 1H), 1.98 – 1.88 (m, 1H), 1.74 – 1.61 (m, 4H), 1.36 – 1.60(m, 5H), 1.32 (d, J = 6.5 Hz, 3H), 1.28 13 –1.24 (m, 2H). C NMR (100 MHz, CDCl3) δ 168.5, 161.1, 157.7, 142.7, 118.9, 105.8, 96.3, 72.0, 55.9, 55.3, 32.3, 30.6, 30.1, 26.5, 25.4, 24.2, 21.2, 19.5. HRMS (ESI+): m/z + [M+H] calc. for C18H27O4: 307.1904, found: 307.1897. [α]D= +10.5 (c= 0. 1, CHCl3) 11d 11b 12 [Lit = +8.7 (c= 1.63, CHCl3), Lit = +9 (c= 1.0, CHCl3), Lit = +4.2 (c= 0.18,

CHCl3)].

39 Chapter 2

2.7 Reference and notes

1. (a) S. A. Hitchcock, G. Pattenden, J. Chem. Soc. Perkin Trans. 1, 1992, 1323–1328. (b) E. Keinan, S. Sinha, A. Sinhabachi, J. Chem. Soc. Perkin Trans.1, 1991, 3333–3339. (c) K. C. Nicolaou, N. Winssinger, J. Pastor, F. Murphy, Angew. Chem., 1998, 110, 2677; Angew. Chem. Int. Ed., 1998, 37, 2534–2537.(d) D. Taub, N. N. Girotra, R. D. Hoffsommer, C. H. Kuo, H. L. Slates, S. Weber, N. L. Wendler, Tetrahedron, 1968, 24, 2443– 2461. 2. M. P. Baggelaar, Master Report, University of Groningen, 2012. 3. W. H. Utian, Br. Med. J., 1973, 1, 579–581. 4. V. Helwig, A. Mayer-Bartschmid, H. Mueller, G. Greif, G. Kley mann, W. Zitzmann, H. V. Tichy, M. Stadler, J. Nat. Prod.,2003, 66, 829. 5. M. Isaka, C. Suyarnsestakorn, M. Tanticharoen, P. Kongsaeree, Y. Thebtaranonth, J. Org. Chem., 2002, 67, 1561. 6. (a) D. C. Aldridge, S. Galt, D. Giles, B. Turner, J. Chem. Soc. C., 1971, 1623. (b) R. C. Cambie, A. R. Lal, P. S. Rutledge, P. D. Woodgate, Phytochemistry, 1991, 30, 287. (c) K.-H. Lee, N. Hayashi, M. Okano, I. H. Hall, R.-Y. Wu, A. T. McPhail, Phytochemistry, 1982, 21, 1119. 7. Y. Xin-Sheng, Y. Ebizuka, H. Noguchi, F. Kiuchi, Y. Litaka, U. Sankawa, H. Seto, Tetrahedron Lett., 1983, 2407. 8. P. M. Dewick, Medicinal Natural Products: a biosynthetic approach, Wiley, 2009. 9. (a) T. Kashima, K. Takahashi, H. Matsuura, K. Nabeta, Biosci. Biotechnol. Biochem., 2009, 73, 1118. (b) T. Kashima, K. Takahashi, H. Matsuura, K. Nabeta, Biosci. Biotechnol. Biochem., 2009, 73, 2522. 10. (a) S. J. Danishefsky, S. J. Etheredge, J. Org. Chem., 1979, 44, 4716–4717. (b) A. Fürstner, O. R. Thiel, N. Kindler, B. Bartkowska, J. Org. Chem., 2000, 65, 7990–7995. (c) H. Gerlach, A. Thalmann, Helv. Chim. Acta., 1977, 60, 2866– 2866. (d) T. Takahashi, K. Kasuga, J. Tsuji, Tetrahedron Lett., 1978, 19, 4917–4920. 11. (a) F. Bracher, B. J. Schulte, J. Chem. Soc. Perkin Trans. 1, 1996, 2619–2622. (b) M. Braun, U. Mahler, S. Houben, Liebigs Ann. Chem., 1990, 513–517. (c) A. Fürstner, N. Kindler, Tetrahedron Lett., 1996, 37, 7005–7008. (d) A. Fürstner, G. Seidel, N. Kindler, Tetrahedron, 1999, 55, 8215–8230. 12. A. R. Solladié, M. C. Carreno, J. L. G. Ruano, Tetrahedron: Asymmetry, 1990, 1, 187–198. 13. G. B. Jones, R. S. Huber, Synlett., 1993, 367–368. 14. M. Fuchs, M. Toesch, M. Schober, C. Wuensch, K. Faber, Eur. J. Org. Chem., 2013, 356–361. 15. K. Geurts, S. P. Fletcher, B. L. Feringa, J. Am. Chem. Soc., 2006, 128, 15572–15573. 16. H. Abe, K. Nishioka, S. Takeda, M. Arai, Y. Takeuchi, T. Harayama, Tetrahedron Lett., 2005, 46, 3197.

40 Chapter 2

17. S. R. Chemler, D. Trauner, S. J. Danishefsky, Angew. Chem. Int. Ed., 2001, 40, 4544. 18. L. S. -M. Wong, M. S. Sherburn, Org. Lett., 2003, 5, 3603. 19. A. Fürstner, M. Bindl, L. Jean, Angew. Chem. Int. Ed., 2007, 46, 9275. 20. G. A. Olah, M. Nojima, I. Kerekes, Synthesis, 1973, 487. 21. M. T. Crimmins, E. A. Tabet, J. Am. Chem. Soc., 2000, 122, 5473. 22. K. J. Quinn, J. M. Curto, K. P. McGrath, N. A. Biddick, Tetrahedron Lett., 2009, 50, 7121. 23. The final product 22 seems unstable upon storing at room temperature.

41 Chapter 2

42 Chapter 3

Chapter 3

A Concise Asymmetric Synthesis of (–)‐Rasfonin

In this chapter the catalytic asymmetric formal synthesis of (–)-Rasfonin is described. CuBr/JosiPhos catalyzed iterative asymmetric conjugate addition of MeMgBr and Feringa’s butenolide are the key strategic elements in this synthesis.

Parts of this chapter have been published: Y. Huang, A. J. Minnaard, B. L. Feringa, Org. Biomol. Chem., 2012, 10, 29–31.

43 Chapter 3

3.1 Introduction

Natural products containing α-pyranones (δ-lactones) show a variety of interesting biological properties (Figure 1). For example, PD 113.271, which was isolated by the group of Tunac in 1983, exhibits promising cytotoxic activities.1 EBC-23, which was found in the fruit of Cinnamomum laubatii, shows excellent in vitro anticancer activities.2 (+)-Goniotriol, which belongs to the styryllactones, shows significant cytotoxicity against several human tumor cell lines.3 As a prime example, rasfonin 1,4 isolated from the fungus Trichurus terrophilus and the fermented mycelium of Taleromyces species 3565-A1, has been reported as an active apoptosis inducer in ras-dependent cells. In connection with this finding, significant proliferation suppression of mouse splenic lymphocytes stimulated with mitogens, concanavalin A and lipopolysaccharide, was reported.

OH

HO

H HO OH O O HO OH O O P OH O Ph - + OH OH O Na O H O O HO H O O 12 EBC-23 (+)-Goniotriol PD 113.271

O O

O OH O

OH Rasfonin 1

Figure 1. Natural products containing α‐pyranones.

2.2 Previous total syntheses of rasfonin

Due to the potential use of (–)-1 in the development of cancer chemotherapeutics, a versatile synthetic route to rasfonin is required to establish which parts of the molecule are important for activity, and to pin down its target protein(s). The first total synthesis of (–)-1, reported by Ishibashi and co-workers in 2003, aimed at structure elucidation and absolute configuration determination.5 A second synthesis, reported by the group of Boeckman in 2006,6 was based on the use of camphor lactam chiral auxiliaries (2a, 2b, 44 Chapter 3

2c) in order to allow the synthesis of different stereoisomers and cationic chiral oxazaborolidine catalyst 3 in the key assembly of butenolide 4 via an asymmetric vinylogous Mukaiyama aldol addition (Figure 2).

Figure 2. Boeckman’s total synthesis of Rasfonin 1.

Recently, the group of Nanda7 reported a chemoenzymatic asymmetric synthesis of rasfonin 1. Enantioselective enzymatic desymmetrization (EED) to form ester 5 and 6 and Gluconobacter oxydans mediated oxidative kinetic resolution (OKR) to prepare 7 have been applied for the introduction of 3 stereocenters (Figure 3).

45 Chapter 3

Figure 3. Nabda’s methodology to the synthesis of rasfonin 1.

As no follow-up appeared in chemical biology, we felt that a concise synthesis of 1, taking advantage of highly efficient and selective catalytic methods and making this compound and analogs more readily available could greatly stimulate biological studies. Herein, we report the asymmetric synthesis of (–)-1 in a highly efficient and selective manner.

2.3 Total synthesis of Rasfonin 2.3.1 Retrosynthetic analysis

O O O TBDPSO S O O OH 8 10 O OH O O OTBS O 1 OH HO O OMenthyl 9 OTBS 11

Figure 4. Retrosynthesis of Rasfonin 1.

In our retrosynthetic analysis, (–)-rasfonin 1 was disconnected into upper half 8 and lower half 9. The former was planned to be obtained from 10, in turn prepared via our iterative catalytic asymmetric conjugate addition protocol to deoxypropionates,8 in combination with a stereospecific Achmatowicz rearrangement. The lower part, 9, should in principle be accessable starting from readily available enantiopure Feringa’s menthyl butenolide 11.9

46 Chapter 3

2.3.2 Synthesis of upper half of Rasfonin

Figure 5. Synthesis of the upper half of Rasfonin 1.

The synthesis of the upper half of Rasfonin 1 started with the preparation of syn-1,3-dimethyl thioester 10 in an excellent 57% overall yield starting from ethylene glycol 12 by CuBr/JosiPhos catalyzed iterative asymmetric conjugate addition of MeMgBr.10 Reduction of 10 with DIBAL afforded the corresponding aldehyde which was used immediately to form the terminal alkyne 13 by addition of lithiated trimethylsilyldiazomethane involving a Colvin rearrangement (Figure 6).11 Vinyl iodide 14 was subsequently prepared by Zr-catalyzed methylalumination,12 followed by Negishi 11 cross coupling with ZnMe2 to afford alkene 15, which after deprotection with TBAF provided alcohol 16 in 87% yield. Ley oxidation13 of 16 afforded the corresponding aldehyde which was treated with 2-furyl lithium to give the corresponding furyl alcohol in 88% yield (syn/anti= 2/1). Subsequent Ley oxidation of this mixture gave ketone 17 in 98% yield which was in turn treated with (S)-CBS reagent and borane14 to afford furyl alcohol 18 in an excellent 94% yield and a syn/anti ratio >98:2. Stereospecific Achmatowicz rearrangement (Figure 7)15 of 18 with vanadyl acetylacetonate and

47 Chapter 3 tert-butylhydroperoxide afforded the hemi-acetal in 69% yield which was subsequently oxidized by Jones’ reagent16 to the corresponding ketolactone. Finally, Luche reduction17 gave the upper half 8 as the only diastereomer.

Figure 6. Mechanism of the Colvin rearrangement.

Figure 7. Mechanism of the Achmatowicz rearrangement.

2.3.3 Synthesis of lower half of Rasfonin 1

Figure 8. Preparation of vinyl iodide 21.

The synthesis of lower half of Rasfonin 1 started with the preparation of vinyl iodide 20. Zr-catalyzed methylalumination (Figure 8) of alcohol 19 afforded vinyl iodide 20 in only 53% yield, which was followed by protection to form the desired fragment 21 in high yield. Due to the low yield of the first step, the reverse synthesis sequence was tried (Figure 9). Protection using p-MeOBnCl afforded 22 in good yield, however, the subsequent Zr-catalyzed methylalumination of 22 gave deprotected product 20.

48 Chapter 3

Figure 9. Preparation vinyl iodide 21.

Fortunately, stannyl cupration18 solved the problem (Figure 10). By mixing CuCN, n-Bu3SnH and n-butyl lithium, the complex (Bu3Sn)2CuCNLi2 formed, followed by syn-addition on the alkyne to give vinyltin cuprate 23. Coupling with methyl iodide afforded vinyltin 24 followed by iodinolysis gave the desired compound 21 in overall 82% yield in one pot!

Figure 10. Stannyl cupration of alkyne 22.

Initial conjugate addition (Table 1) of vinyl iodide 21 on Feringa’s Butenolide 11 using nBuLi and CuI gave a complex mixture of products; both starting materials seemed to decompose during the reaction. Similar results were obtained by changing Li-source

(from nBuLi to tBuLi), using additives (nBu3P, TMSCl and BF3•Et2O) or even applying CuCN.19

49 Chapter 3

Table 1. Conjugate addition of vinyl iodide 21 on Feringa’s butenolide 11.

Entry Condition Yield

1 1. n‐BuLi, ‐78 oC; 2. CuI, ‐78 oC ‐‐

2 1. t‐BuLi, ‐78 oC; 2. CuI, ‐78 oC ‐‐

o o 3 1. t‐BuLi, ‐78 C; 2. CuI, n‐Bu3P, ‐78 C ‐‐

o o 4 1. t‐BuLi, ‐78 C; 2. CuBr•SMe2, TMSCl, ‐78 C ‐‐

o o 5 1. t‐BuLi, ‐78 C; 2. CuI, BF3•Et2O, ‐78 C ‐‐

6 1. t‐BuLi, ‐78 oC; 2. CuCN, ‐78 oC ‐‐

The new route towards the synthesis of lower half 9 started with Michael addition (Figure 11) of lithium bis(phenylthio)methane to butenolide 11 to provide trans-26 as the single diastereomer in 86% yield. Full reduction of 26 by LiAlH4 afforded diol 27 in 90% yield which after protection provided 28 in 96% yield. Unmasking dithiane 28 by HgCl2 and HgO in a mixture of acetonitrile and water went smoothly and afforded the free aldehyde 29 in 85% yield. Initial preparation of the internal alkyne 32 by Corey-Fuchs reaction20 gave only 40% overall yield which was due to the low yield of the first step to the formation of bis-bromide 31. The Bestmann-Ohira reaction21 gave similar results as Corey-Fuchs. Fortunately, Colvin rearrangement improved the yield by 20%.

50 Chapter 3

Figure 11. New route towards lower half of Rasfonin 1.

Initial transformation of the internal alkyne 32 to vinyl iodide 34 using stannyl cupration gave no conversion at all. The application of Schwartz’s hydrozirconation22 afforded a complex mixture of products at gram scale. However, hydrostannation23 improved the result. The first attempt (Table 2, entry 1) using 5 mol% Pd(PPh3)2Cl2 with nBu3SnH in pentane gave 33 in 33% yield as the only regioisomer after iodinolysis. Switching to a polar solvent (THF) didn’t improve the yield a lot. Interestingly it was observed that 1 equiv. of catalyst gave a similar yield as 5 mol% (entry 3), the catalyst seems deactivated after 5 min. Reformation the catalyst every 5 min also didn’t help. Fortunately, the method proposed by Semmelhack and Hooley24 strongly improved the outcome.

Switching to catalytic Pd(OAc)2 and tricyclohexyl phosphine, with hexane as the solvent, led to complete conversion and 80% isolated yield in 20 min!

51 Chapter 3

Table 2. Preparation of vinyl iodide 34 from internal alkyne 32.

Entry Conditions Yield (58)

1 Pd(PPh3)2Cl2 (5 mol%), nBu3SnH, n‐pentane 33%

2 Pd(PPh3)2Cl2 (5 mol%), nBu3SnH, THF 41%

3 Pd(PPh3)2Cl2 (1 equiv.), nBu3SnH, n‐pentane 40%

4 Pd(OAc)2(5 mol%), PCy3, nBu3SnH, n‐hexane 80%

Stille coupling of acid 3525 and 34 provided the lower half 9 in 87% yield. The coupling of the upper half 8 with the lower half 9 of (–)-rasfonin was achieved by Yamaguchi esterification26 in 80% yield. Desilylation initially was not satisfactory as treatment with camphorsulfonic acid gave only 40% yield. Fortunately, switching to aq. HF in acetonitrile27 resulted in quantitative formation of (–)-rasfonin 1.

Figure 12. Completion of the synthesis of the lower half and coupling with the upper half.

52 Chapter 3

2.4 Conclusion

In conclusion, a very efficient total synthesis of the apoptosis inducer (–)-rasfonin has been developed. This synthetic route took 21 linear steps with 10.8% overall yield (16 linear steps with 12.7% overall yield for Boeckman’s route, however, it contains a recycling sequence). CuBr/JosiPhos catalyzed iterative asymmetric conjugate addition of MeMgBr has been employed to install the stereogenic centers in the upper half side chain with excellent yield and stereoselectivity. The hydroxy-lactone core could be prepared by a subsequent stereospecific hydroxy-directed Achmatowicz rearrangement followed by an oxidation-reduction sequence. The synthesis of the lower half 8 makes use of the perfect transfer of chirality in the conjugate addition to butenolide 11 followed by selective construction of the E,E-diene-ester part. The availability of an effective route to rasfonin now allows to study its role in inhibiting the Ras signalling pathway, in addition, it provides access to functional analogs and might lead to the identification of its target protein.

2.5 Experimental section

Starting materials were purchased from Aldrich, Alfa Aesar or Acros and used as received unless stated otherwise. All solvents were reagent grade and, if necessary, dried and distilled prior to use. Column chromatography was performed on silica gel (Aldrich 60, 230-400 mesh) or on aluminium oxide (Merck, aluminium oxide 90 neutral activated).

TLC was performed on silica gel 60/Kieselguhr F254.

1H and 13C NMR spectra were recorded on a Varian VXR300 (299.97 MHz for 1H, 75.48 MHz for 13C) or a Varian AMX400 (399.93 MHz for 1H, 100.59 MHz for 13C) spectrometer in CDCl3 unless stated otherwise. Chemical shifts are reported in δ values 1 13 (ppm) relative to the residual solvent peak (CHCl3, H = 7.24, C = 77.0). Carbon assignments are based on 13C and APT 13C experiments. Splitting patterns are indicated as follows: s (singlet), d (doublet), t (triplet), q (quartet), m (multiplet), br (broad).

High resolution mass spectra (HRMS) were recorded on an AEI-MS-902 and FTMS orbitrap (Thermo Fisher Scientific) mass spectrometer. Optical rotations were measured on a Schmidt+ Haensch polarimeter (Polartronic MH8) with a 10 cm cell (c given in g/100 mL). Enantiomeric excess was determined by HPLC (Chiralcel OB, 250*4.6, 10 μm), (Chiralcel OD, 250*4.6, 10 μm).

53 Chapter 3

(–)-tert-Butyl((2S,4R)-2,4-dimethylhept-6-ynyloxy)diphenylsilane (13): To a stirred mixture of 108 (1.98 g, 4.52 mmol) in dry DCM (100 mL)

TBDPSO was added DIBALH (5.89 mL, 5.89 mmol, 1.0 M solution in DCM) at –65 °C under nitrogen. Stirring was continued until TLC showed complete conversion (2-3 h). The reaction mixture was quenched with 60 mL saturated aqueous Rochelle salt (potassium sodium tartrate) and stirred for 30 min. The phases were separated and the aqueous layer was extracted with DCM (3 x 50 mL).

The combined organic phases were dried over Na2SO4 and concentrated under reduced pressure to yield crude aldehyde which was purified by flash chromatography (eluent pentane/ether) to give the desired aldehyde used in the next step without complete removal of the eluent. To a THF (45 mL) solution of trimethylsilyldiazomethane (4.53 mL, 9.06 mmol, 2.0 M solution in hexanes) was added n-BuLi (3.26 mL, 8.16 mmol, 2.5 M solution in hexanes) at –78 ºC. After being stirred for 30 min, above aldehyde, dissolved in 20 mL of THF, was added. The mixture was stirred for 0.5 h at –78 ºC and then warmed to room temperature overnight. The mixture was quenched with saturated aqueous NH4Cl (20 mL) and extracted with Et2O (3 x 20 mL). The combined organic layers were dried over

Na2SO4, filtered, concentrated and purified by flash chromatography (eluent 1 pentane/ether) to give 13 as a colorless oil (1.46 g, 85%): H NMR (400 MHz, CDCl3) δ 7.69- 7.67 (m, 4H), 7.41- 7.38 (m, 6H), 3.52 (dd, J= 5.4, 9.8 Hz, 1H), 3.43 (dd, J= 6.3, 9.8 Hz, 1H), 2.19- 2.13 (m, 1H), 2.04- 1.98 (m, 1H), 1.93 (t, J= 2.7 Hz, 2H), 1.77- 1.69 (m, 2H), 1.53 (dd, J= 6.6, 13.7 Hz, 1H), 1.06 (s, 9H), 0.97 (d, J= 6.7 Hz, 3H), 0.94 (d, J= 13 6.7 Hz, 3H); C NMR (100 MHz, CDCl3) δ 135.9, 134.3, 129.7, 127.8, 83.4, 69.4, 69.0,

40.1, 33.4, 29.9, 27.1, 25.8, 20.3, 19.6, 17.7; HRMS (APCI+) calculated for C25H35OSi:

379.2452, found: 379.2448; [α]D = –9.2 (c= 1.6, CHCl3).

(-)-tert-Butyl ((2S, 4R, E)-7-iodo-2, 4, 6-trimethyl hept-6-enyloxy) diphenyl silane

(14): H2O (28.6 μL, 1.59 mmol) was added to a solution of Me3Al (7.95 mL, 15.9 mmol)

and ZrCp2Cl2 (233 mg, 0.79 mmol) in DCM (20 mL) at o TBDPSO I –78 C. The mixture was warmed to room temperature and stirred for 30 min, then cooled to –78 oC again. Alkyne 13 (1.20 g, 3.18 mmol) in 12 mL DCM was added slowly to the mixture. After stirring for 3 h at room temperature, the reaction mixture was treated with a solution of I2 (1.61 g, 6.36 mmol) in THF (10 mL) at –78oC. After stirring for 30 min at –78oC, the reaction mixture was warmed to room temperature, quenched with saturated aqueous Na2S2O3 and extracted with Et2O (3 x 20 mL). The combined organic layers were dried over Na2SO4, filtered, concentrated and purified by flash chromatography (eluent pentane/ether) to give 54 Chapter 3

1 14 as a colorless oil (1.46 g, 88%): H NMR (400 MHz, CDCl3) δ 7.70- 7.66 (m, 4H), 7.46- 7.37 (m, 4H), 5.82 (s, 1H), 3.50 (dd, J= 5.3, 9.9 Hz, 1H), 3.43 (dd, J= 6.2, 9.8 Hz, 1H), 2.19 (dd, J= 4.7, 13.3 Hz, 1H), 1.90 (dd, J= 9.3, 13.2 Hz, 1H), 1.77 (d, J= 1.0 Hz, 3H), 1.75- 1.66 (m, 2H), 1.39- 1.30 (m, 2H), 1.07 (s, 9H), 0.94 (d, J= 6.7 Hz, 3H), 0.77 13 (d, J= 6.5 Hz, 3H); C NMR (100 MHz, CDCl3) δ 147.4, 135.9, 135.8, 134.2, 129.8, 127.8, 75.4, 68.9, 47.7, 41.3, 33.4, 28.7, 27.1, 24.0, 20.1, 19.6, 18.1; HRMS (APCI+) calculated for C26H38OSi: 521.1731, found: 521.1721; [α]D = –8.7 (c= 6.2, CHCl3).

(–)-tert-Butyldiphenyl((2S,4R,E)-2,4,6-trimethyloct-6-enyloxy)silane (15): Me2Zn (2.3 mL, 2.67 mmol, 1.2 M in heptane) was added dropwise to

TBDPSO a THF solution (30 mL) of vinyl iodide 14 (1.17 g, 2.22

mmol) and Pd(PPh3)2Cl2 (79.0 mg, 0.112 mmol) at 0 ºC. The reaction mixture was allowed to warm to room temperature slowly, protected from light and stirred overnight. The reaction was quenched with water, diluted with ether and subsequently extracted with ether (3 x 30 mL), The combined organic layers were dried over Na2SO4, filtered, concentrated and purified by flash chromatography (eluent 1 pentane/ether) to give 15 as a colorless oil (0.854 g, 96%): H NMR (400 MHz, CDCl3) δ 7.69- 7.67 (m, 4H), 7.45- 7.35 (m, 6H), 5.16 (q, J= 7 Hz, 1H), 3.51 (dd, J= 5.1, 9.8 Hz, 1H), 3.42 (dd, J= 6.4, 9.8 Hz, 1H), 1.99 (d, J= 8.3 Hz, 1H), 1.80- 1.73 (m, 1H), 1.67- 1.61 (m, 2H), 1.57(d, J= 6.8 Hz, 3H), 1.53 (s, 3H), 1.38- 1.27 (m, 2H), 1.06 (s, 9H), 0.95 13 (d, J= 6.7 Hz, 3H), 0.75 (d, J= 6.1 Hz, 3H); C NMR (100 MHz, CDCl3) δ 135.9, 135.0, 134.4, 129.7, 127.8, 119.9, 69.1, 48.0, 41.4, 33.5, 28.4, 27.1, 20.3, 19.6, 18.2, 15.8, 13.6;

HRMS (APCI+) calculated for C27H41OSi: 409.2921, found: 409.2915; [α]D = –7.8 (c=

1.0, CHCl3).

(-)-(2S,4R,E)-2,4,6-Trimethyloct-6-en-1-ol (16): To a stirred mixture of 15 (0.856 g, 2.1 mmol) in THF (25 mL) was added TBAF (1.0 M solution in HO THF, 6.29 mL, 6.29 mmol). The resulting solution was stirred

for 4 h, and then quenched with sat. aq. NH4Cl and extracted with EtOAc (3 x 20 mL). The combined organic layers were dried over Na2SO4, filtered, concentrated and purified by flash chromatography (eluent pentane/ether) to give 16 as a 1 colorless oil (0.436 g, 87%): H NMR (400 MHz, CDCl3) δ 5.17 (dd, J= 6.2, 12.6 Hz, 1H), 3.52 (dd, J= 5.1, 10.5 Hz, 1H), 3.37 (dd, J= 6.8, 10.5 Hz, 1H), 2.04- 1.97 (m, 1H), 1.77- 1.63 (m, 3H), 1.57 (d, J= 8.5 Hz, 3H), 1.55 (s, 3H), 1.53 (br, 1H), 1.33- 1.25 (m, 13 2H), 0.93 ( d, J= 6.7 Hz, 3H), 0.81 (d, J= 6.1 Hz, 3H); C NMR (100 MHz, CDCl3) δ 134.8, 120.1, 68.5, 47.8, 41.2, 33.4, 28.3, 20.4, 17.7, 15.8, 13.6; HRMS (APCI+) calculated for C11H23O: 171.1743, found: 171.1739; [α]D = –4.2 (c= 1.1, CHCl3). 55 Chapter 3

(+)-(2S,4R,E)-1-(Furan-2-yl)-2,4,6-trimethyloct-6-en-1-one (17): To a stirred solution O of alcohol 16 (311 mg, 1.83 mmol) in DCM (15 mL) were O added molecular sieves 4Å (1.0 g), NMO (648 mg, 5.48 mmol) and TPAP (44.0 mg, 128 μmol). The reaction mixture was stirred at rt for 1 h, filtered through a silica pad, concentrated under reduced pressure and purified by flash chromatography (eluent pentane/ ether) to afford the aldehyde as a 1 colourless oil (277 mg, 90% yield): H NMR (400 MHz, CDCl3) δ 9.56 (d, J= 2.6 Hz, 1H), 5.18 (dd, J= 6.6, 13.2 Hz, 1H), 2.48- 2.41 (m, 1H), 1.96 (dd, J= 5.8, 13.0 Hz, 1H), 1.78- 1.62 (m, 4H), 1.57 (d, J= 6.7 Hz, 3H), 1.54 (d, J= 1.0 Hz, 3H), 1.08 (d, J= 5.5 Hz, 13 3H), 0.83 (d, J= 6.4 Hz, 3H); C NMR (100 MHz, CDCl3) δ 205.7, 134.2, 120.6, 47.9, 44.4, 38.3, 28.6, 19.9, 15.7, 14.5, 13.6. To a stirred solution of distilled furan (194 mg, 2.85 mmol) in THF was added n-BuLi (0.61 mL, 1.52 mmol) at –78 oC. The reaction mixture was stirred for 3 h, and then a THF solution (3 mL) of above aldehyde (160 mg, 0.95 mmol) was added slowly. The reaction mixture was warmed to room temperature and stirred for 1 h. The mixture was quenched with sat. aq. NH4Cl and extracted with ether (3 x 10 mL), The combined organic layers were dried over Na2SO4, filtered, concentrated and purified by flash chromatography (eluent pentane/ether) to give the alcohol as a brown oil (0.197 g, 88%). To a stirred solution of above alcohol (166 mg, 0.702 mmol) in DCM (15 mL) were added molecular sieves 4Å (0.6 g), NMO (249 mg, 2.11 mmol) and TPAP (12 mg, 35 μmol). The reaction mixture was stirred at rt for 1 h, filtered through a silica pad, concentrated under reduced pressure and purified by flash chromatography (eluent pentane/ ether) to afford 17 as a brown oil (161 mg, 98% yield): 1H NMR (400 MHz,

CDCl3) δ 7.58 (s, 1H), 7.18 (d, J= 3.5 Hz, 1H), 6.52 (dd, J= 1.7, 3.5 Hz, 1H), 5.16 (dd, J= 6.6, 13.3 Hz, 1H), 3.41- 3.32 (m, 1H), 1.98 (dd, J= 6.1, 13.1 Hz, 1H), 1.85 (ddd, J= 5.4, 8.8, 14.1 Hz, 1H), 1.72 (dd, J= 8.2, 13.1 Hz, 1H), 1.64- 1.48 (m, 1H), 1.55(d, J= 6.6 Hz, 3H), 1.49 (s, 3H), 1.17 (d, J= 6.9 Hz, 3H), 1.13- 1.08 (m, 1H), 0.79 (d, J= 6.6 Hz, 13 3H); C NMR (100 MHz, CDCl3) δ 193.8, 152.8, 146.6, 134.6, 120.2, 117.3, 112.3, 48.2,

40.9, 39.4, 28.8, 20.0, 18.4, 15.6, 13.6; HRMS (APCI+) calculated for C15H23O2:

235.1693, found: 235.1685; [α]D = +30.5 (c= 0.4, CHCl3).

(–)-(1R,2S,4R,E)-1-(Furan-2-yl)-2,4,6-trimethyloct-6-en-1-ol (18): To a stirred

OH solution of (S)-2-methyl-CBS- oxazaborolidine (43 μL , 0.043 O mmol, 1.0 M solution in THF) in THF (0.6 mL) was added borane-dimethylsufide complex (47 μL, 0.094 mmol, 2 M 56 Chapter 3 solution in THF) followed by a solution of 17 (20 mg, 0.085 mmol) in THF (2 mL) at

0 °C and under N2. After 4 h, the mixture was quenched with sat. aq. NH4Cl and extracted with ether (3 x 10 mL), The combined organic layers were dried over Na2SO4, filtered, concentrated and purified by flash chromatography (eluent pentane/ether) to give 1 18 as a colorless oil (14.9 mg, 94%, syn/anti>98/2): H NMR (400 MHz, CDCl3) δ 7.37 (s, 1H), 6.33 (dd, J= 1.8, 3.2 Hz, 1H), 6.22 (dd, J= 0.6, 3.2 Hz, 1H), 5.15 (q, J= 7 Hz, 1H), 4.51 (d, J= 5.6 Hz, 1H), 2.10- 1.99 (m, 2H), 1.57 (d, J= 6.9 Hz, 3H), 1.54 (s, 3H), 1.33- 1.21 (m, 4H), 0.97 (d, J= 6.8 Hz, 3H), 0.79 (d, J= 6.1 Hz, 3H); 13C NMR (100 MHz,

CDCl3) δ 156.6, 141.8, 134.8, 120.1, 110.3, 106.5, 72.3, 47.4, 41.0, 35.8, 28.4, 20.5, 15.8,

15.6, 13.6; HRMS (ESI+) calculated for C15H25O2: 237.1854, found: 237.0879; [α]D =

–5.4 (c= 0.7, CHCl3).

(-)-(5R,6R)-6-((2S,4R,E)-4,6-Dimethyloct-6-en-2-yl)-5-hydroxy-5,6-dihydro-2H-pyra n-2-one (8): To a stirred solution of furyl alcohol 18 (14.9 mg, 0.063 mmol) in DCM (1 mL) at 0 °C was added vanadyl acetylacetonate (0.8 mg, O O 0.003 mmol) followed by dropwise addition of

OH tert-butylhydroperoxide (0.012 mL, 0.063 mmol, 5.5 M in decane). The solution was warmed to room temperature and stirred for 40 min. The mixture was filtered through a silica pad, concentrated under reduced pressure and the residue was purified by flash chromatography (eluent pentane/ ether) to afford the hemiacetal as a colourless oil (7.3 mg, 69% yield). Jones’ reagent (0.021 mL, 2.7 M) was added dropwise to an ice-cold solution of above hemiacetal (7.0 mg, 0.029 mmol) in acetone (1 mL). The resulting mixture was stirred for 1 h at room temperature. The mixture was diluted with tert-butyl methyl ether (5 mL) and washed with water; the organic phase was dried over Na2SO4, filtered, and the solvent was evaporated to give the crude product keto-lactone which was used directly for the 1 next step: H NMR (400 MHz, CDCl3) δ 6.90 (d, J= 10.2 Hz, 1H), 6.78 (d, J= 10.2 Hz, 1H), 5.19 (dd, J= 6.6, 13.0 Hz, 1H), 4.90 (d, J= 2.4 Hz, 1H), 2.46- 2.37 (m, 1H), 2.02 (d, J= 5.2, 12.4 Hz, 1H), 1.77- 1.53 (m, 4H), 1.58 (d, J= 7.0 Hz, 3H), 1.56 (s, 3H), 0.88 (d, J= 6.8 Hz, 3H), 0.84 (d, J= 6.3 Hz, 3H).

To a solution of above keto-lactone in 1 mL of DCM was added NaBH4 (1.5 mg, 0.039 o mmol) and CeCl3 (0.13 mL, 0.052 mmol, 0.4 M solution in MeOH) at –78 C. The mixture was stirred for 0.5 h, warmed to room temperature and diluted with ether. The mixture was quenched with 5 mL H2O, and extracted with ether (3 x 3 mL), The combined organic layers were dried over Na2SO4, filtered, concentrated and purified by flash chromatography (eluent pentane/ether) to give 8 as a colorless oil (6 mg, 80% based 1 on the keto-lactone): H NMR (400 MHz, CDCl3) δ 7.01 (dd, J= 6.1, 9.6 Hz, 1H), 6.10 (d, 57 Chapter 3

J= 9.6 Hz, 1H), 5.20 (dd, J= 6.0, 11.3 Hz, 1H), 4.22 (d, J= 4.1 Hz, 1H), 3.91 (dd, J= 2.3, 9.3 Hz, 1H), 2.27- 2.13 (m, 2H), 1.85- 1.73 (m, 1H), 1.58 (s, 6H), 1.45- 1.38 (m, 1H), 1.28- 1.22 (m, 1H), 1.14 (d, J= 6.5 Hz, 3H), 1.07- 0.98 (m, 1H), 0.84 (d, J= 6.5 Hz, 3H); 13 C NMR (100 MHz, CDCl3) δ 164.4, 144.6, 134.8, 123.2, 120.3, 85.6, 60.9, 46.6, 40.1,

31.5, 28.2, 21.2, 16.0, 15.8, 13.6; HRMS (ESI+) calculated for C15H24O3Na: 275.1617, found: 275.1616; [α]D = –117.1 (c= 0.6, CHCl3).

(-)-(4R,5R)-4-(Bis(phenylthio)methyl)-5-((1R,2S,5R)-2-isopropyl-5-methylcyclohexyl oxy)dihydrofuran-2(3H)-one (26): To a stirred solution of bis(phenylthio)methane (1.95

O g, 8.4 mmol) in 50 mL THF was added n-BuLi (4.73 mL, 7.56 mmol) dropwise at –78oC. The reaction mixture was stirred for 0.5 O PhS h, followed by the addition of 11 in 20 mL THF. The mixture was OMenthyl SPh quenched with sat. aq. NH4Cl after TLC showed complete conversion (2- 3 h). The mixture was extracted with ether (3 x 50 mL). The combined organic layers were dried over Na2SO4, filtered, concentrated and the residue was purified by flash chromatography (eluent pentane/ether) to give 26 as a white solid (1.71 1 g, 86% as the single diastereomer): H NMR (400 MHz, CDCl3) δ 7.50- 7.31 (m, 10H), 5.86 (s, 1H), 4.34 (d, J= 5.2 Hz, 1H), 3.52 (dt, J= 4.2, 10.7, 10.6 Hz, 1H), 2.87- 2.68 (m, 3H), 2.11- 1.98 (m, 2H), 1.68- 1.58 (m, 2H), 1.44- 1.29 (m, 1H), 1.23- 1.16 (m, 1H), 1.06- 0.81 (m, 3H), 0.93 (d, J= 6.5 Hz, 3H), 0.86 (d, J= 7.0 Hz, 3H), 0.77 (d, J= 6.9 Hz, 13 3H); C NMR (100 MHz, CDCl3) δ 175.2, 133.9, 133.4, 133.3,133.3, 133.1, 129.5, 129.5, 128.9, 128.7, 102.4, 77.7, 61.3, 48.0, 46.7, 40.1, 34.5, 32.4, 31.6, 25.7, 23.3, 22.5,

21.1, 15.9; HRMS (APCI+) calculated for C21H29O3S: 361.1832, found: 361.1812; [α]D =

–83.5 (c= 1.3, CHCl3).

(+)-(R)-2-(Bis(phenylthio)methyl)butane-1,4-diol (27): To a stirred solution of 26 (1.51

OH g, 3.2 mmol) in 100 mL THF was added LiAlH4 (3.2 mL, 12.8 mmol, 4 M solution in diethylether) dropwise at 0 oC. The mixture was stirred OH PhS for an additional 0.5 h at 0 oC and then warmed to room temperature. SPh The mixture was quenched with 10 mL H2O after complete conversion shown by TLC. The resulting mixture was subsequently filtered, and extracted with ether

(3 x 20 mL). The combined organic layers were dried over Na2SO4, filtered, concentrated and the residue was purified by flash chromatography (eluent pentane/ether) to give 27 as 1 a white solid (0.856 g, 90%): H NMR (400 MHz, CDCl3) δ 7.45- 7.24 (m, 10H), 4.67 (d, J= 3.5 Hz, 1H), 3.83 (t, J= 6.1 Hz, 2H), 3.78- 3.71 (m, 1H), 3.65- 3.58 (m, 1H), 3.22 (br, 1H), 2.92 (br, 1H), 2.23- 2.20 (m, 1H), 2.10- 2.07 (m, 1H), 1.82- 1.70 (m, 1H); 13C NMR

(100 MHz, CDCl3) δ 134.9, 134.8, 132.7, 132.5, 129.3, 128.0, 127.9, 64.6, 63.0, 61.6, 58 Chapter 3

44.3, 32.5; HRMS (ESI+) calculated for C17H19O2S2: 319.0826, found: 319.0801; [α]D =

+24.5 (c= 0.2, CHCl3).

(+)-(R)-6-(Bis(phenylthio)methyl)-2,2,3,3,10,10,11,11-octamethyl-4,9-dioxa-3,10-disil adodecane (28): To a solution of 27 (0.752 g, 2.35 mmol) in anhydrous dichloromethane

OTBS (30 mL) was added imidazole (1.28 g, 18.8 mmol) followed by tert-butyl-dimethylsilyl chloride (2.83 g, 18.8 mmol), and the OTBS PhS resulting white suspension was stirred at rt overnight. The reaction SPh mixture was quenched with 20 mL of water and extracted with ether

(3 x 20 mL). The combined organic layers were dried over Na2SO4, filtered, concentrated and purified by flash chromatography (eluent pentane/ether) to give 28 as a colorless oil 1 (1.24 g, 96%): H NMR (400 MHz, CDCl3) δ 7.47- 7.20 (m, 10H), 5.05 (d, J= 2.7 Hz, 1H), 3.82- 3.79 (m, 2H), 3.66 (t, J= 6.1 Hz, 2H), 2.30- 2.27 (m, 1H), 2.11- 2.05 (m, 1H), 1.55- 1.44 (m, 1H), 0.87- 0.85 (m, 18H), 0.02- 0.00 (m, 12H); 13C NMR (100 MHz,

CDCl3) δ 136.2, 135.7, 131.8, 131.3, 129.0, 127.3, 127.0, 63.2, 62.0, 60.5, 43.3, 30.5,

26.1, 26.0, 18.5, 18.3, -5.2, -5.1; HRMS (APCI+) calculated for C23H44O2SSi2: 440.2595, found: 440.2538; [α]D = +44.9 (c= 0.8, CHCl3).

(+)-(S)-6-Ethynyl-2, 2, 3, 3, 10, 10, 11,11-octamethyl-4,9-dioxa-3,10- disilado decane (30): To a solution of thioacetal 28 (4.00 g, 7.29 mmol) in acetonitrile-water (4:1, 50 mL)

OTBS at room temperature was added HgCl2 (3.96 g, 14.6 mmol) and HgO (3.16 g, 14.6 mmol), and the mixture was stirred for 3 h. Subsequent OTBS fitration through Celite, followed by removal of the solvents under reduced pressure. Purification by flash chromatography (eluent 1 pentane/ether) gave 29 as a colorless oil (2.14 g, 85%): H NMR (400 MHz, CDCl3) δ 9.74 (d, J= 1.7 Hz, 1H), 3.93- 3.82 (m, 2H), 3.73- 3.62 (m, 2H), 2.63- 2.57 (m, 1H), 2.01- 1.91 (m, 1H), 1.75- 1.65 (m, 1H), 0.87 (s, 18 H), 0.04 (s, 12H). To a THF solution (45 mL) of trimethylsilyldiazomethane (10.93 mL, 21.86 mmol, 2.0 M solution in hexanes) was added n-BuLi (12.8 mL, 20.4 mmol, 1.6 M solution in hexanes) at –78 ºC. After being stirred for 30 min, a solution of 29 in THF (20 mL) was added. The mixture was stirred for 0.5 h at –78 ºC and then warmed to room temperature overnight.

The mixture was quenched with saturated aqueous NH4Cl (20 mL) and extracted with

Et2O (3 x 20 mL). The combined organic layers were dried over Na2SO4, filtered, concentrated and the product was purified by flash chromatography (eluent pentane/ether) 1 to give 30 as a colorless oil (1.42 g, 67%): H NMR (400 MHz, CDCl3) δ 3.80- 3.75 (m, 2H), 3.71 (dd, J= 5.7, 9.7 Hz, 1H), 3.58 (dd, J= 7.3, 9.7 Hz, 1H), 2.73- 2.63 (m, 1H), 2.03 (d, J= 2.4 Hz, 1H), 1.93- 1.82 (m, 1H), 1.61- 1.49 (m, 1H), 0.89 (s, 18H), 0.06 (s, 12H); 59 Chapter 3

13 C NMR (100 MHz, CDCl3) δ 85.4, 70.1, 66.1, 60.9, 34.5, 31.6, 26.2, 26.1, 18.5, -5.1,

-5.1; HRMS (APCI+) calculated for C18H39O2Si2: 343.2483, found: 343.2472; [α]D =

+5.7 (c= 0.9, CHCl3).

(+)-(S)-2,2,3,3,10,10,11,11-Octamethyl-6-(prop-1-ynyl)-4,9-dioxa-3,10-disiladodecane (32): Alkyne 30 (1.40 g, 4.09 mmol) was dissolved in dry THF (20 mL) and n-BuLi (5.1 o OTBS mL, 8.17 mmol) was added at –78 C under N2. After 10 min, MeI (1.78 mL, 4.06 g, 28.6 mmol) was added. The resulting solution was OTBS allowed to warm to 0 oC over 2 h, after which TLC showed complete conversion. The reaction was quenched with saturated aqueous

NH4Cl (20 mL) and the mixture was extracted with Et2O (3 x 20 mL). The combined organic layers were dried over Na2SO4, filtered, concentrated and the residue was purified by flash chromatography (eluent pentane/ether) to give 32 as a colorless oil (1.33 1 g, 91%): H NMR (400 MHz, CDCl3) δ 3.83- 3.72 (m, 2H), 3.67 (dd, J= 5.6, 9.7 Hz, 1H), 3.50 (dd, J= 7.3, 14.1 Hz, 1H), 2.65- 2.52 (m, 1H), 1.93- 1.82 (m, 1H), 1.78 (d, J= 2.3 Hz, 13 3H), 1.53- 1.41 (m, 1H), 0.89 (s, 18H), 0.05 (s, 12H); C NMR (100 MHz, CDCl3) δ 80.4, 80.0, 66.6, 61.3, 34.9, 31.9, 26.2, 26.1, 18.6, 3.7, -5.1; HRMS (APCI+) calculated for C19H40O2Si2Na: 379.2459, found: 379.2447; [α]D = +30.2 (c= 0.9, CHCl3).

(+)-(S,E)-6-(2-Iodoprop-1-enyl)-2,2,3,3,10,10,11,11-octamethyl-4,9-dioxa-3,10-disilad odecane (34): To an oven-dried, nitrogen-filled flask was added Pd(OAc)2 (15.71 mg, OTBS 0.07 mmol, 5 mol%) and tricyclohexylphosphine (39.26 mg, 0.14

OTBS mmol, 10 mol%) followed by freshly distilled hexane (25 mL) and I the resulting mixture was stirred for 20 min until the solids were dissolved. Alkyne 32 (500 mg, 1.40 mmol) in hexane (10 mL) was added slowly, followed by slow addition of neat Bu3SnH (1.56 mL, 5.61 mmol) over 5 min. The reaction was finished after 20 min (TLC analysis) and the mixture was subsequently transferred to a silica gel column and rapidly eluted with hexane, followed by 1 hexane/ether to provide 33 as a colorless oil (711 mg, 80%): H NMR (400 MHz, CDCl3) δ 5.25 (dd, J= 1.1, 9.1 Hz, 1H), 3.66- 3.41 (m, 4H), 2.86- 2.73 (m, 1H), 1.92- 1.76 (m, 2H), 1.84 (s, 3H), 1.60- 1.43 (m, 7H), 1.36- 1.24 (m, 12H), 0.91- 0.83 (m, 9H), 0.88 (s, 13 18H), 0.03 (s, 12H); C NMR (100 MHz, CDCl3) δ 142.5, 139.6, 66.9, 61.6, 37.4, 35.2, 29.4, 27.6, 26.2, 26.1, 19.9, 18.5, 13.9, 9.3, -5.1. To a solution of 33 (622 mg, 0.96 mmol) in dichloromethane (20 mL) was added a o solution of I2 (487 mg, 1.92 mmol, 1.3 eq) in dichloromethane (15 mL) at –78 C under nitrogen. The resulting solution was stirred for 10 min at –78 oC and then allowed to warm to room temperature. The solution was quenched with saturated aqueous Na2S2O3 60 Chapter 3

and extracted with Et2O (3 x 20 mL). The combined organic layers were dried over

Na2SO4, filtered, concentrated and the product was purified by flash chromatography (eluent pentane/ether) to give 34 as a colorless oil (348.6 mg, 75%): 1H NMR (400 MHz,

CDCl3) δ 5.93 (d, J= 10.1 Hz, 1H), 3.66- 3.52(m, 2H), 3.49- 3.46 (m,2H), 2.74- 2.61 (m, 1H), 2.39 (s, 3H), 1.75- 1.63 (m, 1H), 1.43- 1.30 (m, 1H), 0.88 (s, 18H), 0.03 (s, 12H); 13 C NMR (75 MHz, CDCl3) δ 143.0, 95.5, 66.1, 60.8, 40.6, 34.3, 28.5, 26.2, 26.1, 18.5,

-5.2; HRMS (ESI+) calculated for C19H42O2Si2I: 485.1763, found: 485.1752; [α]D =

+28.5 (c= 1.2, CHCl3).

(+)-(S,2E,4E)-8-(tert-Butyldimethylsilyloxy)-6-((tert-butyldimethylsilyloxy)methyl)-4 -methylocta-2,4-dienoic acid (9): To a solution of vinyl iodide 34 (188.8 mg, 0.39 mmol) O and carboxylic acid 35 (288 mg, 0.779 mmol) in OTBS HO N-methylpyrrolidinone (7 mL) was added

OTBS diisopropylethylamine (339 μL, 1.95 mmol) and Pd2dba3 (38 mg, 0.039 mmol) at room temperature under nitrogen. The flask was covered with aluminum foil, and the reaction mixture was stirred overnight. The reaction was quenched with saturated aqueous NH4Cl (20 mL) and extracted with Et2O (3 x 20 mL).

The combined organic layers were dried over Na2SO4, filtered, concentrated and the product was purified by flash chromatography (eluent pentane/ether) to give 9 as a 1 colorless oil (145 mg, 87%): H NMR (300 MHz, CDCl3) δ 7.40 (d, J = 15.6 Hz, 1H), 5.80 (d, J = 15.8 Hz, 1H), 5.75 (d, J = 12.4 Hz, 1H), 3.62-3.47 (m, 4H), 2.89-2.88 (m, 1H), 1.82 (s, 3H), 1.54-1.37 (m, 2H), 0.87 (s, 18H), 0.01 (s, 12H); 13C NMR (75 MHz,

CDCl3) δ 173.3, 152.0, 145.2, 134.3, 115.3, 66.3, 61.0, 38.6, 34.8, 26.1, 26.0, 18.4, 13.8,

12.8, -5.1, -5.2; HRMS (ESI+) calculated for C22H44O4Si2Na: 451.7432, found: 451.2652;

[α]D = +39.4 (c= 0.6, CHCl3).

(-)-(S,2E,4E)-((2R,3R)-2-((2S,4R,E)-4,6-Dimethyloct-6-en-2-yl)-6-oxo-3,6-dihydro-2 H-pyran-3-yl)-8-(tert-butyldimethylsilyloxy)-6-((tert-butyldimethylsilyloxy)methyl)-4 -methylocta-2,4-dienoate (36): To a solution of acid 9 (12.76 mg, 0.030 mmol, 1.5 eq) in dry toluene (0.47 mL) was added HPLC grade O O triethylamine (8.3 μL, 0.059 mmol, 3.0 equiv) at O room temperature followed by dropwise addition of OTBS O 2,4,6-trichlorobenzoyl chloride (6.4 μL, 0.040

OTBS mmol, 2.0 equiv). The resulting solution was stirred for 1 h upon which TLC showed complete conversion. Alcohol 8 (5.0 mg, 0.02 mmol, 1.0 eq) in dry toluene (0.5 mL) was then added, followed by DMAP (8.5 mg, 0.069 mmol, 3.5 equiv). After 2 h, the mixture was transferred to a silica gel column and eluted with 61 Chapter 3

1 pentane/ether to yield 36 as a colorless oil (10.5 mg, 80%): H NMR (400 MHz, CDCl3) δ 7.34 (d, J = 15.7 Hz, 1H), 7.04 (dd, J = 6, 9.6 Hz, 1H), 6.20 (d, J = 9.6 Hz, 1H), 5.77 (d, J= 15.8 Hz, 1H), 5.73 (d, J= 5.5 Hz, 1H), 5.34 (dd, J= 2.4, 6.0 Hz, 1H), 5.11 (q, J = 7 Hz, 1H), 4.12 (dd, J= 2.4, 8.8 Hz, 1H), 3.63-3.44 (m, 4H), 2.89-2.78 (m, 1H), 2.22-2.13 (m, 1H), 2.05 (br d, J = 10.9 Hz, 1H), 1.79 (s, 3H), 1.74-1.61 (m, 1H), 1.52 (s, 3H), 1.48-1.36 (m, 1H), 1.30-0.94 (m, 11H), 0.87 (s, 9H), 0.86 (s, 9H), 0.78 (d, J = 6.5 Hz, 3H), 13 0.01-0.00 (m, 12H); C NMR (75 MHz, CDCl3): δ 166.6, 163.6, 151.8, 145.6, 140.8, 134.4, 134.2, 125.0, 120.3, 114.3, 83.5, 66.2, 61.9, 60.9, 46.6, 40.2, 38.6, 34.8, 31.7, 28.2, 26.1, 20.8, 18.5, 16.1, 15.5, 13.6, 12.8, -5.1 ; HRMS (ESI+) calculated for

C37H66O6Si2Na: 685.4290, found: 685.4245; [α]D = –157.1 (c= 0.5, CHCl3).

(-)-(S,2E,4E)-((2R,3R)-2-((2S,4R,E)-4,6-Dimethyloct-6-en-2-yl)-6-oxo-3,6-dihydro-2 H-pyran-3-yl)-8-hydroxy-6-(hydroxymethyl)-4-methylocta-2,4-dienoate (1): To a solution of 36 (3.9 mg, 0.0059 mmol) in MeCN (1 O O mL) was added one drop of HF (48 wt.% in H2O) at O room temperature. The resulting mixture was stirred OH O for 20 min, and then quenched with saturated

OH aqueous NH4Cl (5 mL) and extracted with Et2O (3 x

5 mL). The combined organic layers were dried over Na2SO4, filtered, concentrated and the product was purified by flash chromatography (eluent EtOAc/Heptane) to give 1 as a 1 colorless oil (2.6 mg, 100%): H NMR (400 MHz, CDCl3) δ 7.34 (d, J = 15.7 Hz, 1H), 7.04 (dd, J = 5.9, 9.5 Hz, 1H), 6.21 (d, J = 9.6 Hz, 1H), 5.81 (d, J = 15.8 Hz, 1H), 5.77 (d, J = 11.3 Hz, 1H), 5.35 (dd, J= 2.4, 6.0 Hz, 1H), 5.12 (q, J = 7 Hz, 1H), 4.12 (dd, J= 2.9, 6.7 Hz, 1H), 3.77- 3.71 (m, 1H), 3.65-3.57 (m, 2H), 2.93-2.84 (m, 1H), 2.25-2.13 (m, 1H), 2.05 (br d, J = 10.9 Hz, 1H), 1.84 (s, 3H), 1.80-1.75 (m, 1H), 1.71-1.66 (m, 1H), 1.65- 1.59 (m, 1H), 1.55 (d, J = 6.7 Hz, 3H), 1.53 (s, 3H), 1.44- 1.41 (m, 2H), 1.34- 1.24 (m, 5H), 1.15 (d, J = 6.6 Hz, 3H), 1.01, 0.78 (d, J = 6.6 Hz, 3H); 13C NMR (100 MHz,

CDCl3): δ 163.5, 160.7, 148.3, 140.7, 138.0, 132.1, 131.6, 122.4, 117.5, 112.5, 80.7, 63.3, 59.1, 58.1, 43.7, 37.4, 36.6, 32.1, 28.8, 27.7, 27.1, 18.0, 13.3, 10.8, 10.0; HRMS (ESI+) calculated for C25H38O6Na: 457.2561, found: 457.2531; [α]D = –164.8 (c= 0.1, CHCl3) [Lit.6= –162.8 (c=0.43, DCM), Lit.5b= –170 (c= 0.09, MeOH)]. The optical and spectroscopic data are in agreement with the reported values.5,6

62 Chapter 3

2.6 References

1. (a) D. S. Lewy, C.-M. Gauss, D. R. Soenen, D. L. Boger, Curr. Med. Chem., 2002, 9, 2005. (b) R. C. Jackson, D. W. Fry, T. J. Boritzki, B. J. Roberts, K. E. Hook, W. R. Leopold, Adv. Enzyme Regul., 1985, 23, 193. (c) W. Scheithauer, D. D. von Hoff, G. M. Clark, J. L. Shillis, E. F. Elslager, Eur. J. Clin. Oncol., 1986, 22, 921. 2. P. W. Reddell, V. A. Gordon, WO 2007070984 A1 20070628 PCT Int. Appl. 2007. 3. (a) M. A. Blazquez, A. Bermejo, M. C. Zafra-Polo, D. Cortes, Phytochem. Anal., 1999, 10, 161–170. (b) H. B. Mereyala, M. Joe, Curr. Med. Chem. Anti-Cancer Agents, 2001, 1, 293–300. (c) P. Tuchinda, B. Munyoo, M. Pohmakotr, P. Thinapong, S. Sophasan, T. Santisuk, V. Reutrakul, J. Nat. Prod., 2006, 69, 1728–1733. (d) Z. Tian, S. Chen, Y. Zhang, M. Huang, L. Shi, F. Huang, C. Fong, M. Yang, P. Xiao, Phytomedecine, 2006, 13, 181–186. 4. T. Tomikawa, K. Shin-Ya, K. Furihato, T. Kinoshita, A. Miyajima, H. Seto, Y. Hayakawa, J. Antibiot., 2000, 53, 848. 5. (a) K. Akiyama, S. Kawamoto, H. Fujimoto, M. Ishibashi, Tetrahedron Lett., 2003, 44, 8427. (b) K. Akiyama, S. Yamamoto, H. Fujimoto, M. Ishibashi, Tetrahedron, 2005, 61, 1827. 6. R. K. Boeckman, Jr., J. E. Pero, D. J. Boehmler, J. Am. Chem. Soc., 2006, 128, 11032. 7. R. Bhuniya, S. Nanda, Tetrahedron, 2013, 69, 1153–1165. 8. (a) R. D. Mazery, M. Pullez, F. López, S. R. Harutyunyan, A. J. Minnaard, B. L. Feringa, J. Am. Chem. Soc., 2005, 127 , 9966. (b) B. ter Horst, B. L. Feringa, A. J. Minnaard, Chem. Commun., 2010, 46, 2535. 9. (a) B. L. Feringa, B. D. Lange, J. C. de Jong, J. Org. Chem., 1989, 54, 2471. (b) A. van Oeveren, B. L. Feringa, J. Org. Chem., 1996, 61, 2920. (c) A. van Oeveren, J. F. G. A. Jansen, B. L. Feringa, J. Org. Chem., 1994, 59, 5999. 10. B. ter Horst, B. L. Feringa, A. J. Minnaard, Org. Lett., 2007, 9, 3013. 11. B. M. Trost, J. Waser, A. Meyer, J. Am. Chem. Soc., 2007, 129, 14556. 12. G. Zhu, E. Negishi, Chem. Eur. J., 2008, 14, 311. 13. O. Robles, F. E. McDonald, Org. Lett., 2009, 11, 5498. 14. G. E. Keck, C. E. Knutson, S. A. Wiles, Org. Lett., 2001, 3, 707. 15. S. F. Sabes, R. A. Urbanek, C. J. Forsyth, J. Am. Chem. Soc., 1998, 120, 2534. 16. J. A. Henderson, K. L. Jackson, A. J. Phillips, Org. Lett., 2007, 9, 5299 17. A. Furstner, T. Nagano, J. Am. Chem. Soc., 2007, 129, 1906. 18. (a) A. Barbero, F. J. Pulido, Chem. Soc. Rev., 2005, 34, 913–920. (b) M. G. Organ, S. Bratovanov, Tetrahedron Lett., 2000, 41, 6945–6949. 19. Manfred Schlosser, Organometallics in Synthesis Third Manual, Wiley, 2013. 20. E. J. Corey, P. L. Fuchs, Tetrahedron Lett., 1972, 13, 3769–3772. 21. S. Müller, B. Liepold, G. J. Roth, H. J. Bestmann, Synlett, 1996, 521–522.

63 Chapter 3

22. J. Schwartz, J. A. Labinger, Angew. Chem. Int. Ed., 2003, 15, 330–340. 23. F. -Y. Yang, M. Shanmugasundaram, S. -Y. Chuang, P. -J. Ku, M. -Y. Wu, C.-H. Cheng, J. Am. Chem. Soc., 2003, 125, 12576-12583. 24. M. F. Semmelhack, R. J. Hooley, Tetrahedron Lett., 2003, 44, 5737. 25. J. Thibonnet, V. Launay, M. Abarbri, A. Duchene, J. Parrain, Tetrahedron Lett., 1998, 39, 4277. 26. K. Ghosh, Y. Wang, J. T. Kim, J. Org. Chem., 2001, 66, 8973. 27. R. F. Newton, D. P. Reynolds, Tetrahedron Lett., 1979, 20, 3981.

64 Chapter 4

Chapter 4

A Novel Catalytic Asymmetric Route towards Skipped Dienes with a Methyl‐Substituted Central Stereogenic Carbon

In this chapter a highly efficient method for the enantioselective synthesis of 1,4-dienes (skipped dienes) with a methyl-substituted central stereogenic carbon using copper-catalyzed asymmetric allylic alkylation of diene bromides is described. Excellent regio- and enantioselectivity (up to 97: 3 SN2’/SN2 ratio and 99% ee) were achieved with broad substrate scope.

Parts of this chapter have been published: Y. Huang, M. Fañanás‐Mastral, A. J. Minnaard, B. L. Feringa, Chem. Commun., 2013, 49, 3309—3311.

65 Chapter 4

4.1 Introduction

Natural products containing 1,4-dienes (skipped dienes) such as polyunsaturated fatty acids have important biological functions.1 Particular interesting molecules with a 1,4-diene bearing a methyl-substituted central stereogenic carbon include Hennoxazole A,2 Ansalactam A,3 Ambruticin S,4 Iejimalide5 and Phorbasin E,6 shown in Figure 1 which are potent antibiotic, antifungal and cytotoxic agents.

Figure 1. Skipped polyenes found in diverse natural products.

4.2 Previous methodologies

The efficient preparation of these structural motifs remains a major challenge in organic chemistry, although multi-step syntheses7 were reported. An elegant synthesis of the above motif was reported by the group of Micalizio8 using a titanium-promoted reductive cross-coupling reaction (Figure 2) between vinylcyclopropanes and alkynes (or vinylsilanes). To the best of our knowledge, the only catalytic asymmetric synthesis of the 3-methyl substituted 1,4-diene unit with broad substrate scope was reported by the group of RajanBabu (Figure 3) by hydrovinylation of 1,3-dienes with excellent regio-

66 Chapter 4 and enantioselectivity.9

Figure 2. Reductive cross‐coupling of vinylcyclopropanes with alkynes or vinylsilanes.

Figure 3. Hydrovinylation of 1,3‐dienes.

Some examples have been reported in the literature for the synthesis of related structures using copper-catalysed asymmetric allylic alkylation (AAA)10 mainly with longer alkyl chains at the central position. The group of Hoveyda (Figure 4) described a copper-catalysed AAA of allylic phosphates with diethylzinc reagent using peptide-based ligand L7 including one example of a skipped diene.11 Li and Alexakis (Figure 5) reported an copper-catalyzed asymmetric allylic substitution of enyne chlorides with Grignard reagents which was also extended to two diene chlorides for the synthesis of 3-ethyl and 3-phenethyl substituted skipped dienes.12 Recently Mauduit et.al. reported a single example of a 3-methyl substituted skipped diene using copper-catalyzed AAA of diene allylic phosphates with dimethylzinc (Figure 6).13 Since the introduction of a methyl branch remains a highly wanted goal in view of its importance in natural product synthesis, we report here a highly efficient catalytic methodology to prepare this structural motif with broad substrate scope and excellent enantioselectivity using copper-catalysed AAA of diene bromides with the readily available methylmagnesium bromide (Figure 7).

67 Chapter 4

Figure 4. Hoveyda’s approach.

Figure 5. Alexakis’s approach.

Figure 6. Mauduit’s approach.

Figure 7. Goal of my research.

4.3 Synthesis of Starting materials

To explore the substrate scope of the reaction, a series of linear non-branched E,E-diene allylic bromides were synthesized followed by some methyl-branched ones. Finally two substrates with different double bond geometry were prepared.

Synthesis of linear non-branched substrate 1a The first substrate 1a (Figure 8) was prepared from commercial available E,E-diene acid

4. Initial reduction of the acid 4 using LiAlH4 and borane resulted in complicated products or reduction of the double bond. Switching to a two step sequence via the preparation of the methyl ester 5 followed by reduction using DIBAL gave the allyl alcohol 6 in good yield. Bromination of 6 with NBS and dimethyl sulfide gave the desired product 1a in high yield.

68 Chapter 4

Figure 8. Synthesis of phenyl substituted E,E‐diene bromide 1a.

Synthesis of linear non-branched substrate 1b The synthesis of substrate 1b (Figure 9) started with Horner–Wadsworth–Emmons reaction of aldehyde 7 and a E/Z-mixture of phosphonate 8 to give E,E-diene ester 9 in 62% yield. Reduction of ester 9 using DIBAL afforded allyl alcohol 10 in high yield.

Initial bromination using PBr3 gave an E/Z mixture of 1b (E/Z=90/10), fortunately, employing NBS and dimethyl sulfide improved the result and no isomerization of the double bond occurred.

Figure 9. Synthesis of E,E‐diene bromide 1b.

Synthesis of linear non-branched substrate 1c The first attempt to prepare 1c (Figure 10) started from esterification of 11 to form methyl ester 12 followed by the radical reaction using NBS and AIBN, however, complicated products were obtained. Fortunately alkene metathesis using allyl bromide 13 gave the desired product 1c.

69 Chapter 4

Figure 10. Synthesis of E,E‐diene bromide 1c.

Synthesis of linear non-branched substrate 1d Synthesis of substrate 1d (Figure 11) started from acid 14. Esterification of acid 14 using HCl and methanol gave di-ester 15 in high yield followed by reduction using DIBAL to form diol 16. Mono-protection of diol 16 using TBSCl afforded alcohol 17 in moderate yield. First attempt to prepare 1d using PBr3 failed probably due to the acidity of the reaction media. Employing NBS and dimethyl sulfide, the desired product 1d was obtained in high yield. O O HCl, MeOH DIBAL OH O OH HO O HO r.t., o.n. DCM, -78oC O O 86% 83% 14 15 16

TBSCl,imidazole NBS, Me S OTBS 2 HO DMF, rt, 16h DCM, -20oC 53% after 2 columns 17 OTBS 85% Br 1d PBr 3 Figure 11. Synthesis of E,E‐diene bromide 1d.

Synthesis of linear methyl-branched substrate 1e The synthesis of substrate 1e (Figure 12) started from the preparation of diol 19. Initial reduction of di-ester 18 using DIBAL resulted in low yield with large amounts of an unknown product. While LiAlH4 gave fully reduced product 20. It’s interesting to see that reduction of internal alkyne 21 using LiAlH4 stereoselectively gave E-diol 19 in high yield. Mono-esterification of diol 19 gave alcohol 22 followed by PCC oxidation to form the aldehyde 23. Wittig reaction of aldehyde 23 and ylide 24 gave diene ester 25 in overall 64% yield for above 2 steps. Mild hydrolysis of the acetate group using K2CO3 in ethanol afforded the desired alcohol 26 in 93% yield. Final bromination using NBS and dimethyl sulfide gave the desired product 1e.

70 Chapter 4

Figure 12. Synthesis of E,E‐diene bromide 1e.

Synthesis of linear methyl-branched substrate 1f Substrate 1f (Figure 13) was synthesized from Horner–Wadsworth –Emmons reaction of methyl ketone 27 and phosphonate 28 in water to form ester 29 as a mixture of isomers (E/Z=2/1). HCl promoted hydrolysis of the acetal group and isomerization of the internal double bond formed the aldehyde 30 which was followed by Wittig reaction with ylide

31 gave aldehyde 32. Aldehyde 32 was used immediately for reduction using NaBH4 to alcohol 33. Bromination of 33 gave the desired product 1f in high yield.

71 Chapter 4

Figure 13. Synthesis of E,E‐diene bromide 1f.

Synthesis of linear methyl-branched substrate 1g by Stille cross coupling The initial synthesis of vinyltin alcohol 36 started from radical reaction of alcohol 38 using AIBN and tributyltin hydride without any solvent.14 However, the reaction gave complicated products. Similar results were obtained using hexane and benzene as the solvent. Stannyl cupration15 using CuCN, tributyltin hydride and butyllithium and palladium catalyzed hydrostannation16 gave no conversion at all. Surprising to see that changing from tributyltin hydride to bis-tributyltin (Figure 14),17 the stannyl cupration gave full conversion in 4 h although providing a mixture of regioisomers (36/39=1/1).

Figure 14. Synthesis of vinyltin alcohol 36.

Due to a separation problem of above regioisomers, a Piers’ modification18 was performed (Figure 15). Stannyl cupration using CuBr•SMe2 instead of CuCN on alkyne 40 selectively gave the E-vinyltin 41 in quant. yield. Reduction of the vinyltin ester 41 using DIBAL afforded the desired vinyltin alcohol 36 in high yield.

Figure 15. Piers’ modification of stannyl cupration.

The substrate 1g (Figure 16) was prepared from the synthesis of Z-vinyliodide 35 from ester 34 using LiI in acetic acid in high yield. Initial Stille cross coupling of iodide 35 and

72 Chapter 4

vinyltin 36 using Pd2dba3 in NMP gave a mixture of stereoisomers of 37 due to the double bond isomerization catalyzed by palladium. However, a catalytic amount of triphenyl arsine18 improved the result considerably as no isomerization occurred. And the product 37 could be obtained in 76% yield. Final bromination of 37 using NBS and dimethyl sulfide gave the substrate 1g in good yield.

Figure 16. Synthesis of diene bromide 1g.

Attempted synthesis of linear methyl-branched substrate 1h The synthesis of substrate 1h (Figure 17) started from alkyne 38. Zr-catalyzed methylalumination of 38 gave vinyl iodide 42 followed by protection to form compound 43. Stille cross coupling of vinyl tin 41 and vinyl iodide 43 gave diene ester 44 in good yield. Reduction of the ester group by DIBAL afforded the allyl alcohol 45 in 80% yield. However, all attempts to synthesize 1h failed probably due to its instability.

Figure 17. Attempted synthesis of substrate 1h.

Attempted synthesis of linear methyl-branched substrate 1i The synthesis of substrate 1i (Figure 18) started with the preparation of vinyl iodide 48 from alkyne 46. The first attempt using palladium catalyzed hydrostannation gave no

73 Chapter 4 conversion at all. The classic Schwartz’s hydrozirconation19 (the Schwartz’s reagent was prepared from Cp2ZrCl2 and LiAlH4) gave a mixture of regioisomers (48/49=1/1). Fortunately, the Negishi’s modification20 (the Schwartz’s reagent was prepared from

Cp2ZrCl2 and DIBAL) improved the result a lot, only the desired regioisomer 48 was obtained. Stille cross coupling of vinyl iodide 48 and vinyl tin 41 gave the desired diene ester 50 in 89% yield. Reduction of the ester 50 using DIBAL formed the allyl alcohol 51. However, all attempts towards the synthesis of the substrate 1i failed probably due to the instability of the substrate. Although the synthesis of substrates 1h and 1i failed, the corresponding products after AAA (skipped dienes) could be easily made by the AAA of esters 1e and 1f followed by reduction and protection.

Figure 18. Attempted synthesis of substrate 1i.

4.4 Results of the Cu-catalyzed AAA and discussion

Our investigation started with the asymmetric allylic alkylation of 1a with MeMgBr in dichloromethane at –80 oC employing copper bromide dimethylsulfide complex

(CuBr•SMe2) and L1 as ligand (Table 1, entry 1). Product 2a was isolated in 65% yield with 85% ee and the ratio of 2a:3a (SN2’/SN2) was 79:21. To increase both the regio- and enantioselectivity of the reaction, a series of catalysts based on the chiral ligands depicted in Table 1 was tested. Both Tol-BINAP (L2) and JosiPhos type ligands (L4 and L5) afforded lower ee than L1. We then used the combination of CuBr•SMe2 with TaniaPhos (L3), which has emerged as an excellent catalyst for the introduction of the methyl unit via copper-catalysed AAA.21 We were pleased to see that the use of this catalytic system led to product 2a in 66% yield with >99% ee and with excellent regioselectivity (2a:3a =

74 Chapter 4

95:5). It is important to note that only 1,3-substitution happened and no 1,5-substitution adduct was detected. Table 1. Optimization of the copper‐catalysed AAA of 1a.

CuBr•SMe2 5mol% Br Ligand 6 mol% + MeMgBr 1.2 equiv DCM, -80 oC, o.n. 1a 2a 3a

Ph O P N P(Tol)2 O P(Tol)2 Ph

(S,R,R)-L1 (R)-Tol-BINAP L2 N PPh2 Ph2P

PCy2 Ph2P Fe Fe PPh2 Cy2P Fe

(S,S)-TaniaPhos L3 (R,S) -Josiphos L4 (S,R)-rev. Josiphos L5

a b c d Entry Ligand Solvent Yield [%] SN2’/SN2 ee [%] 1 L1 DCM 65 79:21 85 2 L2 DCM 68 36:64 ‐50f 3 L3 DCM 66 95:5 >99 4 L4 DCM 75 50:50 ‐62f 5 L5 DCM 51 23:77 32 6 L3 Toluene 25e 80:20 95 7 L3 THF 34e 91:9 87 e 8 L3 Et2O 46 8:92 71 a Reaction conditions: MeMgBr (0.3 mmol) was added to a stirred solution of CuBr•SMe2 and ligand in dry solvent at –80 oC; 1a (0.25 mmol) in 1 mL of dry solvent was added dropwise over 1 h. bisolated combined yield. cdetermined by 1H NMR or GC. ddetermined by chiral GC. emixture of products. fThe negative ee value indicates that the opposite enantiomer was formed.

With this highly selective catalyst in hand, we studied the solvent effects on the reaction. We found that dichloromethane was still the most effective solvent. When we used toluene, product 2a was obtained with 95% ee but with lower regioselectivity (Table 1, entry 6). The use of THF gave comparable regioselectivity as DCM but the ee decreased to 87% (entry 7). The situation in diethyl ether was even worse; the enantioselectivity decreased and the regioselectivity was totally switched, with linear product 3a being the main product of the reaction (entry 8). To study the scope of this new enantioselective

75 Chapter 4 transformation, previous synthesized substrates were tested under the optimized conditions (Table 1, entry 3). Excellent regio- and enantioselectivity were obtained in all cases (Figure 19).

Figure 19. Copper‐catalysed AAA of diene bromides 1. Reaction conditions: MeMgBr (0.3 o mmol) was added to a stirred solution of CuBr•SMe2 and TaniaPhos (L3) in dry DCM at –80 C; substrate 1 (0.25 mmol) in 1 mL of dry DCM was added dropwise over 1 h. Yield represents combined isolated yield. Regioselectivity was determined by 1H NMR or GC analysis. Ee was a determined by chiral GC or HPLC. 1 g scale, 1 mol% of catalyst; 58% yield, SN2’/SN2 95:5, 99% ee. bVolatile products

This new methodology proved to be also very efficient for an alkyl substituted substrate such as 1b (R1 = iso-Bu) which afforded 1,4-diene 2b with excellent selectivity. It should be noted that product 2b represents the side chain of Phorbasins (Figure 1). Notably, both the regio- and enantioselectivity of the reaction dropped considerably when we used a (2E,4E)/(2Z,4E) isomeric mixture of substrate 1b. The E-geometry of the double bond next to the bromide seems crucial for achieving high ee and regioselectivity (Figure 20). A remarkable result was obtained with ester-substituted diene bromide 1c which led exclusively to product 2c, without any traces of the 1,2-, 1,4- or 1,6-addition to the carbonyl moiety nor of the 1,5-substitution product. Moreover, product 2c is a core unit of Iejimalides (Figure 1). The versatile substituent TBSOCH2, as present in diene 1d, had no influence on the enantioselectivity and diene 2d could be obtained with 99% ee although a slightly lower regioselectivity was observed. The more substituted substrates

76 Chapter 4

1e and 1f, with methyl groups at R2 or R3, could also be used in this transformation. Again, 2e and 2f were obtained exclusively with excellent selectivity. We also tested the effect of the remote double bond geometry as the presence of a Z-double bond is common in some natural products like Hennoxazole A. We were pleased to find that the reaction also proceeded succesfully with (Z,E)-1g affording similar regio- and enantioselectivity as with (E,E)-1f, while no double bond isomerization was detected in 2g. These remarkable results show that the geometry of the double bond remote from the bromide seems to have no effect on both the regio- and enantioselectivity (Figure 20). An important feature is the scalability of this reaction. Synthesis of 2a was executed on a larger scale (1 gram) using only 1 mol% of catalyst and still excellent ee and regioselectivity were obtained, with similar yield.

Figure 20. Effect of double bond geometry on ee and regioselectivity.

Finally we tried further functionalization of the 1,4-diene 2d via cross metathesis22 (Figure 21) for future synthetic applications. The initially attempted cross metathesis with ethyl acrylate 52 using Grubbs’ first and second-generation catalysts and Hoveyda–Grubbs first-generation catalyst under different conditions led to complicated products. However, Hoveyda–Grubbs second-generation catalyst significantly improved the outcome and afforded the product 53 as single E,E-isomer. The presence of a protected alcohol and ester group facilitates the use of optically active 53 as a versatile multifunctional building block in natural product synthesis allowing for chain elongation on each end of the molecule.

Figure 21. Cross metathesis between 1,4‐diene 2d and ethyl acrylate 52.

77 Chapter 4

4.5 Conclusion

In summary, a copper-catalysed asymmetric allylic alkylation with methylmagnesium bromide as nucleophile employing prochiral diene allylic bromides as substrates was developed. The reaction leads to important chiral 3-methyl substituted 1,4-diene building blocks with excellent regio- and enantioselectivity (ee values up to 99%; SN2’/SN2 ratio up to 97:3) in nearly all cases. Application of this methodology to the total synthesis of Phorbasins B is ongoing (Chapter 5).

4.6 Experimental section

Starting materials were purchased from Aldrich, Alpha Aesar or Acros and used as received unless stated otherwise. All solvents were reagent grade and, if necessary, dried and distilled prior to use. All reactions were carried out under a nitrogen atmosphere using oven dried glassware and using standard Schlenk techniques. Column chromatography was performed on silica gel (Aldrich 60, 230-400 mesh). TLC was performed on silica gel 60/Kieselguhr F254. 1H and 13C NMR spectra were recorded on a Varian VXR300 (299.97 MHz for 1H, 75.48 MHz for 13C) or a Varian AMX400 (399.93 MHz for 1H, 100.59 MHz for 13C) spectrometer in CDCl3 unless stated otherwise. Chemical shifts are reported in δ values 1 13 (ppm) relative to the residual solvent peak (CDCl3, H = 7.24, C = 77.0). Carbon assignments are based on 13C and APT 13C experiments. Splitting patterns are indicated as follows: s (singlet), d (doublet), t (triplet), q (quartet), m (multiplet), br (broad). High resolution mass spectra (HRMS) were recorded on an AEI-MS-902 and FTMS orbitrap (Thermo Fisher Scientific) mass spectrometer. Optical rotations were measured on a Schmidt+ Haensch polarimeter (Polartronic MH8) with a 10 cm cell (c given in g/100 mL). Enantiomeric excesses were determined by HPLC analysis using a Shimadzu LC-10ADVP HPLC equipped with a Shimadzu SPD-M10AVP diode array detector (Chiralcel OD-H, 250*4.6, 10 μm) or by capillary GC analysis (HP 6890, CP-Chiralsil-Dex-CB column (25 m * 0.25 mm)) using a flame ionization detector. Racemic products were synthesized by reaction of the allyl bromide and the corresponding organomagnesium reagent at –80 °C in dry dichloromethane in the presence of CuBr•SMe2 (10 mol%) and PPh3 (20 mol%).

78 Chapter 4

(2E,4E)-Methyl-5-phenylpenta-2,4-dienoate (5).23 To a stirred solution of O 5-phenylpenta-2,4-dienoic acid 4 (3.91 g, 22.4 mmol) in 50 mL O of methanol was added HCl (20 ml, 60 mmol, 3M in methanol) at room temperature. The solution was stirred for 16 h, concentrated and purified by flash chromatography (eluent pentane/ether = 10/1) to give 1 5 as a white solid (4.07 g, 97% yield). H NMR (400 MHz, CDCl3) δ 7.45 – 7.21 (m, 6H), 13 6.87 – 6.73 (m, 2H), 5.93 (d, J = 15.3 Hz, 1H), 3.70 (s, 3H); C NMR (101 MHz, CDCl3) δ 167.5, 144.8, 140.5, 136.0, 129.0, 128.8, 127.2, 126.2, 120.8, 51.6.

(2E,4E)-5-Phenylpenta-2,4-dien-1-ol (6).23 To a stirred solution of the methyl ester 5 (4.00 g, 21.2 mmol) in 50 mL of dry DCM was added OH DIBAL-H (63.8 ml, 1 M in DCM, 63.8 mmol, 3 eq) over 0.5 h at –78 oC. The reaction mixture was stirred for 4 h when TLC showed full conversion. The mixture was quenched with 60 mL of saturated aqueous Rochelle salt (potassium sodium tartrate) and stirred for 30 min. The phases were separated and the aqueous layer was extracted with DCM (3 x 50 mL). The combined organic phases were dried over Na2SO4, filtered, concentrated and purified by flash chromatography (eluent pentane/ether = 4/1) to give 6 as a white solid (3.0 g, 88% yield). 1 H NMR (400 MHz, CDCl3) δ 7.38 – 7.05 (m, 5H), 6.72 (dd, J = 15.6, 10.5 Hz, 1H), 6.49 (d, J = 15.6 Hz, 1H), 6.36 (dd, J = 15.1, 10.5 Hz, 1H), 5.90 (dt, J = 15.1, 5.9 Hz, 1H), 13 4.19 (d, J = 5.9 Hz, 2H), 1.31 (br, 1H); C NMR (101 MHz, CDCl3) δ 137.1, 132.8, 132.4, 131.6, 128.6, 128.1, 127.6, 126.4, 63.5.

((1E,3E)-5-Bromopenta-1,3-dienyl)benzene (1a). To a stirred solution of NBS (4.17 g, o 23.4 mmol, 1.3 eq) in DCM (40 mL) at –20 C was added Me2S Br (0.980 g, 1.2 mL, 28.9 mmol, 1.83 eq) slowly over 5 min. The reaction mixture was stirred for 15 min before a solution of allylic alcohol 9 (2.50 g, 15.6 mmol) in 10 mL of DCM was added dropwise over 10 min.

The mixture was quenched with a saturated aqueous NH4Cl solution (50 mL) when TLC showed full conversion and after the mixture was warmed up to room temperature, the layers were separated. The organic layer was washed with water (3 x 20 mL), dried over

Na2SO4, filtered and concentrated to give crude 1a as a white solid (2.83 g, 81%) which was used in the next step (asymmetric allylic alkylation) immediately due to instability. 1 H NMR (400 MHz, CDCl3) δ 7.54 – 7.15 (m, 5H), 6.77 (dd, J = 15.6, 10.4 Hz, 1H), 6.60 (d, J = 15.6 Hz, 1H), 6.46 (dd, J = 14.8, 10.4 Hz, 1H), 6.08 – 5.90 (m, 1H), 4.11 (d, J = 13 8.0 Hz, 2H); C NMR (101 MHz, CDCl3) δ 136.7, 135.2, 134.5, 128.9, 128.7, 128.0, 127.3, 126.6, 33.4.

79 Chapter 4

(2E,4E)-Ethyl‐7-methylocta-2,4-dienoate (9).24 To a stirred solution of compound 8 (mixture of E/Z isomers, 5.0 g, 20 mmol) in 30 mL of dry THF was added LiHDMS (20 mL, 20 mmol, 1M in THF) at –78 oC under nitrogen. The mixture was stirred for about 30 min when a THF solution (5 mL) of aldehyde 7 (1.72 g, 20 mmol, 1 eq) was added dropwise over 10 min. The resulting solution was stirred overnight and quenched with a o saturated aqueous NH4Cl solution (50 mL) at –78 C. The mixture was warmed up to room temperature and the layers were separated. The aqueous layer was washed with ether (3 x 30 mL) and the organic layers were dried over Na2SO4, filtered and concentrated and purified by flash chromatography (eluent pentane/ether = 10/1) to give 1 9 as a colorless oil (2.3 g, 62% yield). H NMR (400 MHz, CDCl3) δ 7.25 – 7.12 (m, 1H), 6.17 – 5.94 (m, 2H), 5.72 (d, J = 15.5 Hz, 1H), 4.13 (q, J = 7.1 Hz, 2H), 2.01 – 1.97 (m, 2H), 1.66 – 1.63 (m, 1H), 1.23 (t, J = 7.1 Hz, 3H), 0.84 (d, J = 6.7 Hz, 6H); 13C NMR

(101 MHz, CDCl3) δ 167.3, 145.0, 143.5, 129.4, 119.2, 60.2, 42.3, 28.3, 22.3, 14.3.

(2E,4E)-7-Methylocta-2,4-dien-1-ol (10). To a stirred solution of the ethyl ester 9 (2.0 g, 11 mmol) in 30 mL of dry DCM was added DIBAL-H (33 ml, 1 OH M in DCM, 33 mmol, 3 eq) over 0.5 h at –78 oC. The reaction mixture was stirred for 4 h when TLC showed full conversion. The reaction mixture was quenched with 30 mL saturated aqueous Rochelle salt (potassium sodium tartrate) and stirred for 30 min. The phases were separated and the aqueous layer was extracted with DCM (3 x 30 mL). The combined organic phases were dried over Na2SO4, filtered, concentrated and purified by flash chromatography (eluent pentane/ether = 4/1) to give 10 as a colorless oil (1.5 g, 95% yield). 1H NMR (400 MHz,

CDCl3) δ 6.23 (dd, J = 15.1, 10.4 Hz, 1H), 6.03 (dd, J = 15.1, 10.4 Hz, 1H), 5.81 – 5.63 (m, 2H), 4.16 (dd, J = 6.1, 0.9 Hz, 2H), 2.02 – 1.92 (m, 2H), 1.65 – 1.62 (m, 1H), 1.38 (br, 13 1H), 0.89 (d, J = 6.7 Hz, 6H); C NMR (101 MHz, CDCl3) δ 134.5, 132.1, 130.4, 129.3, + 63.5, 42.0, 28.5, 22.3. HRMS (APCI+) calculated for C9H15[M–OH] : 123.1174, found: 123.1116.

(2E,4E)-1-Bromo-7-methylocta-2,4-diene (1b). To a stirred solution of NBS (1.07 g, 6 mmol, 1.3 eq) in DCM (20 mL) at –20 oC was slowly added Br Me2S (460 mg, 0.54 mL, 7.4 mmol, 1.83 eq) over a 5 min period. The reaction mixture was stirred for 15 min before a solution of allylic alcohol 10 (560 mg, 4 mmol) in 10 mL of DCM was added dropwise over 10 min.

The mixture was quenched with a saturated aqueous NH4Cl solution (50 mL) when TLC

80 Chapter 4 showed full conversion and after the mixture was warmed up to room temperature, the layers were separated. The organic layer was washed with water (3 x 20 mL), dried over

Na2SO4, filtered and the solvent was concentrated to give crude 1b as colorless oil (0.69 1 g, 85%) used in the next step immediately due to instability. H NMR (400 MHz, CDCl3) δ 6.32 – 6.07 (m, 1H), 5.99 – 5.88 (m, 1H), 5.78 – 5.63 (m, 2H), 4.02 – 3.85 (m, 2H), 1.96 – 1.87 (m, 2H), 1.60 – 1.57 (m, 1H), 0.82 (d, J = 6.7 Hz, 6H); 13C NMR (101 MHz,

CDCl3) δ 136.5, 135.4, 129.8, 126.1, 42.0, 33.9, 28.4, 22.3.

(2E,4E)-Methyl-hexa-2,4-dienoate (12).25 To a stirred solution of acid 11 (4.0 g, 36 mmol, 1 eq) in 50 mL of methanol was added HCl (24 mL, 72 mmol, O 3M in methanol, 2 eq) at room temperature. The resulting solution O was stirred for 16 h, concentrated and purified by flash chromatography (eluent pentane/ether = 10/1) to give 12 as colorless oil (4.22 g, 93% 1 yield). H NMR (400 MHz, CDCl3) δ 7.25 – 7.10 (m, 1H), 6.19 – 5.98 (m, 2H), 5.71 (d, J 13 = 15.4 Hz, 1H), 3.67 (s, 3H), 1.78 (d, J = 5.6 Hz, 3H); C NMR (101 MHz, CDCl3) δ 167.8, 145.2, 139.4, 129.7, 118.5, 51.4, 18.6.

(2E,4E)-Methyl‐6-bromohexa-2,4-dienoate (1c).26 To a solution of methyl sorbate 12 (1.0 g, 8 mmol) and allyl bromide 13 (4.84 g, 40 mmol, 5 eq) in O Br 80 mL of CH2Cl2 (c = 0.1 M) was added the Hoveyda-Grubbs-II O catalyst (C11H38Cl2N2ORu, 50 mg, 0.08 mmol, 0.01 equiv) at room temperature. After stirring for 24 h, silica was added to the reaction mixture and after evaporation of the solvent, the pad of silica was loaded on top of a silica gel column and the product was quickly purified by a flash chromatography (eluent pentane/ether = 1 4/1) to give 1c as a colorless oil (483 mg, 30% yield). H NMR (400 MHz, CDCl3) δ 7.32 – 7.21 (m, 1H), 6.39 (dd, J = 15.0, 10.9 Hz, 1H), 6.24 (dd, J = 15.4, 10.9 Hz, 1H), 5.94 (d, 13 J = 15.4 Hz, 1H), 4.06 – 3.99 (m, 2H), 3.75 (s, 3H); C NMR (101 MHz, CDCl3) δ 166.9, 142.8, 136.7, 131.8, 122.7, 51.7, 31.2.

(2E,4E)-Dimethyl‐hexa-2,4-dienedioate (15).27 To a stirred solution of acid 14 (4.0 g,

O 28 mmol) in 50 mL of methanol was added HCl (19 mL, 57 O O mmol, 3M in methanol, 2 eq) at room temperature. The O resulting solution was stirred for 16 h, concentrated and purified by crystallization from ether to give 15 as a white solid (4.13 g, 86% yield). 1H NMR 13 (400 MHz, CDCl3) δ 7.33 – 7.21 (m, 2H), 6.21 – 6.07 (m, 2H), 3.72 (s, 6H); C NMR

(101 MHz, CDCl3) δ 166.3, 140.9, 128.0, 51.9.

81 Chapter 4

(2E,4E)-Hexa-2,4-diene-1,6-diol (16).28 To a stirred solution of the methyl ester 15 (1.0 g, 5.9 mmol) in 30 mL of dry DCM was added DIBAL-H (36 ml, OH HO 1 M in DCM, 36 mmol, 6 eq) over 0.5 h at –78 oC. The reaction mixture was stirred for 8 h when TLC showed full conversion. The reaction mixture was quenched with 30 mL saturated aqueous Rochelle salt (potassium sodium tartrate) followed by stirring for 30 min. The phases were separated and the aqueous layer was extracted with DCM (3 x 30 mL). The combined organic phases were dried over Na2SO4, filtered, concentrated and purified by flash chromatography (eluent pentane/EtOAc = 2/1) 1 to give 16 as a white solid (560 mg, 83% yield). H NMR (400 MHz, CDCl3) δ 6.30 – 6.08 (m, 2H), 5.91 – 5.66 (m, 2H), 4.14 (t, J = 5.4 Hz, 4H), 1.29 (t, J = 5.4 Hz, 2H); 13C

NMR (101 MHz, CDCl3) δ 132.4, 130.4, 63.2.

(2E,4E)-6-((tert-Butyldimethylsilyl)oxy)hexa-2,4-dien-1-ol (17).29 To a solution of 16 (300 mg, 2.63 mmol) in dry dichloromethane (20 mL) was OTBS HO added imidazole (269 mg, 3.95 mmol, 1.5 eq) followed by tert-butyl-dimethylsilyl chloride (436 mg, 2.89 mmol, 1.1 eq), and the resulting white suspension was stirred at room temperature for 12 h. The reaction mixture was quenched with 20 mL of water and extracted with ether (3 x 20 mL). The combined organic layers were dried over Na2SO4, filtered, concentrated and purified by flash chromatography (eluent pentane/EtOAc = 4/1) to give 17 as a colorless oil (319 mg, 53%). 1H NMR (400

MHz, CDCl3) δ 6.24 – 6.11 (m, 2H), 5.81 – 5.65 (m, 2H), 4.15 (d, J = 4.5 Hz, 2H), 4.11 (t, 13 J = 5.6 Hz, 2H), 1.50 (br, 1H), 0.84 (s, 9H), 0.00 (s, 6H); C NMR (101 MHz, CDCl3) δ 133.3, 131.4, 131.0, 128.8, 63.4, 63.3, 25.9, 18.4, –5.2.

((2E,4E)-6-Bromohexa-2,4-dienyloxy)(tert-butyl)-dimethyl silane (1d). To a stirred

OTBS solution of NBS (267 mg, 1.5 mmol, 1.5 eq) in DCM (20 mL) Br o at –20 C was slowly added Me2S (115 mg, 0.14 mL, 1.85 mmol, 1.85 eq) over a 5 min period. The reaction mixture was stirred for 15 min before a solution of allylic alcohol 17 (228 mg, 1 mmol) in 5 mL of DCM was added dropwise over 10 min. The mixture was quenched with a saturated aqueous NH4Cl solution (20 mL) when TLC showed full conversion and subsequently the mixture was warmed up to room temperature and the layers separated. The organic layer was washed with water (3 x 10 mL), dried over Na2SO4, filtered and concentrated to give the crude 1d as colorless oil (248 mg, 85%) immediately used in the next step due to instability. 1H NMR (400 MHz,

CDCl3) δ 6.29 – 6.10 (m, 2H), 5.77 (m, 2H), 4.16 (dd, J = 5.0, 1.1 Hz, 2H), 3.96 (d, J = 13 7.9 Hz, 2H), 0.84 (s, 9H), 0.00 (s, 6H); C NMR (101 MHz, CDCl3) δ 135.2, 134.4, 127.9, 127.9, 63.1, 33.3, 25.9, 18.1, –5.3.

82 Chapter 4

(E)-But-2-ene-1,4-diol (21).30 To a stirred solution of diol 19 (2.6 g, 30 mmol) in dry

OH THF (40 mL) was added solid LiAlH4 (1.44 g, 36 mmol, 1.2 eq) at 0 HO oC under nitrogen. The mixture was heated to reflux for 4 h, followed o by quenching with a saturated aqueous NH4Cl solution (20 mL) at 0 C. The mixture was allowed to warm to room temperature and the phases were separated. The aqueous layer was extracted with EtOAc (3 x 30 mL). The combined organic phases were dried over

Na2SO4, filtered, concentrated and purified by flash chromatography (eluent pentane/EtOAc = 1/1) to give 21 as colorless oil (660 mg, 20% yield, soluble in water). 1 H NMR (400 MHz, CDCl3) δ 5.89 – 5.79 (m, 2H), 4.17 – 4.05 (m, 4H), 1.38 (br, 2H); 13 C NMR (101 MHz, CDCl3) δ 130.5, 62.9.

(E)-4-Hydroxybut-2-en-1-yl acetate (22).31 To a stirred solution of but-2-ene-1,4-diol

OAc 21 (1.0 g, 11 mmol) in dry THF (20 mL) was added solid NaH (440 HO mg, 60% in mineral oil, 11 mmol, 1 eq) at 0 oC. The mixture was stirred at room temperature for 1 h followed by the addition of a THF solution (5 mL) of acetyl chloride (863 mg, 0.81 mL, 1 eq). The reaction mixture was stirred for 3 h after which time TLC (diethyl ether) showed a mixture of starting compound, monoacetate and diacetate. H2O (10 mL) was added, and the mixture was extracted with diethyl ether

(3 x 30 mL). The combined organic phases were dried over Na2SO4, filtered, concentrated and purified by flash chromatography (eluent pentane/EtOAc = 4/1) to give 1 22 as colourless oil (930 mg, 65% yield). H NMR (400 MHz, CDCl3) δ 5.93 – 5.81 (m, 1H), 5.80 – 5.73 (m, 1H), 4.52 (dd, J = 5.8, 1.2 Hz, 2H), 4.12 (dd, J = 5.0, 1.2 Hz, 2H), 13 2.01 (s, 3H), 1.63 (br, 1H); C NMR (101 MHz, CDCl3) δ 171.6, 133.5, 125.1, 64.2, 62.7, 20.9.

(2E,4E)-Ethyl-6-acetoxy-2-methylhexa-2,4-dienoate (25).31 4-Acetoxy- 2-buten-1-ol

O 22 (0.91 g, 7 mmol) was added to a suspension of pyridinium OAc EtO chlorochromate (1.81 g, 8.4 mmol) in 20 mL of DCM at 0 °C. The mixture was stirred at room temperature until TLC showed full conversion. The solids were removed by flash chromatography, washed with diethyl ether and the organic solvents were removed in vacuo to give 4-acetoxy-crotonaldehyde 23 used immediately in the next step. The aldehyde 23 was dissolved in 10 mL of DCM followed by the addition of phosphorane 24 (2.54 g, 7.0 mmol) at room temperature. The solution was stirred for 30 min until TLC showed full conversion. The solvent was removed in vacuo. The product was purified by flash chromatography (eluent pentane/ether = 4/1) to give 25 as colorless

83 Chapter 4

1 oil (945 mg, 64% yield over 2 steps). H NMR (400 MHz, CDCl3) δ 7.10 (d, J = 11.4 Hz, 1H), 6.58 – 6.45 (m, 1H), 6.02 (dt, J = 15.2, 6.1 Hz, 1H), 4.62 (d, J = 6.1 Hz, 2H), 4.15 (q, J = 7.1 Hz, 2H), 2.03 (s, 3H), 1.88 (d, J = 0.7 Hz, 3H), 1.24 (t, J = 7.1 Hz, 3H).

31 (2E,4E)-Ethyl-6-hydroxy-2-methylhexa-2,4-dienoate (26). K2CO3 (1.23 g, 8.9 mmol)

O was added to a solution of 25 (945 mg, 4.45 mmol) in 15 mL of OH EtO ethanol at room temperature. The resulting mixture was stirred about 2 h when TLC showed full conversion. The reaction was quenched with an aqueous saturated NaCl solution (20 mL). The phases were separated and the aqueous layer was extracted with EtOAc (3 x 30 mL). The combined organic phases were dried over Na2SO4, filtered, concentrated and purified by flash chromatography (eluent pentane/EtOAc = 4/1) to give 26 as colourless oil (707 mg, 93% 1 yield). H NMR (400 MHz, CDCl3) δ 7.13 (d, J = 11.4 Hz, 1H), 6.53 (dd, J = 15.2, 11.4 Hz, 1H), 6.11 (dt, J = 15.2, 5.2 Hz, 1H), 4.28 – 4.20 (m, 2H), 4.15 (q, J = 7.1 Hz, 2H), 1.89 (d, J = 0.7 Hz, 3H), 1.45 (br, 1H), 1.24 (t, J = 7.1 Hz, 3H); 13C NMR (101 MHz,

CDCl3) δ 168.4, 139.2, 137.1, 127.8, 125.7, 63.1, 60.6, 14.3, 12.7.

(2E,4E) -Ethyl- 6- bromo -2- methylhexa-2,4-dienoate (1e). To a stirred solution of o O NBS (470 mg, 2.64 mmol, 1.3 eq) in DCM (20 mL) at –20 C Br EtO was slowly added Me2S (197 mg, 0.23 mL, 3.17 mmol, 1.83 eq) over a 5 min period. The reaction mixture was stirred for 15 min before a solution of allylic alcohol 26 (300 mg, 1.76 mmol) in 5 mL of DCM was added dropwise over 10 min. The mixture was quenched with a saturated aqueous NH4Cl solution (50 mL) when TLC showed full conversion and after the mixture was warmed to room temperature, the layers were separated. The organic layer was washed with water (3 x 10 mL), dried over Na2SO4, filtered and concentrated to give the crude 1e as colorless oil (323 mg, 79%) used in the next step immediately due to instability. 1H NMR (400

MHz, CDCl3) δ 7.09 (dd, J = 11.4, 0.9 Hz, 1H), 6.59 – 6.44 (m, 1H), 6.13 (dt, J = 15.4, 7.8 Hz, 1H), 4.15 (q, J = 7.1 Hz, 2H), 4.01 (d, J = 7.8 Hz, 2H), 1.90 (d, J = 1.3 Hz, 3H), 13 1.24 (t, J = 7.1 Hz, 3H); C NMR (101 MHz, CDCl3) δ 167.9, 136.0, 135.1, 129.6, 129.5, 60.8, 32.0, 14.3, 12.8.

Ethyl‐4,4-dimethoxy-3-methylbut-2-enoate (29).32 A mixture of 1,1-dimethoxyacetone

OMe 27 (5.91 g, 50 mmol) and ethyl 2-(diethoxyphosphoryl)acetate 28 (13.45 g, 60 mmol) was added dropwise to a suspension of K CO (17.28 g) in 10 OMe 2 3 OEt mL of water at room temperature. The reaction mixture was stirred for 16 h. O The insoluble material was then removed by filtration and washed with

84 Chapter 4 ether. The organic phase was separated and washed with brine (50 mL). The organic layer was dried over Na2SO4, filtered, concentrated and purified by flash chromatography (eluent pentane/ether) to give product 29 as colorless oil (8.0 g, 85%, mixture of E and Z acetal esters).

(2E,4E)- Ethyl -6- hydroxy-3- methylhexa- 2,4-dienoate (33).33 HCl (3 N, 3.5 mL

OEt of an aq. solution) was added dropwise to a stirred solution of the O above obtained esters 29 (2.00 g, 10.6 mmol) in 15 mL DCM at

0 °C. The resulting mixture was stirred for 2 h. The organic layer OH was separated and washed with a saturated aqueous solution of

NaHCO3 (30 mL) and brine (20 mL), dried over Na2SO4, filtered and concentrated. The crude product was purified by vacuum distillation to yield the aldehyde 30 which was used directly for the next step. Compound 31 (3.20 g, 10.6 mmol) was added to a stirred solution of the aldehyde 30 in DCM (10 mL) at room temperature. The reaction mixture was stirred for 15 h and subsequently concentrated. The resulting mixture was dissolved in pentane and the solids were filtered. The solution was concentrated to give the crude aldehyde 32 which was used in the next step. A solution of the aldehyde 32 in 5 mL of ethanol was added to a solution of sodium borohydride (1.0 g, 25 mmol) in EtOH/H2O (1:1, 20 mL) at 0 °C. The reaction mixture was stirred at room temperature for 20 min followed by quenching with aqueous saturated NaCl. The aqueous layer was separated and extracted with Et2O (5 x 20 mL).

The combined organic layers were dried over Na2SO4, filtered, concentrated and purified by flash chromatography (eluent pentane/ether = 1/1) to give product 33 as colorless oil 1 (0.87 g, 50% yield over three steps). H NMR (400 MHz, CDCl3) δ 6.26 (d, J = 15.8 Hz, 1H), 6.17 (m, 1H), 5.71 (s, 1H), 4.24 (d, J = 5.2 Hz, 2H), 4.11 (q, J = 7.1 Hz, 2H), 13 2.22 (s, 3H), 1.51 (br, 1H), 1.22 (t, J = 7.1 Hz, 3H). C NMR (101 MHz, CDCl3) δ 167.0, 151.3, 134.3, 133.8, 119.7, 63.1, 59.8, 14.3, 13.8.

(2E,4E)-Ethyl-6-bromo-3-methylhexa-2,4-dienoate (1f). To a stirred solution of NBS o OEt (470 mg, 2.64 mmol, 1.3 eq) in DCM (20 mL) at –20 C was slowly O added Me2S (197 mg, 0.23 mL, 3.17 mmol, 1.83 eq) over a 5 min period. The reaction mixture was stirred for 15 min before a solution Br of allylic alcohol 33 (300 mg, 1.76 mmol) in 5 mL of DCM was added dropwise over 10 min. The mixture was quenched with a saturated aqueous NH4Cl solution (50 mL) when TLC showed full conversion and subsequently the mixture was warmed up to room temperature and the layers were separated. The organic layer was

85 Chapter 4

washed with water (3 x 10 mL), dried over Na2SO4, filtered and concentrated to give crude 1f as colorless oil (354 mg, 86%) used in the next step immediately due to 1 instability. H NMR (400 MHz, CDCl3) δ 6.29 – 6.13 (m, 2H), 5.73 (s, 1H), 4.11 (q, J = 7.1 Hz, 2H), 3.99 (d, J = 7.1 Hz, 2H), 2.20 (d, J = 1.2 Hz, 3H), 1.22 (t, J = 7.1 Hz, 3H); 13 C NMR (101 MHz, CDCl3) δ 166.6, 150.3, 137.6, 130.7, 121.1, 59.9, 32.0, 14.3, 13.8.

(Z)-Ethyl-3-iodobut-2-enoate (35).34 To a mixture of ethyl but-2-ynoate 34 (2.34 g, 20

OEt mmol) and lithium iodide (4.4 g, 32 mmol, 1.6 eq) was added acetic acid o I O (6 mL) at room temperature. The reaction mixture was heated to 70 C and stirred for 16 h followed by dilution with ether (50 mL). The organic phase was washed with water (20 mL), saturated aqueous NaHCO3 (20 mL), and saturated aqueous

Na2S2O3 (20 mL), dried over Na2SO4, filtered and concentrated and purified by flash chromatography (eluent pentane/ether = 10/1) to give product 35 as a colorless oil (2.1 g, 1 88% yield). H NMR (400 MHz, CDCl3) δ 6.29 (q, J = 1.4 Hz, 1H), 4.22 (q, J = 7.1 Hz, 2H), 2.73 (d, J = 1.4 Hz, 3H), 1.29 (t, J = 7.1 Hz, 3H).

(2Z,4E)-Ethyl‐6-hydroxy-3-methylhexa-2,4-dienoate (37). A solution of Pd2(dba)3 (45

OEt mg, 0.005 mmol) in 20 mL of NMP was treated with triphenyl

O arsine (59 mg, 0.2 mmol) at room temperature. The solution was stirred for 10 min followed by the addition of a solution of iodide 35 HO (466 mg, 1.94 mmol) in 5 mL of NMP. The solution was further stirred for 10 min followed by the addition of a solution of stannane 3613 (674 mg, 1.94 mmol) in 5 mL of NMP. The reaction mixture was stirred for 16 h followed by quenching with saturated aqueous potassium fluoride solution (20 mL). The reaction mixture was diluted with diethyl ether (20 mL). The organic layer was separated and washed with saturated aqueous potassium fluoride solution (10 mL), dried over Na2SO4, filtered and concentrated. The product was purified by column chromatography (eluent pentane/ether 1 = 2/1) to give 37 as a colourless oil (252 mg, 76%). H NMR (400 MHz, CDCl3) δ 7.75 (dd, J = 16.0, 0.7 Hz, 1H), 6.23 (dt, J = 16.0, 5.6 Hz, 1H), 5.71 (s, 1H), 4.32 (td, J = 5.6, 1.4 Hz, 2H), 4.16 (q, J = 7.1 Hz, 2H), 2.02 (d, J = 1.3 Hz, 3H), 1.63 (br, 1H), 1.28 (t, J = 13 7.1 Hz, 3H); C NMR (101 MHz, CDCl3) δ 166.2, 150.0, 135.8, 127.8, 117.9, 63.6, 59.8, + 21.0, 14.3. HRMS (APCI+) calculated for C9H13O2[M–OH] :153.0910, found:153.0909.

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(2Z,4E)-Ethyl-6-bromo-3-methylhexa-2,4-dienoate (1g). To a stirred solution of NBS o OEt (313 mg, 1.76 mmol, 1.3 eq) in DCM (20 mL) at –20 C was slowly

O added Me2S (132 mg, 0.16 mL, 2.12 mmol, 1.83 eq) over a 5 min period. The reaction mixture was stirred for 15 min before a solution Br of allylic alcohol 37 (200 mg, 1.18 mmol) in 5 mL of DCM was added dropwise over 10 min. The mixture was quenched with a saturated aqueous NH4Cl solution (50 mL) after TLC showed full conversion and subsequently the mixture was warmed up to room temperature and the layers were separated. The organic layer was washed with water (3 x 10 mL), dried over anhydrous Na2SO4, filtered and concentrated to give the crude 1g as colorless oil (214 mg, 78%) used in the next step immediately due 1 to instability. H NMR (400 MHz, CDCl3) δ 7.80 (dd, J = 15.6, 0.9 Hz, 1H), 6.23 (dt, J = 15.6, 7.8 Hz, 1H), 5.72 (s, 1H), 4.16 (q, J = 7.1 Hz, 2H), 4.10 (dd, J = 7.8, 0.9 Hz, 2H), 13 2.00 (d, J = 1.3 Hz, 3H), 1.28 (t, J = 7.1 Hz, 3H); C NMR (101 MHz, CDCl3) δ 165.9, 149.0, 132.0, 131.1, 119.2, 59.9, 32.6, 20.8, 14.3.

(E)-3-Iodo-2-methylprop-2-en-1-ol (42):35 To a flame-dried, nitrogen-purged round

bottom flask (500 mL) was added dry DCM (100 mL) and Cp2ZrCl2 (3.78 I OH g, 12.9 mmol). The above solution was cooled to 0°C and Me3Al (2 M in toluene, 77.6 mL, 155 mmol) was added dropwise followed by addition of a solution of propargyl alcohol 38 (3 mL, 51.5 mmol) in 10 mL of DCM. The mixture was warmed up to room temperature and stirred overnight. The solution was cooled to -30 °C and I2 (19.6 g, 77.3 mmol) was added. After 3 h stirring at room temperature, the solution was quenched by sat. potassium sodium tartrate to give a heterogeneous solution. The mixture was filtered through celite and filter cake was further rinsed with ether. The aqueous layer was washed with DCM (3 x 50 mL) and the combined organic layers were washed with brine, dried over anhydrous MgSO4, filtered, and concentrated via rotary evaporation to give crude product 42 which was used directly for the next step.

(E)-tert-Butyl(3-iodo-2-methylallyloxy)dimethylsilane (43):35 The mixture 42 obtained from above step was dissolved in dry DCM (100 mL) followed by the I OTBS addition of TBSCl (3.60 g, 24.5 mmol) and imidazole (1.70 g, 24.5 mmol). The solution was stirred at room temperature for 3 h and quenched with 30 mL of water and extracted with ether (3 x 30 mL). The combined organic layers were dried over

Na2SO4, filtered, concentrated and purified by flash chromatography (eluent 1 pentane/ether) to give 43 as colorless oil (3.49 g, 78% yield). H NMR (400 MHz, CDCl3) δ 6.13 (dd, J = 2.6, 1.3 Hz, 1H), 4.03 (d, J = 1.0 Hz, 2H), 1.71 (d, J = 0.6 Hz, 3H), 0.84 (s, 13 9H), 0.00 (s, 6H); C NMR (101 MHz, CDCl3) δ 146.8, 75.9, 67.1, 25.8, 21.1, 18.3, -5.5.

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(E)-Methyl-3-(tributylstannyl)acrylate (41):18 To a stirred solution of n-butyllithium (43 mL, 1.6 M solution in hexanes, 68.7 mmol) in 50 mL of dry THF was added dropwise bistributyltin (34.7 mL, 68.7 mmol) at 0 oC under nitrogen. The solution was stirred for 20 min before transferred by cannula to a precooled solution of copper bromide dimethylsulfide complex (14.2 g, 68.9 mmol) in 30 mL of dry THF at -50 oC. The resulting black mixture was stirred for 25 min before cooling to -78 oC. A THF solution of methyl propiolate (2.0 mL, 22.9 mmol) was added dropwise and the reaction mixture was stirred for 4 h. Dry methanol (69 mL) was added and the reaction was warmed to room temperature. The reaction mixture was partitioned between diethyl ether (100 mL) and water (100 mL) and filtered through Celite to remove the dark coloration. The phases were separated and the aqueous fraction was extracted with ether (3 x 50 mL). The combined organic layers were washed with brine (150 mL), dried over MgSO4, and concentrated. The product was purified by column chromatography (eluent pentane/ether) to afford stannane 41 as 1 colorless oil (9.9 g, quant. yield). H NMR (400 MHz, CDCl3) δ 7.68 (d, J = 19.4 Hz, 1H), 6.24 (d, J = 19.4 Hz, 1H), 3.68 (s, 3H), 1.44 (dd, J = 12.0, 4.4 Hz, 6H), 1.23 (dt, J = 14.1, 7.1 Hz, 6H), 0.96 – 0.86 (m, 6H), 0.83 (td, J = 7.3, 4.5 Hz, 9H).

(2E,4E)-Methyl-6-(tert-butyldimethylsilyloxy)-5-methylhexa-2,4-dienoate (44): A solution of Pd -(dba) (157 mg, 0.17 mmol) in 50 mL of MeO 2 3 OTBS NMP was treated with triphenyl arsine (105 mg, 0.34 mmol) O at room temperature. The solution was stirred for 10 min followed by the addition of a solution of iodide 43 (1.20 g, 3.77 mmol) in 5 mL of NMP. The solution was further stirred for 10 min followed by the addition of a solution of stannane 41 (1.29 g, 3.43 mmol) in 5 mL of NMP. The reaction mixture was stirred overnight followed by quenching with saturated aqueous potassium fluoride solution (30 mL). The reaction was diluted with diethyl ether (30 mL). The organic layer was separated and washed with saturated aqueous potassium fluoride solution (10 mL), dried and concentrated. The product was purified by column chromatography (eluent 1 pentane/ether) to give 44 as colorless oil (750 mg, 81%). H NMR (400 MHz, CDCl3) δ 7.53 (dd, J = 15.2, 11.8 Hz, 1H), 6.19 (dd, J = 11.8, 0.8 Hz, 1H), 5.78 (d, J = 15.2 Hz, 1H), 4.05 (d, J = 0.6 Hz, 2H), 3.67 (s, 3H), 1.75 (d, J = 0.7 Hz, 3H), 0.84 (s, 9H), 0.00 (s, 13 6H); C NMR (101 MHz, CDCl3) δ 167.9, 147.7, 140.5, 120.9, 119.7, 67.3, 51.4, 25.9,

18.4, 14.4, -5.4. HRMS (APCI+) calculated for C14H27O3Si:271.1724, found:271.1718.

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(2E,4E)-6-(tert-Butyldimethylsilyloxy)-5-methylhexa-2,4-dien-1-ol (45): To a stirred solution of the methyl ester 44 (1.0 g, 3.7 mmol, 1 equiv.) in HO OTBS 30 mL of DCM was added DIBAL-H (2 ml, 1.60 g, 11.1 mmol, 3 equiv.) over 0.5 h at -78 oC under nitrogen. The reaction mixture was stirred for about 4 h when TLC showed full conversion. The reaction mixture was quenched with 30 mL saturated aqueous Rochelle salt (potassium sodium tartrate) and stirred for 30 min. The phases were separated and the aqueous layer was extracted with DCM (3 x 30 mL). The combined organic phases were dried over

Na2SO4, concentrated and purified by flash chromatography (eluent pentane/ether) to 1 give 45 as colorless oil (720 mg, 80% yield). H NMR (400 MHz, CDCl3) δ 6.43 (ddt, J = 15.0, 11.0, 1.4 Hz, 1H), 6.02 (d, J = 11.1 Hz, 1H), 5.74 (dt, J = 15.1, 6.0 Hz, 1H), 4.14 (d, J = 6.0 Hz, 2H), 4.00 (s, 2H), 1.66 (s, 3H), 1.37 (d, J = 5.2 Hz, 1H), 0.85 (s, 9H), 0.00 13 (s, 6H); C NMR (101 MHz, CDCl3) δ 138.1, 130.9, 127.5, 122.5, 67.9, 63.7, 25.9, 18.4, + 13.9, -5.4. HRMS (APCI+) calculated for C13H25O3Si[M-OH] :225.1669, found:225.1667.

(But-2-ynyloxy)(tert-butyl)dimethylsilane (46):36 To a stirred solution of but-2-yn-1-ol (930 mg, 13.3 mmol, 1 equiv.) in anhydrous dichloromethane (50 mL) was added imidazole (1.00 g, 14.6 mmol, 1.1 equiv.) followed by OTBS tert-butyl-dimethylsilyl chloride (2.2 g, 14.6 mmol, 1.1 equiv.), and the resulting white suspension was stirred at room temperature for overnight. The reaction mixture was quenched with 30 mL of water and extracted with ether (3 x 20 mL). The combined organic layers were dried over Na2SO4, filtered, concentrated and purified by flash chromatography (eluent pentane/EtOAc) to give 46 as colorless oil (2.32 g, 95% 1 yield). H NMR (400 MHz, CDCl3) δ 4.16 (d, J = 2.3 Hz, 2H), 1.72 (s, 3H), 0.80 (s, 9H), 13 0.00 (s, 6H); C NMR (101 MHz, CDCl3) δ 80.9, 77.8, 52.0, 25.9, 18.4, 3.6, -5.2.

(E)-tert-Butyl(3-iodobut-2-enyloxy)dimethylsilane (48):20 To a stirred solution of

ZrCp2Cl2 (4.4 g, 15 mmol) in dry THF (50 mL) was added slowly o TBSO I iBu2AlH (2.13 g, 2.73 mL, 15 mmol) at 0 C under nitrogen. The resultant suspension was stirred for 30 min at 0 oC, followed by addition of a THF solution of alkyne 46 (850 mg, 4.61 mmol). The mixture was warmed to room temperature and stirred until a homogeneous solution resulted (about 2 h) and then o cooled to –78 C, followed by the addition of a THF solution of I2 (3.81 g, 15 mmol). After stirring for 30 min, the reaction mixture was quenched with 1N HCl, extracted with ether, washed successively with saturated Na2S2O3, NaHCO3 and brine, dried over

MgSO4, filtered, and concentrated. Flash chromatography (eluent, hexanes) to give 48 as

89 Chapter 4

1 colorless oil (0.84 g, 58% yield). H NMR (400 MHz, CDCl3) δ 6.33 – 6.11 (m, 1H), 4.05 (d, J = 6.5 Hz, 2H), 2.34 (s, 3H), 0.83 (d, J = 1.1 Hz, 9H), 0.00 (d, J = 1.1 Hz, 6H); 13C

NMR (101 MHz, CDCl3) δ 140.6, 95.9, 60.7, 28.1, 25.9, 18.4, -5.2.

(2E,4E)-Methyl 6-(tert-butyldimethylsilyloxy)-4-methylhexa-2,4-dienoate (50): A

solution of Pd2-(dba)3 (60 mg, 0.066 mmol) in 50 mL of MeO OTBS NMP was treated with triphenyl arsine (67 mg, 0.22 mmol) O at room temperature. The solution was stirred for 10 min followed by the addition of a solution of iodide 48 (0.840 g, 2.19 mmol) in 5 mL of NMP. The solution was further stirred for 10 min followed by the addition of a solution of stannane 41 (0.83 g, 2.2 mmol) in 5 mL of NMP. The reaction was stirred for overnight followed by quenching with saturated aqueous potassium fluoride solution (30 mL). The reaction was diluted with diethyl ether (30 mL). The organic layer was separated and washed with saturated aqueous potassium fluoride solution (10 mL), dried and concentrated. The product was purified by column chromatography (eluent pentane/ether) 1 to give 50 as colorless oil (607 mg, quant. yield). H NMR (400 MHz, CDCl3) δ 7.24 (d, J = 15.5 Hz, 1H), 5.88 (dd, J = 6.3, 5.7 Hz, 1H), 5.77 (d, J = 15.7 Hz, 1H), 4.29 (d, J = 6.0 Hz, 2H), 3.68 (s, 3H), 1.69 (d, J = 1.0 Hz, 3H), 0.83 (s, 9H), 0.00 (s, 6H); 13C NMR

(101 MHz, CDCl3) δ 167.7, 148.8, 140.5, 132.4, 116.6, 60.4, 51.5, 25.9, 18.3, 12.4, -5.2.

HRMS (APCI+) calculated for C14H27O3Si:271.1724, found:271.1722.

(2E,4E)-6-(tert-Butyldimethylsilyloxy)-4-methylhexa-2,4-dien-1-ol (51): To a stirred solution of the methyl ester 50 (0.900 g, 3.33 mmol, 1 equiv.)

HO in 30 mL of DCM was added DIBAL-H (1.8 ml, 1.42 g, 9.98 OTBS mmol, 3 equiv.) over 0.5 h at -78 oC. The reaction mixture was stirred for about 4 h when TLC showed full conversion. The reaction mixture was quenched with 30 mL saturated aqueous Rochelle salt (potassium sodium tartrate) and stirred for 30 min. The phases were separated and the aqueous layer was extracted with

DCM (3 x 30 mL). The combined organic phases were dried over Na2SO4, concentrated and purified by flash chromatography (eluent pentane/ether) to give 51 as colorless oil 1 (647 mg, 80% yield). H NMR (400 MHz, CDCl3) δ 6.19 (d, J = 15.7 Hz, 1H), 5.72 (dt, J = 15.7, 6.0 Hz, 1H), 5.52 (t, J = 6.3 Hz, 1H), 4.25 (d, J = 6.3 Hz, 2H), 4.14 (dd, J = 6.0, 0.8 Hz, 2H), 1.68 (d, J = 0.9 Hz, 3H), 1.39 (br, 1H), 0.83 (s, 9H), 0.00 (s, 6H); 13C NMR

(101 MHz, CDCl3) δ 135.8, 133.3, 132.1, 126.9, 63.8, 60.2, 25.9, 18.4, 12.7, -5.1. HRMS + (APCI+) calculated for C9H17O2Si [M-tBu] : 185.0998, found:185.1322.

90 Chapter 4

General procedure for the copper-catalyzed allylic alkylation of an allylic bromide with organomagnesium reagents

A Schlenk tube equipped with septum and stirring bar was charged with CuBr•SMe2 (0.0125 mmol, 2.6 mg, 5 mol%) and the appropriate ligand (0.015 mmol, 6 mol%). Dry dichloromethane (3 mL) was added and the solution was stirred under nitrogen at room temperature for 15 min and cooled to –80 °C. Then, the corresponding organomagnesium reagent (0.3 mmol, 1.2 eq) was added under nitrogen. Allylic bromide (0.25 mmol) was dissolved in 1 mL of DCM and added dropwise to the reaction mixture over 1 h using a syringe pump. The reaction was quenched after overnight with a saturated aqueous

NH4Cl solution (2 mL) and the mixture was warmed up to room temperature, diluted with ether and the layers were separated. The aqueous layer was extracted with dichloromethane (3 x 5 mL) and the combined organic layers were dried over anhydrous

Na2SO4, filtered and carefully concentrated (note: several products are highly volatile). The crude product was purified by flash chromatography on silica gel using different mixtures of n-pentane: Et2O as the eluent.

Note: Gas chromatography analysis was carried out to determine the SN2’:SN2 ratio’s and ee’s on a sample obtained after aqueous extraction with dichloromethane, which has been passed through a short plug of silica gel to remove transition metal residues.

(R,E)-(3-Methylpenta-1,4-dienyl)benzene (2a): The title compound was prepared from 1a (55 mg, 0.25 mmol) following the general procedure for the copper-catalyzed asymmetric allylic alkylation. Purification by

column chromatography (SiO2, Pentane) afforded product (26 mg,

66% yield, 99% ee) as a colourless oil as a mixture of two regioisomers 2a/3a (SN2’/SN2) 1 1 in 95:5 ratio (determined by H NMR at 20 °C and GC). H NMR (400 MHz, CDCl3) δ 7.40 – 7.14 (m, 5H), 6.39 (d, J = 16.0 Hz, 1H), 6.18 (dd, J = 16.0, 7.0 Hz, 1H), 5.88 (ddd, J = 17.0, 10.3, 6.5 Hz, 1H), 5.05 (m, 2H), 3.11 – 2.99 (m, 1H), 1.21 (d, J = 6.9 Hz, 3H); 13 C NMR (101 MHz, CDCl3) δ 142.4, 137.6, 134.3, 128.6, 128.5, 127.0, 126.1, 113.3, 20 40.6, 19.8. HRMS (APCI+) calculated for C12H15:159.1168, found: 159.1157. [α]D = 13 20 –56.5 (c = 2.3, CHCl3), [Lit. (S isomer, er = 96:4) [α]D = +55 (c = 0.87, CHCl3)]. Enantiomeric excess was determined by chiral GC analysis, CP-Chiralsil-Dex-CB (25 m x 0.25 mm), initial temperature 80 ºC for 80 min, then 1oC/min to 140oC (hold for 5 min), then 10oC/min to 180oC (final temp), retention times (min.): 25.8 (major) and 26.9 (minor); retention time 3a: 72.0 min. In analogy to this result, the absolute configuration of the other products is assumed to be (R).

91 Chapter 4

(R,E)-3,7-Dimethylocta-1,4-diene (2b): The title compound was prepared from 1b (50 mg, 0.25 mmol) following the general procedure for the copper-catalyzed asymmetric allylic alkylation. Purification by column chromatography (SiO2, Pentane) afforded product (11 mg, 32% yield, 99% ee) as a colourless oil as a mixture of two regioisomers 2b/3b (SN2’/SN2) in 96:4 ratio 1 1 (determined by H NMR at 20 °C and GC). H NMR (400 MHz, CDCl3) δ 5.80 – 5.66 (m, 1H), 5.38 – 5.23 (m, 2H), 4.97 – 4.81 (m, 2H), 2.77 – 2.73 (m, 1H), 1.82 (dd, J = 9.7, 3.6 Hz, 2H), 1.55 – 1.50 (m, 1H), 1.01 (d, J = 6.9 Hz, 3H), 0.80 (d, J = 6.6 Hz, 6H); 13C

NMR (101 MHz, CDCl3) δ 143.4, 135.0, 128.0, 112.4, 41.9, 40.3, 28.4, 22.2, 20.0. + 20 HRMS (APCI+) calculated for C7H11 [M–iso-propyl] : 95.0861, found: 95.0814. [α]D =

+4.8 (c = 0.25, CHCl3). Enantiomeric excess was determined by chiral GC analysis, CP-Chiralsil-Dex-CB (25 m x 0.25 mm), initial temperature 60 ºC for 20 min, then 0.5oC/min to 100oC (hold for 10 min), then 1oC/min to 140oC (final temp), retention times (min.): 15.1 (major) and 15.5 (minor); retention time 3b: 34.6 min.

(R,E)-Methyl-4-methylhexa-2,5-dienoate (2c):37 The title compound was prepared from 1c (51 mg, 0.25 mmol) following the general procedure for the O copper-catalyzed asymmetric allylic alkylation. Purification by O column chromatography (SiO2, Pentane/Et2O = 10/1) afforded product (15 mg, 43% yield, 97% ee) as a colorless oil as a mixture of two regioisomers 1 1 2c/3c (SN2’/SN2) in 94:6 ratio (determined by H NMR at 20 °C and GC). H NMR (400

MHz, CDCl3) δ 6.86 (dd, J = 15.7, 6.9 Hz, 1H), 5.79 – 5.63 (m, 2H), 5.05 – 4.94 (m, 2H), 13 3.66 (s, 3H), 3.02 – 2.87 (m, 1H), 1.10 (d, J = 6.9 Hz, 3H); C NMR (101 MHz, CDCl3) 20 δ 167.1, 152.1, 140.0, 119.7, 114.8, 51.4, 40.0, 18.8. [α]D = –20 (c = 0.65, CHCl3). Enantiomeric excess was determined by chiral GC analysis, CP-Chiralsil-Dex-CB (25 m x 0.25 mm), initial temperature 60 ºC for 20 min, then 0.5oC/min to 100oC (hold for 10 min), then 1oC/min to 140oC (final temp), retention times (min.): 35.9 (major) and 37.6 (minor); retention time 3c: 64.0 min.

(R,E)-tert-Butyldimethyl(4-methylhexa-2,5-dienyloxy)silane (2d): The title compound was prepared from 1d (73 mg, 0.25 mmol) following the general TBSO procedure for the copper-catalyzed asymmetric allylic alkylation.

Purification by column chromatography (SiO2, Pentane/Et2O = 10/1) afforded product (52 mg, 92% yield, 99% ee) as a colorless oil as a mixture of two regioisomers 2d/3d 1 1 (SN2’/SN2) in 91:9 ratio (determined by H NMR at 20 °C and GC). H NMR (400 MHz,

CDCl3) δ 5.72 (ddd, J = 17.1, 10.3, 6.6 Hz, 1H), 5.58 – 5.39 (m, 2H), 4.91 (m, 2H), 4.08

92 Chapter 4

(dt, J = 4.7, 1.0 Hz, 2H), 2.87 – 2.72 (m, 1H), 1.03 (d, J = 6.9 Hz, 3H), 0.84 (s, 9H), 0.00 13 (s, 6H); C NMR (101 MHz, CDCl3) δ 142.6, 134.5, 128.2, 112.9, 64.0, 39.9, 26.0, 19.6, 20 18.4, –5.1. HRMS (ESI+) calculated for C13H27OSi:227.1826, found: 227.1820. [α]D =

–4.4 (c = 2.5, CHCl3). Enantiomeric excess was determined by chiral GC analysis, CP-Chiralsil-Dex-CB (25 m x 0.25 mm), initial temperature 80 ºC for 80 min, then 1oC/min to 140oC (hold for 5 min), then 10oC/min to 180oC (final temp), retention times (min.): 23.6 (major) and 24.4 (minor); retention time 3d: 48.2 min.

(R,E)-Ethyl‐2,4-dimethylhexa-2,5-dienoate (2e): The title compound was prepared from 1e (58 mg, 0.25 mmol) following the general procedure for O the copper-catalyzed asymmetric allylic alkylation. Purification by O column chromatography (SiO2, Pentane/Et2O = 10/1) afforded product (38 mg, 90% yield, 98% ee) as a colorless oil as a mixture of two regioisomers 1 1 2e/3e (SN2’/SN2) in 97:3 ratio (determined by H NMR at 20 °C and GC). H NMR (400

MHz, CDCl3) δ 6.51 (dd, J = 9.6, 1.4 Hz, 1H), 5.69 (ddd, J = 16.6, 10.3, 6.4 Hz, 1H), 5.02 – 4.87 (m, 2H), 4.12 (q, J = 7.1 Hz, 2H), 3.16 – 3.10 (m, 1H), 1.79 (d, J = 1.3 Hz, 13 3H), 1.23 (t, J = 7.1 Hz, 3H), 1.07 (d, J = 6.8 Hz, 3H); C NMR (101 MHz, CDCl3) δ 168.3, 144.5, 140.6, 127.0, 113.7, 60.5, 37.0, 19.7, 14.3, 12.4. HRMS (APCI+) calculated 20 for C10H17O2:169.1223, found: 169.1219. [α]D = –2.8 (c = 1.85, CHCl3). Enantiomeric excess was determined by chiral GC analysis, CP-Chiralsil-Dex-CB (25 m x 0.25 mm), initial temperature 60 ºC for 20 min, then 0.5oC/min to 100oC (hold for 10 min), then 1oC/min to 140oC (final temp), retention times (min.): 54.6 (major) and 55.8 (minor); retention time 3e: 86.3 min.

(R,E)-Ethyl-3,4-dimethylhexa-2,5-dienoate (2f): The title compound was prepared from 1f (58 mg, 0.25 mmol) following the general procedure for O the copper-catalyzed asymmetric allylic alkylation. Purification by O column chromatography (SiO2, Pentane/Et2O = 10/1) afforded product (31 mg, 74% yield, 98% ee) as a colorless oil as a mixture of two regioisomers 1 1 2f/3f (SN2’/SN2) in 95:5 ratio (determined by H NMR at 20 °C and GC). H NMR (400

MHz, CDCl3) δ 5.75 – 5.60 (m, 2H), 5.05 – 4.96 (m, 2H), 4.09 (q, J = 7.1 Hz, 2H), 2.89 – 2.77 (m, 1H), 2.05 (d, J = 1.3 Hz, 3H), 1.21 (t, J = 7.1 Hz, 3H), 1.10 (d, J = 6.9 Hz, 3H); 13 C NMR (101 MHz, CDCl3) δ 167.0, 162.2, 140.3, 115.2, 115.0, 59.6, 47.2, 17.8, 16.8, 20 14.3. HRMS (APCI+) calculated for C10H17O2:169.1223, found: 169.1220. [α]D = +19.0

(c = 0.6, CHCl3). Enantiomeric excess was determined by HPLC analysis (Chiralpak OD-H: 0.5 mL/min, n-heptane, 40 °C isotherm, 214 nm), retention times: 54.2 min (major), 57.9 min (minor).

93 Chapter 4

(R,Z)-Ethyl‐3,4-dimethylhexa-2,5-dienoate (2g): The title compound was prepared

O O from 1g (58 mg, 0.25 mmol) following the general procedure for the copper-catalyzed asymmetric allylic alkylation. Purification by

column chromatography (SiO2, Pentane/Et2O = 10/1) afforded product (32 mg, 74% yield with trace of ether, 99% ee) as a colorless oil as a mixture of 1 two regioisomers 2g/3g (SN2’/SN2) in 96:4 ratio (determined by H NMR at 20 °C and 1 GC). H NMR (400 MHz, CDCl3) δ 5.77 (ddd, J = 16.4, 10.4, 6.0 Hz, 1H), 5.58 (s, 1H), 5.10 – 4.92 (m, 2H), 4.63 – 4.50 (m, 1H), 4.08 (q, J = 7.1 Hz, 2H), 1.71 (d, J = 1.0 Hz, 13 3H), 1.21 (t, J = 7.1 Hz, 3H), 1.08 (d, J = 7.0 Hz, 3H); C NMR (101 MHz, CDCl3) δ 166.1, 162.0, 140.4, 116.1, 114.3, 59.5, 37.9, 20.0, 17.1, 14.3. HRMS (APCI+) calculated 20 for C10H17O2:169.1223, found: 169.1222. [α]D = +130.1 (c = 1.5, CH2Cl2). Enantiomeric excess was determined by chiral GC analysis, CP-Chiralsil-Dex-CB (25 m x 0.25 mm), initial temperature 60 ºC for 20 min, then 0.5oC/min to 100 oC (hold for 10 min), then 1 oC/min to 140 oC (final temp), retention times (min): 56.1 (major) and 56.5 (minor); retention time 3g: 80.6 min.

Cross metathesis between 2d and 52.

(S,2E,5E)-Ethyl-7-((tert-butyldimethylsilyl)oxy)-4-methylhepta-2,5-dienoate (53): To a stirred solution of compound 2d/3d (20 mg, 0.088 mmol, 91/9) in 4 mL of dry DCM was added ethyl acrylate 52 (9 mg, 0.09 mmol) and Hoveyda–Grubbs second-generation catalyst22 (0.6 mg, 0.00088 mmol, 1 mol%) under a nitrogen atmosphere at room temperature. The reaction mixture was stirred for 2 d and the product was purified by flash chromatography (pentane/Et2O = 10/1) to give 53 as a colorless oil (15 mg, 57%) 1 which was contaminated by trace of compound 54. H NMR (400 MHz, CDCl3) δ 6.84 (dd, J = 15.7, 6.9 Hz, 1H), 5.72 (dd, J = 15.7, 1.4 Hz, 1H), 5.55 – 5.44 (m, 2H), 4.14 – 4.07 (m, 4H), 2.99 – 2.94 (m, 1H), 1.22 (t, J = 7.1 Hz, 3H), 1.10 (d, J = 6.9 Hz, 3H), 0.84 13 (s, 9H), 0.00 (s, 6H); C NMR (101 MHz, CDCl3) δ 166.8, 152.1, 132.0, 129.8, 119.9, 63.6, 60.2, 38.7, 26.0, 19.1, 18.4, 14.3, -5.2. HRMS (APCI+) calculated for 20 C16H31O3Si:299.2042, found: 299.2037. [α]D = +8.2 (c = 0.7, CH2Cl2).

94 Chapter 4

4.7 References and notes

1. M. S. F. L. K. Jie, M. K. Pasha, M. S. K. Syed-Rahmatulla, Nat. Prod. Rep., 1997, 14, 163–189. 2. (a) T. Ichiba, W. Y. Yoshida, P. J. Scheuer, T. Higa, J. Am. Chem. Soc., 1991, 113, 3173–3174; (b) T. Higa, J. Tanaka, A. Kitamura, T. Koyama, M. Takahashi, T. Uchida, Pure Appl. Chem., 1994, 66, 2227–2230. 3. M. C. Wilson, S.-J. Nam, T. A. M. Gulder, C. A. Kauffman, P. R. Jensen, W. Fenical, B. S. Moore, J. Am. Chem. Soc., 2011, 133, 1971–1977. 4. (a) D. T. Connor, R. C. Greenough, M. von Strandtmann, J. Org. Chem., 1977, 42, 3664–3669; (b) S. M. Ringel, R. C. Greenough, S. Roemer, D. Connor, A. L. Gutt, B. Blair, G. Kanter, M. von Strandtmann, J. Antibiot., 1977, 371–375. 5. (a) J. Kobayashi, J. Cheng, T. Ohta, H. Nakamura, S. Nozoe, Y. Hirata, Y. Ohizumi, T. Sasaki, J. Org. Chem., 1988, 53, 6147; (b) Y. Kikuchi, M. Ishibashi, T. Sasaki, J. Kobayashi, Tetrahedron Lett., 1991, 32, 797. 6. (a) D. Vuong, R. J. Capon, J. Nat. Prod.,2000, 63, 1684; M. McNally, R. J. Capon, J. Nat. Prod.,2001, 64, 645; (b) H.-S. Lee, S. Y. Park, C. J. Sim, J.-R. Rho, Chem. Pharm. Bull, 2008, 56, 1198. 7. (a) T. K. Macklin, G. C. Micalizio, J. Am. Chem. Soc., 2009, 131, 1392–1393; (b) S. Hanessian, T. Focken, X. Mi, R. Oza, B. Chen, D. Ritson, R. Beaudegnies, J. Org. Chem., 2010, 75, 5601–5618. 8. T. K. Macklin, G. C. Micalizio, Nature Chem., 2010, 2, 638–643. 9. (a) R. K. Sharma, T. V. RajanBabu, J. Am. Chem. Soc., 2010, 132, 3295–3297; (b) A. Zhang, T. V. RajanBabu, J. Am. Chem. Soc., 2006, 128, 54–53; (c) C. R. Smith, T. V. RajanBabu, Org. Lett.,2008, 10, 1657–1659. 10. For reviews on Cu-catalysed AAA, see: (a) S. Harutyunyan, T. den Hartog, K. Geurts, A. J. Minnaard and B. L. Feringa, Chem. Rev., 2008, 108, 2824; (b) A. Alexakis, J. E. Bäckvall, N. Krause, O. Pàmies and M. Diéguez, Chem. Rev., 2008, 108, 2796. (c) J.-B. Langlois, A. Alexakis, In Transition Metal Catalyzed Allylic Substitution in Organic Synthesis (Ed. U. Kazmaier,) Springer-Verlag, Berlin, 2012, pp. 235–268. 11. M. A. Kacprzynski, A. H. Hoveyda, J. Am. Chem. Soc., 2004, 126, 10676–10681. 12. H. Li, A. Alexakis, Angew. Chem. Int. Ed., 2012, 51, 1055 –1058. 13. M. Magrez, Y. L. Guen, O. Baslé, C. Crévisy, M. Mauduit, Chem. Eur. J., 2013, 19, 1199–1203. 14. H. Oda, T. Kobayashi, M. Kosugi, T. Migita, Tetrahedron, 1995, 51, 695–702; G. Pattenden, D. A. Stoker, Synlett, 2009, 11, 1800–1802.

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15. F. -Y. Yang, M. Shanmugasundaram, S. -Y. Chuang, P. -J. Ku, M. -Y. Wu, C.-H. Cheng, J. Am. Chem. Soc., 2003, 125, 12576–12583. 16. A. Barbero, F. J. Pulido, Chem. Soc. Rev., 2005, 34, 913–920. 17. R. J. Payne, F. Peyrot, O. Kerbarh, A. D. Abell, C. Abell, ChemMedChem, 2007, 2, 1015–1029. 18. (a) E. Piers, J. M. Chong, Can. J. Chem., 1988, 66, 1425 – 1429. (b) E. Piers, J. M. Chong, H. E. Morton, Tetrahedron, 1989, 45, 363 –380. 19. J. Schwartz, J. A. Labinger, Angew. Chem. Int. Ed., 2003, 15, 330–340. 20. Z. Huang, E. Negishi, Org. Lett., 2006, 8, 3675–3678. 21. For selected examples, see: (a) F. López, A.W. van Zijl, A. J. Minnaard, B. L. Feringa, Chem. Commun., 2006, 4, 409–411; (b) K. Geurts, S. P. Fletcher, B. L. Feringa, J. Am. Chem. Soc. 2006, 128, 15572-15573; (c) M. Fañanás-Mastral, B. ter Horst, A. J. Minnaard , B. L. Feringa, Chem. Commun., 2011, 47, 5843–5845; (d) M. Pérez, M. Fañanás-Mastral, P. H. Bos, A. Rudolph, S. R. Harutyunyan, B. L. Feringa, Nature Chem., 2011, 3, 377–38. 22. (a) S. J. Connon, S. Blechert, Angew. Chem. Int. Ed., 2003, 42, 1900–1923; (b) A. H. Hoveyda, A. R. Zhugralin, Nature, 2007, 450, 243–251; (c) C. Samojlowicz, M. Bieniek, K. Grela, Chem. Rev. 2009, 109, 3708–3742; (d) G. C. Vougioukalakis, R. H. Grubbs, Chem. Rev. 2010, 110, 1746–1787. 23. D. D. Kim, S. J. Lee, P. Beak, J. Org. Chem., 2005, 70, 5376–5386. 24. T. den Hartog, S. R. Harutyunyan, D. Font, A. J. Minnaard, B. L. Feringa, Angew. Chem. Int. Ed., 2008, 47, 398–401. 25. B. Narasimhan, V. Judge, R. Narang, R. Ohlan, S. Ohlan, Bioorg. Med. Chem. Lett., 2007, 17, 5836–5845. 26. L. Ferrié, D. Amans, S. Reymond, V. Bellosta, P. Capdevielle, J. Cossy, J. Organomet. Chem., 2006, 691, 5456–5465. 27. L. Boisvert, F. Beaumier, C. Spino, Org. Lett., 2007, 9, 5361–5363. 28. V. Gudipati, D. P. Curran, Tetrahedron Lett., 2011, 52, 2254–2257. 29. S. B. Han, A. Hassan, I. S. Kim, M. J. Krische, J. Am. Chem. Soc., 2010, 132, 15559–15561. 30. L. Zhao, X. Lu, W. Xu, J. Org. Chem., 2005, 70, 4059–4063. 31. A. A. C. van Wijk, J. Lugtenburg, Eur. J. Org. Chem., 2002, 4217–4221. 32. Q. Y. Hu, P. D. Rege, E. J. Corey, J. Am. Chem. Soc., 2004, 126, 5984–5986. 33. P. Li, J. Li, F. Arikan, W. Ahlbrecht, M. Dieckmann, D. Menche, J. Am. Chem. Soc., 2009, 131, 11678–11679. 34. W. Zhang, H. Xu, H. Xu, W. Tang, J. Am. Chem. Soc., 2009, 131, 3832–3833. 35. N. Kotoku, N. Tamada, A. Hayashi, M. Kobayashi, Bioorg. Med. Chem. Lett., 2008, 18, 3532–3535. 36. F. R. Wuest, M. Berndt, J. Label. Compd. Radiopharm., 2006, 49, 91–100.

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37. A. Zhang, T. V. RajanBabu, J. Am. Chem. Soc., 2006, 128, 54–53.

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98 Chapter 5

Chapter 5

Towards a Total Synthesis and Structure Elucidation of Phorbasin B

In this chapter an approach towards the catalytic asymmetric synthesis of Phorbasin B is presented. The copper-catalyzed asymmetric allylic alkylation presented in chapter 4 is the key strategic transformation in this synthesis.

99 Chapter 5

5.1 Introduction

The Phorbasins1 (Figure 1) are a class of structurally novel diterpenes isolated from a southern Australian marine sponge (Phorbas sp.). For example, Phorbasin B and C were isolated during scientific trawling operations in the Great Australian Bight, while Phorbasin D-F were isolated during an investigation of anticancer agents from marine organisms. Preliminary biological studies of Phorbasins B, C and E showed that they displayed GI50 (growth inhibition) values against several cancer cell lines in the range of 5-15 μM.1c As no follow-up appeared in chemical biology, we felt that a concise synthesis of Phorbasins, taking advantage of highly efficient and selective catalytic methods and making these compounds and analogs more readily available could greatly stimulate biological studies.

Figure 1. Representative Phorbasins.

5.2 Previous synthesis of Phorbasins

In 2009 the group of Micalizio2 reported the first and so far the only total synthesis of Phorbasin C (ent). They employed a titanium-mediated reductive cross-coupling3 (Figure 2) of diol 1 with TMS-propyne 2. This reaction proceeds via a formal metallo-[3,3] rearrangement involving intermediate 3 with exquisite selectivity. However, the 1,4-diene unit was prepared in a multiple step sequence. As a starting material the chiral alcohol 4 was used which was synthesized from (S)-Roche ester.4 Oxidation of the alcohol to the corresponding aldehyde followed by a stereoselective Julia-Kocienski olefination to give E-alkene 5. Removal of the THP group to the free alcohol, followed by oxidation to the aldehyde and Takai olefination gave the 1,4-diene unit 6. Compared

100 Chapter 5 with the methodology developed in chapter 4, this synthetic route is less efficient.

I THPO OH THPO

45 6

O B O TMS

O TMS (L) OH n Ti Me 1. 2,2-DMP,PTSA O 2 HO Br 2. OsO4,NMO O O Me 3.Bu SnH, AIBN Ti(OiPr)4 - 3 HO O O Me OH M+ H 1 3

O Me O Me H+ O Me 1. VO(acac)2,TBHPO O TMS TMS O TMS 2. (COCl)2,DMSO

3. CH2=PPh3 - Ti O O (L)n HO O

O Me 1. NIS HO Me O TMS 2. TFA O 1. Pd(PPh3)4,AcOH I Phorbasin C AcO (ent) 2. Ac2O 3. IBX AcO OAc OAc 4. Sc(OTf)2 Figure 2. Total synthesis of Phorbasin C (ent).2

5.3 First retrosynthetic analysis of Phorbasin B

In 1994 the group of Narasaka5 reported the total synthesis of Paniculide A (Figure 3) via a chiral TADDOL-titanium complex catalyzed asymmetric D-A reaction of vinyl borate 7 in high yield and ee. We also envisioned applying this asymmetric D-A reaction in the synthesis of Phorbasin B. In our retrosynthetic analysis (Figure 4), Phorbasin B was disconnected into right part 8 (the 1,4-diene unit) and left part 9 (the cyclohexenone unit). The former (both of the enantiomers) can be provided by copper-catalyzed asymmetric allylic alkylation of diene bromide as described in chapter 4. The left part, 9, in principle could be prepared by the above mentioned catalytic asymmetric D-A reaction followed by further functionalization (Riley oxidation6 and Baylis-Hillman reaction7).

101 Chapter 5

Figure 3. Total synthesis of Paniculide A involving an enantioselective D‐A reaction.

Figure 4. First retrosynthetic analysis of Phorbasin B.

5.4 Results and discussion

The first attempt (Figure 5) towards the synthesis of vinyl borate 7 started from the preparation of acid chloride 13. However, all attempts (PCl5, oxalyl chloride and PCl3) to prepare 13 gave complicated products. O OH HN O Cl 14 O O O oxalyl chloride or O O O N B N O O O PCl3 or PCl5 O 12 13 15 7 Figure 5. First attempt towards vinyl borate 7.

The second attempt (Figure 6) started by the synthesis of vinyl iodide 16 from acid 12

102 Chapter 5 using aq. HI. Chlorination of 16 using oxalyl chloride in neat form (Cl-I exchange happened when the reaction was carried out in solvent) followed by addition of the oxazolidinone 14 resulted in the vinyl iodide 17 in 70% overall yield. However, all attempts (I-Li exchange followed by the synthesis of boronate8 and palladium catalyzed coupling with di-boronate9) towards the synthesis of 7 failed.

Figure 6. Second attempt towards vinyl borate 7.

Figure 7. Final synthesis of vinyl borate 7.

The third route started from propargyl alcohol 18. TMS-protection of the terminal position of alkyne 18 was followed by oxidation to acid 20 using Jones reagent.10 Chlorination in neat form followed by the addition of oxazolidinone 14 resulted in the desired product 21 in 72% yield together with a trace of deprotected product 15. Removal of the silyl group using TBAF gave product 15 in high yield. Hydroboration of alkyne 15 using Ipc2BH resulted in intermediate 22 which was quenched with ethanol to form boronate 23. Exchange of the ethoxy groups with 2,2-dimethylpropane-1,3-diol finished

103 Chapter 5 the synthesis and gave 7 in 67% yield. However, the Diels-Alder reaction (Figure 8) of vinyl borate 7 and diene 11 (mixture of E/Z isomers) catalyzed by TADDOL-titanium complex didn’t work. Employing pure E-diene 11 gave similar results and starting materials were recovered.

Figure 8. Attempted D‐A reaction catalyzed by TADDOL‐titanium complex.

5.5 Second retrosynthesis of Phorbasin B

Figure 9. Second retrosynthetic analysis of Phorbasin B.

In our second retrosynthetic analysis (Figure 9), Phorbasin B was disconnected into right part 8 (the 1,4-diene unit) and left part 9 as before. The former (both of the enantiomers) was provided by copper-catalyzed asymmetric allylic alkylation of diene bromide described in chapter 4. The left part, 9, might be prepared from 25 by a Baylis-Hillman reaction. Compound 25 is anticipated to be accessible via cyclization11 of ketone 26.

104 Chapter 5

5.6 Results and discussion

Figure 10. Second route towards Phorbasin B.

Figure 11. Zimmerman‐Traxler transition state of Evans aldol reaction.

The second synthesis of Phorbasin B started from the preparation of chiral oxazolidinone 30 from acid 27 via chlorination followed by acylated carbamate formation (Figure 10). Evans aldol reaction12 (the preferred transition state is showing in Figure 11 where the dipoles of the enolate and the carbonyl group are opposed, and there is the least number of unfavored steric interactions13) of 30 with aldehyde 31 first resulted in borinate ester 32 which was initially oxidative cleaved by methanol and hydrogen peroxide. However elimination product 33 was the only product. Employing neutral conditions14 (pH=7 phosphate buffer) the oxidative cleavage gave the desired product 34 in 97% yield.

Reduction of 34 using LiBH4 resulted in diol 35 in high yield. To investigate the effect of the protecting group on the cyclization (see Figure 9), linear 105 Chapter 5 and cyclic protecting groups were employed. For the synthesis of a cyclic acetal (Figure 12), initial acetal formation using TsOH and benzaldehyde dimethyl acetal resulted in elimination product 36. Fortunately changing to a weaker acid, PPTS,15 the desired cyclic product 37 was obtained in high yield. Wacker oxidation16 of 37 gave ketone 38 which was transformed to aldehyde 39 by ozonolysis in pure DCM.17

Figure 12. Synthesis of cyclic keto‐aldehyde 39.

The synthesis of the linear keto-aldehyde 42 (Figure 12) followed the same sequence as for the cyclic analogue. Protection using TBSCl gave 40 followed by Wacker oxidation to ketone 41. Finally ozonolysis of the olefin moiety in 41 resulted in the product 42.

Figure 13. Synthesis of linear keto‐aldehyde 42.

Initial cyclization of 39 using aq. NaOH18 (Table 1, entry 1) resulted in only 5% yield. Employing a two step sequence via the formation of hemi-acetal using DBU (0.5 equiv.) followed by the formation of mesylate and elimination gave the desired product in 24% 19 20 yield. The highest yield was obtained using K2CO3 in tBuOH, however, the reaction was very unselective. Finally using DBU under dehydration condition21 resulted in 18% yield of cyclohexenone 43. The attempted cyclization of 42 using various conditions didn’t result in desired product 44. The control of conformation via cyclic acetal is very crucial for the cyclization which is entropically favored.22

106 Chapter 5

Table 1. Cyclization of 39 and 42.

Entry Conditiona Yieldb Conditiona Yieldb

1 aq.NaOH 5% aq.NaOH ‐‐

2 1. DBU, DCM 24% 1. DBU, DCM ‐‐

2. MsCl, Et3N 2. MsCl, Et3N

c 3 K2CO3, tBuOH 37% TsOH, benzene ‐‐ 4 DBU, benzene 18% aThe reaction was stopped after 16 h. boverall yield based on alkene 38. cunselective reaction.

Table 2. Investigating the amount of the base used and reaction time.

Entry Condition Yielda

1 1.DBU(0.5 equiv.), DCM, rt, 1d; 2. MsCl, Et3N, DCM 24%

2 1.DBU(0.5 equiv.), DCM, rt, 2d; 2. MsCl, Et3N, DCM 26%

3 1.DBU(1 equiv.), DCM, rt, 2d; 2. MsCl, Et3N, DCM 40% (full conv.) aOverall yield based on alkene 38.

With the most promising conditions in hand (Table 1, entry 2), we investigated the effect of the amount of the base used and reaction time. Extending the reaction time to two days didn’t improve the yield (Table 2, entry 2). Fortunately by doubling the amount of the base to 1 equiv., the reaction finished in 2 d and gave 40% overall yield. The stereochemistry of the cyclohexenone 43 with two cis-oriented substituents was confirmed by its 1H NMR spectrum, in which the coupling constant between 4-H and 5-H (Table 2) was 2.9 Hz in good accordance with data for analogous cis-disubstituted cyclohexenones.23

107 Chapter 5

Initial α-hydroxylation24 (Figure 14) of 43 using oxirane 46 and NaHMDS (for enolate formation) gave a complex mixture of products. Employing a two step sequence25 (Figure 15) via the formation of silyl enol ether 47 followed by oxidation gave also complex products. We later found that compound 43 was not stable under these basic conditions. 26 Employing mild conditions (Et3N and TESOTf), the silyl enol ether 50 was obtained in quantitative yield (Figure 16), no further attempts to prepare 51 was made due to time constraints.

Figure 14. Attempted α‐Hydroxylation of 43.

Figure 15. Attempted α‐Hydroxylation of 43.

Figure 16. α‐Hydroxylation of 43.

5.7 Conclusion

Several attempts have been made to synthesize fragments 25 and 8 of Phorbasin B. So far we have achieved the synthesis of the right part (the 1,4-diene unit) in a highly enantioselective manner as described in chapter 4. For the left part, we have achieved the synthesis of the cyclohexenone ring 43 by intramolecular cyclization after the failure of

108 Chapter 5 asymmetric D-A approach. The stereochemistry of 43 was controlled by Evans aldol reaction. Further functionalization of the ring especially using a Baylis-Hillman reaction of 51 and Zr-catalyzed methylalumination of the acetylene side chain in 54 are required in the future to finish the fragment 55 (Figure 17). The final key step would be the cross metathesis of 55 and 8 with precise control of E-geometry of the double bond. Comparison of the spectra and optical rotation with the natural product could be employed to deduce the absolute configuration of the methyl group even the absolute stereochemistry of Phorbasin B.

Figure 17. Synthesis route towards Phorbasin B

5.8 Experimental section

Starting materials were purchased from Aldrich, Alpha Aesar or Acros and used as received unless stated otherwise. All solvents were reagent grade and, if necessary, dried and distilled prior to use. Column chromatography was performed on silica gel (Aldrich 60, 230-400 mesh) or on aluminium oxide (Merck, aluminium oxide 90 neutral activated).

TLC was performed on silica gel 60/Kieselguhr F254.

1H and 13C NMR spectra were recorded on a Varian VXR300 (299.97 MHz for 1H, 75.48 MHz for 13C) or a Varian AMX400 (399.93 MHz for 1H, 100.59 MHz for 13C) spectrometer in CDCl3 unless stated otherwise. Chemical shifts are reported in δ values 1 13 (ppm) relative to the residual solvent peak (CHCl3, H = 7.24, C = 77.0). Carbon assignments are based on 13C and APT 13C experiments. Splitting patterns are indicated as follows: s (singlet), d (doublet), t (triplet), q (quartet), m (multiplet), br (broad).

High resolution mass spectra (HRMS) were recorded on an AEI-MS-902 and FTMS

109 Chapter 5 orbitrap (Thermo Fisher Scientific) mass spectrometer. Optical rotations were measured on a Schmidt+ Haensch polarimeter (Polartronic MH8) with a 10 cm cell (c given in g/100 mL).

(E)-3-Iodoacrylic acid (16):27 A mixture of propiolic acid 12 (22 g, 314 mmol) and aq. O HI (72 mL of a 57% w/w (7 M) aqueous solution, 503 mmol) was heated I OH in three Al-foil-wrapped Ace tubes at 95 oC overnight. The resulting mixtures were cooled to ambient temperature. The pressure was released (careful!!!), and the mixture was diluted with water (5 mL) and filtered under vacuum. Washing of the suspended product with light petroleum (20 mL), followed by drying afforded the iodo 1 acid 16 (35.1 g, 60%) as large white needles. H NMR (300 MHz, CDCl3) δ 8.09 (d, J = 14.8 Hz, 1H), 6.90 (d, J = 14.8 Hz, 1H).

(E)-3-(3-Iodoacryloyl)oxazolidin-2-one (17): To a stirred solution of acid 16 (100 mg, O O 0.51 mmol, 1 equiv) and a catalytic amount of DMF was slowly added o I N O oxalyl chloride (0.05 mL, 0.61 mmol, 1.2 equiv) at 0 C under nitrogen. The reaction mixture was warmed to room temperature and stirred for 1 h to form the acid chloride. To another flask, NaH (30 mg, 60% dispersed in mineral oil, 0.72 mmol, 1.4 equiv) was added to a stirred solution of compound 14 (57 mg, 0.66 mmol, 1.3 equiv) in 10 mL of dry THF at 0 oC under nitrogen. The mixture was stirred for 30 min followed by the slow addition of above acid chloride. The reaction mixture was warmed to room temperature and quenched with a saturated aq. NH4Cl solution (5 mL) when TLC showed full conversion. The layers were separated, the aqueous layer was washed with ether (3 x 5 mL) and the organic layers were dried over anhydrous Na2SO4, filtered, concentrated and purified by flash chromatography (eluent pentane/ether) to give 17 as a white solid (93 mg, 70% yield). 1H NMR (300 MHz,

CDCl3) δ 8.28 (d, J = 14.5 Hz, 1H), 8.12 (d, J = 14.5 Hz, 1H), 4.45 (t, J = 8.0 Hz, 2H), 13 4.05 (t, J = 8.0 Hz, 2H); C NMR (101 MHz, CDCl3) δ 162.8, 153.1, 134.8, 101.7, 62.2,

42.5. HRMS (ESI+) calculated for C6H6INO3Na:289.9285, found: 289.9282.

3-(Trimethylsilyl)prop-2-yn-1-ol (19):28 To a stirred solution of propargyl alcohol 18 OH (4.66 mL, 4.5 g, 80 mmol) in THF (100 mL) at –78 °C was added dropwise n-BuLi (1.6 M in hexanes, 100 mL, 160 mmol, 200 mol %). TMS After stirring for 20 min at –78 °C, TMSCl (25 mL, 196 mmol, 220 mol %) was added dropwise. The reaction mixture was stirred for 5 min at –78 °C, warmed to room temperature and stirred for an additional1 h. The reaction was quenched with 50 mL water and then 10% aq. HCl was added to the crude reaction mixture until

110 Chapter 5 complete consumption of the TMS-ether was observed according to TLC analysis. The layers were separated, the aqueous layer was washed with ether (3 x 50 mL) and the organic layers were dried over anhydrous Na2SO4, filtered, concentrated and the crude product was purified by flash chromatography (eluent pentane/ether) to give 19 as 1 colorless oil (9.65 g, 94% yield). H NMR (400 MHz, CDCl3) δ 4.26 (d, J = 6.1 Hz, 2H), 13 1.76 (t, J = 5.3 Hz, 1H), 0.17 (s, 9H); C NMR (101 MHz, CDCl3) δ 103.8, 90.6, 51.6, -0.2.

3-(Trimethylsilyl)propiolic acid (20):29 Jones’ reagent (11.1 mL, 2.7 M) was added OH dropwise to an ice-cold solution of alcohol 19 (1.14 g, 10 mmol) in O acetone (100 mL). The resulting mixture was stirred for 1 h at room TMS temperature. The mixture was diluted with tert-butyl methyl ether (50 mL) and washed with water; the organic phase was dried over Na2SO4, filtered, concentrated and the residue was purified by flash chromatography (eluent pentane/ether) 1 to give 20 as colorless oil (1.24 g, 98% yield). H NMR (400 MHz, CDCl3) δ 10.30 (br, 13 1H), 0.26 (s, 9H); C NMR (101 MHz, CDCl3) δ 157.2, 97.2, 93.8, -1.0.

3-(3-(Trimethylsilyl)propioloyl)oxazolidin-2-one (21): To a stirred solution of acid 20 O O (14.5 g, 100 mmol, 1 equiv) and a catalytic amount of DMF was o N O slowly added oxalyl chloride (9.5 mL, 110 mmol, 1.1 equiv) at 0 C TMS under nitrogen. The reaction mixture was warmed to room temperature and stirred for 1 h to form the acid chloride. To another flask, NaH (6.00 g, 60% dispersed in mineral oil, 150 mmol, 1.5 equiv) was added to a stirred solution of compound 14 (13 g, 150 mmol, 1.5 equiv) in 100 mL of dry THF at 0 oC under nitrogen. The mixture was stirred for 30 min followed by the slow addition of above acid chloride. The reaction mixture was warmed to room temperature and quenched with a saturated aq.

NH4Cl solution (50 mL) after TLC showed full conversion, the layers were separated. The aqueous layer was washed with ether (3 x 50 mL) and the organic layers were dried over anhydrous Na2SO4, filtered, concentrated and purified by flash chromatography (eluent pentane/ether) to give 21 as colorless oil (15.1 g, 72% yield). 1H NMR (400 MHz, 13 CDCl3) δ 4.44 – 4.36 (m, 2H), 4.06 – 3.95 (m, 2H), 0.29 – 0.23 (m, 9H); C NMR (101

MHz, CDCl3) δ 172.3, 150.2, 105.0, 94.1, 61.9, 42.3, -1.0. HRMS (ESI+) calculated for

C9H13NO3SiNa:234.0557, found: 234.0554.

111 Chapter 5

3-Propioloyloxazolidin-2-one (15):30 To a stirred solution of 21 (15.1 g, 72 mmol) in O O THF (50 mL) was added TBAF (1.0 M solution in THF, 108 mL, 108

N O mmol) at room temperature. The resulting solution was stirred for 4 h,

and then quenched with sat. aq. NH4Cl and extracted with EtOAc (3 x 50 mL). The combined organic layers were dried over Na2SO4, filtered, concentrated and the crude product was purified by flash chromatography (eluent pentane/ether) to give 15 as 1 a colorless oil (7.75 g, 77%): H NMR (400 MHz, CDCl3) δ 4.46 (dd, J = 8.5, 7.5 Hz, 2H), 4.06 (dd, J = 8.5, 7.4 Hz, 2H), 3.45 (s, 1H).

Preparation of Ipc2BH: A 50-mL centrifuge vial fitted with a rubber septum and magnetic stirring bar was charged with borane dimethylsulfide (2.58 mL, 25 mmol) and 25 mL of dry THF under nitrogen. The mixture was cooled to 0 oC and (+)-α-pinene (7.94 mL, 50 mmol) was added dropwise with stirring. After the complete addition of α-pinene, the stirring was stopped and the centrifuge vial was stored at 0 oC overnight. The supernatant solution was decanted by using a double-ended needle. The crystalline lumps of (-)-Ipc2BH were broken, washed with dry ether (3 x 10 mL) and dried at room temperature to afford the product: 5.3 g (74% yield).

(E)-3-(3-(5,5-Dimethyl-1,3,2-dioxaborinan-2-yl)acryloyl)oxazolidin-2-one (7):30 Under an argon atmosphere, 15 (2.10 g, 15.1 mmol) was O O O added to a THF (20 mL) suspension of B N O o O diisopinocampheylborane (6.10 g, 22.6 mmol) at 0 C. After stirring for 1.5 h, excess acetaldehyde (12.6 mL, 226 mmol) was added and the mixture was warmed to 40 oC and stirred for 1 h. The mixture was cooled to ambient temperature followed by the addition of 2,2-dimethyl- 1,3-propanediol (2.34 g, 22.6 mmol) and subsequently stirred for 3 h. After removal of the solvent under reduced pressure, ethanol was added to the residue. The resulting precipitates were filtered, washed with ether and then dried to afford the product 7 (3.1 g, 82%) as a white 1 solid. H NMR (300 MHz, CDCl3) δ 7.86 (d, J = 17.7 Hz, 1H), 6.89 (d, J = 17.7 Hz, 1H), 4.44 (t, J = 8.0 Hz, 2H), 4.08 (t, J = 8.0 Hz, 2H), 3.67 (s, 4H), 1.58 (s, 3H), 0.98 (s, 3H).

(R)-4-Benzyl-3-(pent-4-enoyl)oxazolidin-2-one (30): To a stirred solution of acid 27 O (8.05 g, 80.4 mmol, 1.5 equiv) and a catalytic amount of DMF was O O N slowly added oxalyl chloride (10.2 g, 6.9 mL, 80.4 mmol, 1.5 equiv) at 0 oC under nitrogen. The reaction mixture was warmed to room Bn temperature and stirred for 1 h to form acid chloride. To another flask, NaH (2.80 g, 60% dispersed in mineral oil, 64.3 mmol, 1.2 equiv) was added to a stirred

112 Chapter 5 solution of compound 29 (9.50 g, 53.6 mmol, 1 equiv) in 100 mL of dry THF at 0 oC under nitrogen. The mixture was stirred for 30 min followed by the slow addition of above acid chloride. The reaction mixture was warmed to room temperature and quenched with a saturated aq. NH4Cl solution (50 mL) when TLC showed full conversion. The layers were separated, the aqueous layer was washed with ether (3 x 50 mL) and the organic layers were dried over anhydrous Na2SO4, filtered, concentrated and the crude product was purified by flash chromatography (eluent pentane/ether) to give 30 1 as colorless oil (12.21 g, 88% yield). H NMR (400 MHz, CDCl3) δ 7.28 (ddd, J = 30.2, 19.3, 6.8 Hz, 5H), 5.88 (ddt, J = 16.9, 10.3, 6.5 Hz, 1H), 5.07 (ddd, J = 13.7, 11.5, 1.4 Hz, 2H), 4.67 (ddd, J = 10.5, 7.1, 3.5 Hz, 1H), 4.25 – 4.11 (m, 2H), 3.30 (dd, J = 13.4, 3.3 Hz, 1H), 3.06 (qt, J = 17.2, 7.3 Hz, 2H), 2.76 (dd, J = 13.4, 9.6 Hz, 1H), 2.54 – 2.37 (m, 2H); 13 C NMR (101 MHz, CDCl3) δ172.5, 153.6, 136.7, 135.2, 129.4, 128.9, 127.3, 115.7,

66.2, 55.2, 37.9, 34.8, 28.2. HRMS (ESI+) calculated for C15H18O3N:260.1281, found: 20 260.1276. [α]D = -73.8 (c = 3.0, CHCl3).

(R)-3-((2R,3S)-2-Allyl-3-hydroxy-5-methylhex-4-enoyl)-4-benzyloxazolidin-2-one (34): To a stirred solution of compound 30 (2.6 g, 10 mmol, 1 equiv) in 30 mL of dry

O DCM was added dibutylboron triflate (11 mL, 1 M in DCM, 1.1 O o O equiv) at 0 C under nitrogen followed by N OH N,N-di-isopropylethylamine (2 mL, 1.55 g, 1.2 mmol, 1.2 equiv). Bn The resulting solution was stirred for 1 h at that temperature and cooled to -78 oC. A DCM solution of aldehyde 31 (925 mg, 11 mmol, 1 equiv) was slowly added to the above solution, the mixture was warmed to room temperature and stirred overnight. The reaction was quenched with 50 mL of phosphate buffer (pH=7) followed by oxidative workup with aq. H2O2 (10 mL) in methanol. The aqueous layer was washed with ether (3 x 50 mL) and the organic layers were dried over anhydrous Na2SO4, filtered, concentrated and the crude product was purified by flash chromatography (eluent pentane/ether) to give 34 as colorless oil (3.33 g, 97% yield). 1H

NMR (400 MHz, CDCl3) δ 7.44 – 7.16 (m, 5H), 5.95 – 5.76 (m, 1H), 5.36 – 5.25 (m, 1H), 5.07 (ddd, J = 13.6, 11.0, 1.1 Hz, 2H), 4.80 – 4.59 (m, 2H), 4.38 – 4.21 (m, 1H), 4.21 – 4.09 (m, 2H), 3.28 (dd, J = 13.4, 3.3 Hz, 1H), 2.77 – 2.60 (m, 1H), 2.60 – 2.53 (m, 1H), 2.49 (ddd, J = 7.9, 6.1, 3.1 Hz, 1H), 1.74 (d, J = 1.1 Hz, 3H), 1.69 (d, J = 1.2 Hz, 3H); 13 C NMR (101 MHz, CDCl3) δ 174.3, 153.7, 137.3, 135.3, 135.3, 129.4, 128.9, 127.3, 123.8, 117.2, 69.4, 66.0, 55.6, 47.8, 38.0, 32.7, 25.9, 18.4. HRMS (ESI+) calculated for 20 C20H24O3N:326.1751, found: 326.1745. [α]D = -38.6 (c = 0.15, CHCl3).

113 Chapter 5

(2S,3S)-2-Allyl-5-methylhex-4-ene-1,3-diol (35): To a stirred solution of alcohol 34 OH (3.33 g, 9.7 mmol, 1 equiv) in MeOH/THF (40 mL, 1/1 v/v) was added OH o LiBH4 (423 mg, 19.4 mmol, 2 equiv) at 0 C under nitrogen. The mixture

was warmed to room temperature and quenched with a saturated aq. NH4Cl solution (30 mL) when TLC showed full conversion. The layers were separated, the aqueous layer was washed with ether (3 x 30 mL) and the organic layers were dried over anhydrous Na2SO4, filtered, concentrated and the crude product was purified by flash chromatography (eluent pentane/ether) to give 35 as colorless oil (1.37 g, 1 83% yield). H NMR (400 MHz, CDCl3) δ 5.81 (ddt, J = 17.3, 10.1, 7.0 Hz, 1H), 5.34 (d, J = 9.2 Hz, 1H), 5.04 (t, J = 12.2 Hz, 2H), 4.56 (dd, J = 9.2, 4.3 Hz, 1H), 3.76 (dd, J = 10.9, 7.3 Hz, 1H), 3.66 (dd, J = 10.9, 4.1 Hz, 1H), 2.35 (s, 2H), 2.15 – 2.04 (m, 1H), 2.00 (dd, J = 14.3, 7.7 Hz, 1H), 1.95 – 1.86 (m, 1H), 1.76 (s, 3H), 1.68 (s, 3H); 13C NMR (101

MHz, CDCl3) δ 136.9, 136.6, 124.4, 116.4, 71.2, 63.9, 45.1, 31.4, 26.1, 18.4. HRMS 20 (ESI+) calculated for C10H18O2Na:193.1199, found: 193.1211. [α]D = -3.3 (c = 0.65,

CHCl3).

(2R,4S,5S)-5-Allyl-4-(2-methylprop-1-en-1-yl)-2-phenyl-1,3-dioxane (37): To a stirred solution of diol 35 (1.17 g, 6.85 mmol, 1 equiv) in 30 mL of dry DCM was added benzaldehyde dimethyl acetal (1.03 mL, 1.04 g, 6.85 mmol, 1 equiv) and PPTS (pyridinium p-toluenesulfonate, 86 mg, 0.34 mmol, 5 mol%) at room temperature. The resulting solution was stirred overnight and quenched with excess triethylamine (10 mL) followed by a saturated aq. NH4Cl solution (30 mL). The layers were separated, the aqueous layer was washed with ether (3 x 20 mL) and the organic layers were dried over anhydrous Na2SO4, filtered, concentrated and the crude product was purified by flash chromatography (eluent 1 pentane/ether) to give 37 as colorless oil (1.2 g, 68% yield). H NMR (400 MHz, CDCl3) δ 7.60 – 7.27 (m, 5H), 5.94 – 5.75 (m, 1H), 5.61 (s, 1H), 5.38 (d, J = 7.7 Hz, 1H), 5.24 – 5.04 (m, 2H), 4.79 (dd, J = 7.7, 2.2 Hz, 1H), 4.25 (d, J = 11.4 Hz, 1H), 4.00 (d, J = 11.4 Hz, 1H), 2.73 – 2.53 (m, 1H), 2.49 – 2.35 (m, 1H), 1.77 (s, 3H), 1.73 (s, 3H), 1.49 (dd, J 13 = 10.7, 2.3 Hz, 1H); C NMR (101 MHz, CDCl3) δ 138.8, 137.3, 135.3, 128.8, 128.2, 126.3, 123.3, 116.7, 102.2, 76.7, 69.5, 38.1, 29.3, 25.9, 18.6. HRMS (ESI+) calculated 20 for C17H22O2Na:281.1512, found: 281.1508. [α]D = -8.0 (c = 0.65, CHCl3).

114 Chapter 5

1-((2R,4S,5S)-4-(2-Methylprop-1-en-1-yl)-2-phenyl-1,3-dioxan-5-yl)propan-2-one

(38): To a stirred solution of alkene 37 (1.15 g, 4.45 mmol, 1 equiv) in DMA/H2O (30 mL, 1/1 v/v) was added palladium dichloride (79.0 mg, 0.445 mmol, 10

mol%) and Cu(OAc)2•H2O (178 mg, 0.89 mmol, 20 mol%) at room temperature under 1 atmosphere of oxygen. The reaction mixture was

stirred for 2 d and quenched with a saturated aq. NH4Cl solution (50 mL) after TLC showed full conversion. The layers were separated, the aqueous layer was washed with ether (3 x 20 mL) and the organic layers were dried over anhydrous Na2SO4, filtered, concentrated and the crude product was purified by flash chromatography (eluent pentane/ether) to give 38 as colorless oil (0.93 g, 76% yield). 1H

NMR (300 MHz, CDCl3) δ 7.57 – 7.29 (m, 5H), 5.58 (d, J = 3.0 Hz, 1H), 5.29 – 5.06 (m, 1H), 4.85 – 4.67 (m, 1H), 4.09 (d, J = 1.5 Hz, 2H), 3.06 (dt, J = 9.5, 6.3 Hz, 1H), 2.75 (dd, J = 18.6, 3.3 Hz, 1H), 2.19 (d, J = 3.1 Hz, 3H), 2.18 – 2.07 (m, 1H), 1.73 (s, 3H), 13 1.70 (s, 3H); C NMR (75 MHz, CDCl3) δ 208.5, 138.7, 136.4, 129.1, 128.5, 126.3, 102.1, 80.4, 76.8, 71.2, 39.8, 33.8, 30.9, 26.1, 18.9. HRMS (ESI+) calculated for 20 C17H22O3Na:297.1461, found: 297.1457. [α]D = -6.0 (c = 0.2, CHCl3).

(2R,4R,5S)-5-(2-Oxopropyl)-2-phenyl-1,3-dioxane-4-carbaldehyde (39): To a stirred

solution of ketone 38 (521 mg, 1.9 mmol) in DCM was bubbled O3 at -78 oC. The ozonolysis was finished when the solution turned blue. Excess dimethyl sulfide (5 mL) was added and the solution was warmed to room

temperature. The mixture was quenched with a saturated aq. NH4Cl solution (20 mL). The layers were separated, the aqueous layer was washed with ether (3 x 20 mL) and the organic layers were dried over anhydrous

Na2SO4, filtered, concentrated and the crude product was purified by flash chromatography (eluent pentane/ether) to give 39 as colorless oil which was used 1 immediately for the next step. H NMR (300 MHz, CDCl3) δ 9.62 (s, 1H), 7.54 – 7.38 (m, 5H), 5.59 (s, 1H), 4.48 (d, J = 2.6 Hz, 1H), 4.12 (dd, J = 30.7, 11.7 Hz, 2H), 3.06 (dd, J = 13 18.5, 9.1 Hz, 1H), 2.78 – 2.43 (m, 2H), 2.14 (s, 3H); C NMR (75 MHz, CDCl3) δ 207.0, 200.4, 137.6, 129.6, 128.6, 126.3, 102.2, 98.2, 83.7, 71.0, 39.8, 30.6.

(2R,4aS,8aS)-2-Phenyl-4a,5-dihydro-4H-benzo[d][1,3]dioxin-6(8aH)-one (43): To a stirred solution of above aldehyde 39 in dry DCM (20 mL) was added DBU (289 mg, 0.28 mL, 1.9 mmol, 1.0 equiv) at room temperature. The resulting solution was stirred overnight followed by the addition of MsCl (654 mg, 5.7 mmol, 3 equiv) and triethylamine (1.73 g, 2.32 mL, 17.2 mmol, 9 equiv). The mixture was stirred for about 4 h until TLC showed full conversion. The

115 Chapter 5

mixture was quenched with a saturated aqueous NH4Cl solution (20 mL) and the layers were separated. The aqueous layer was washed with ether (3 x 10 mL) and the organic layers were dried over anhydrous Na2SO4, filtered, concentrated and the crude product was purified by flash chromatography (eluent pentane/ether) to give 43 as colorless oil 1 (179 mg, 41% yield). H NMR (400 MHz, CDCl3) δ 7.51 – 7.24 (m, 5H), 6.87 (dd, J = 10.0, 5.7 Hz, 1H), 6.12 (d, J = 10.0 Hz, 1H), 5.53 (s, 1H), 4.49 (dd, J = 5.7, 2.9 Hz, 1H), 4.17 (dd, J = 12.0, 2.9 Hz, 1H), 4.01 (d, J = 11.9 Hz, 1H), 3.15 (dd, J = 16.6, 13.5 Hz, 13 1H), 2.38 (dd, J = 16.6, 4.2 Hz, 1H), 2.07 – 1.90 (m, 1H); C NMR (101 MHz, CDCl3) δ 199.8, 142.7, 137.7, 132.8, 129.2, 128.4, 126.1, 101.9, 70.6, 69.9, 37.1, 32.5. HRMS 20 (ESI+) calculated for C14H15O3:231.1016, found: 231.1011. [α]D = +112.0 (c = 0.1,

CHCl3).

Triethyl(((2R,4aS,8aS)-2-phenyl-4a,8a-dihydro-4H-benzo[d][1,3]dioxin-6-yl)oxy)sila ne (50): To a stirred solution of ketone 43 (20 mg, 0.087 mmol, 1 equiv) in 4 mL of dry DCM was added TESOTf (30.0 mg, 0.113 mmol, 1.3 equiv) and triethylamine (10.5 mg, 0.104 mmol, 1.2 equiv) at 0 oC under nitrogen. The mixture was warmed to room temperature and stirred for 1 h when TLC showed full conversion. The reaction was quenched with a saturated aq.

NH4Cl solution (5 mL), and the layers were separated. The aqueous layer was washed with ether (3 x 5 mL) and the organic layers were dried over anhydrous

Na2SO4, filtered, concentrated to give 50 as colorless oil which was used in the next step 1 immediately (50 mg, quant. yield). H NMR (400 MHz, CDCl3) δ 7.36 (ddd, J = 7.0, 6.4, 2.4 Hz, 5H), 6.07 (dd, J = 9.7, 1.9 Hz, 1H), 6.00 (dd, J = 9.7, 5.7 Hz, 1H), 5.45 (s, 1H), 5.04 (s, 1H), 4.29 (t, J = 5.1 Hz, 1H), 4.22 (d, J = 1.4 Hz, 2H), 2.33 (s, 1H), 0.92 (t, J = 13 7.9 Hz, 6H), 0.51 (q, J = 7.9 Hz, 9H); C NMR (101 MHz, CDCl3) δ 148.6, 138.5, 131.4, 128.9, 128.2, 126.3, 124.2, 106.8, 100.5, 70.7, 69.4, 34.8, 5.8, 4.9.

(5S,6S)-6-Allyl-2,2,3,3,9,9,10,10-octamethyl-5-(2-methylprop-1-en-1-yl)-4,8-dioxa-3, 9-disilaundecane (40): To a stirred solution of diol 35 (200 mg, 1.18 mmol, 1 equiv) in OTBSOTBS dry dichloromethane (20 mL) was added imidazole (640 mg, 9.4 mmol, 8 equiv) followed by tert-butyl-dimethylsilyl chloride (1.4 mg, 9.4 mmol, 8 equiv), and the resulting white suspension was stirred at room temperature overnight. The reaction mixture was quenched with 20 mL of water and extracted with ether (3 x 20 mL). The combined organic layers were dried over Na2SO4, filtered, concentrated and the crude product was purified by flash chromatography (eluent pentane/EtOAc) to give 40 as colorless oil (521 mg, quant. yield). 1 H NMR (400 MHz, CDCl3) δ 5.94 – 5.73 (m, 1H), 5.26 – 5.11 (m, 1H), 5.11 – 4.94 (m,

116 Chapter 5

2H), 4.55 (dd, J = 9.1, 5.1 Hz, 1H), 3.57 (dt, J = 14.1, 7.0 Hz, 1H), 3.53 – 3.46 (m, 1H), 2.27 (dddd, J = 7.8, 4.8, 3.9, 2.4 Hz, 1H), 2.17 – 2.00 (m, 1H), 1.72 (d, J = 1.2 Hz, 3H), 1.65 (d, J = 1.3 Hz, 3H), 1.54 (ddd, J = 8.8, 5.0, 3.1 Hz, 1H), 0.92 (s, 9H), 0.90 (s, 9H), 13 0.04 (s, 6H), 0.04 (s, 6H); C NMR (101 MHz, CDCl3) δ 138.3, 131.6, 128.0, 115.2, 68.7, 61.5, 48.1, 30.6, 25.7, 25.7, 18.3, 18.2, -3.0, -4.2. HRMS (ESI+) calculated for 20 C22H46O2Si2Na:421.2934, found: 421.2964. [α]D = -2.0 (c = 0.6, CHCl3).

(4S,5S)-5-((tert-Butyldimethylsilyl)oxy)-4-(((tert-butyldimethylsilyl)oxy)methyl)-7-m ethyloct-6-en-2-one (41): To a stirred solution of alkene 40 (200 mg, 0.5 mmol, 1 equiv)

OTBSOTBS in DMA/H2O (15 mL, 1/1 v/v) was added palladium dichloride (9 mg,

0.05 mmol, 10 mol%) and Cu(OAc)2-H2O (20 mg, 0.1 mmol, 20 mol%) at room temperature under 1 atmosphere of oxygen. The reaction mixture

O was stirred for 2 d and quenched with a saturated aq. NH4Cl solution (50 mL) when TLC showed full conversion. The layers were separated, the aqueous layer was washed with ether (3 x 10 mL) and the organic layers were dried over anhydrous

Na2SO4, filtered, concentrated and the crude product was purified by flash chromatography (eluent pentane/ether) to give 41 as colorless oil (157 mg, 76% yield). 1 H NMR (400 MHz, CDCl3) δ 5.14 – 5.05 (m, 1H), 4.52 (dd, J = 8.9, 5.5 Hz, 1H), 3.59 (dd, J = 9.8, 5.9 Hz, 1H), 3.48 (dd, J = 9.8, 5.5 Hz, 1H), 2.59 (dd, J = 17.1, 4.7 Hz, 1H), 2.45 (dd, J = 17.1, 8.2 Hz, 1H), 2.14 (d, J = 9.9 Hz, 3H), 2.11 (dt, J = 8.2, 3.8 Hz, 1H), 1.71 (d, J = 1.1 Hz, 3H), 1.64 (d, J = 1.2 Hz, 3H), 0.90 (d, J = 6.2 Hz, 18H), 0.04 (s, 13 12H); C NMR (101 MHz, CDCl3) δ 214.7, 138.1, 133.0, 74.1, 67.8, 49.8, 46.7, 35.9, 31.4, 31.3, 23.9, 23.8, 23.7, 1.4, 0.6, 0.1. HRMS (ESI+) calculated for 20 C22H46O3Si2Na:437.2878, found: 437.2878. [α]D = +2.0 (c = 0.4, CHCl3).

(2R,3S)-2-((tert-Butyldimethylsilyl)oxy)-3-(((tert-butyldimethylsilyl)oxy)methyl)-5-ox ohexanal (42): To a stirred solution of ketone 41 (20 mg, 0.48 mmol) in DCM was o OTBSOTBS bubbled O3 at -78 C. The ozonolysis was finished when the solution O turned blue. Excess dimethyl sulfide (2 mL) was added and the solution was warmed to room temperature. The mixture was quenched with a

O saturated aq. NH4Cl solution (10 mL). The layers were separated, the aqueous layer was washed with ether (3 x 10 mL) and the organic layers were dried over anhydrous Na2SO4, filtered, concentrated and the crude product was purified by flash chromatography (eluent pentane/ether) to give 42 as colorless oil which was used 1 immediately for the next step (cyclization). H NMR (400 MHz, CDCl3) δ 9.49 (d, J = 1.3 Hz, 1H), 4.09 (dd, J = 4.1, 1.4 Hz, 1H), 3.76 (dd, J = 9.8, 4.1 Hz, 1H), 3.42 (dd, J = 9.8, 5.3 Hz, 1H), 2.64 – 2.54 (m, 1H), 2.54 – 2.43 (m, 1H), 2.37 (dd, J = 17.6, 6.4 Hz,

117 Chapter 5

1H), 2.11 (s, 3H), 0.87 (d, J = 17.4 Hz, 18H), 0.03 (dd, J = 12.9, 5.5 Hz, 12H); 13C NMR

(101 MHz, CDCl3) δ 207.4, 203.6, 113.7, 85.6, 61.1, 40.7, 39.9, 30.5, 25.8, 25.7, 18.1, -4.7, -5.1, -5.5, -5.6.

5.9 References and notes

1. (a) D. Vuong, R. J. Capon, J. Nat. Prod., 2000, 63, 1684. (b) M. McNally, R. J. Capon, J. Nat. Prod., 2001, 64, 645. (c) H. Zhang, R. J. Capon, Org. Lett., 2008, 10, 1959. (d) H. Zhang, J. M. Major, R. J. Lewis, R. J. Capon, Org. Biomol. Chem., 2008, 6, 3811. (e) H.-S. Lee, S. Y. Park, C. J. Sim, J.-R. Rho, Chem. Pharm. Bull., 2008, 56, 1198. 2. T. K. Macklin, G. C. Micalizio, J. Am. Chem. Soc., 2009, 131, 1392–1393. 3. (a) F. Kolundzic, G. C. Micalizio, J. Am. Chem. Soc., 2007, 129, 15112. (b) H. L. Shimp, A. Hare, M. McLaughlin, G. C. Micalizio, Tetrahedron, 2008, 64, 6831. (c) M. McLaughlin, H. L. Shimp, R. Navarro, G. C. Micalizio, Synlett., 2008, 735. (d) J. K. Belardi, G. C. Micalizio, J. Am. Chem. Soc., 2008, 130, 16870. 4. I. Paterson, G. J. Florence, Eur. J. Org. Chem., 2003, 2193–2208. 5. I. Yamamoto, K. Narasaka, Bull. Chem. Soc. Jpn., 1994, 67, 3327–3333. 6. H. L. Riley, British Patent, 1931, Aug. 17, 3,547,983,547,98. 7. A. B. Baylis, M. E. D. Hillman, German Patent 2155113, 1972. 8. B. J. Lundy , S. Jansone-Popova, J. A. May, Org. Lett., 2011, 13, 4958–4961. 9. F. Tripoteau, T. Verdelet, A. Hercouet, F. Carreaux, B. Carboni, Chem. Eur. J., 2011, 17, 13670–13675. 10. K. Bowden , I. M. Heilbron , E. R. H. Jones, B. C. L. Weedon, J. Chem. Soc., 1946, 39–45. 11. S. Kuwahara, S. Imada, Tetrahedron Lett., 2005, 46, 547–549. 12. D. A. Evans , J. Bartroli , T. L. Shih, J. Am. Chem. Soc., 1981, 103, 2127–2129. 13. H. E. Zimmerman, M. D. Traxler, J. Am. Chem. Soc., 1957, 79, 1920–1923. 14. F. J. P. Feuillet, M. Cheeseman, M. F. Mahon, S. D. Bull, Org. Biomol. Chem., 2005, 3, 2976–2989. 15. J. Xie, Y. Ma, D. A. Horne, Tetrahedron, 2011, 67, 7485–7501. 16. C. N. Cornell, M. S. Sigman, Org. Lett., 2006, 8, 4117–4120. 17. R. Willand-Charnley, T. J. Fisher, B. M. Johnson, P. H. Dussault, Org. Lett., 2012, 14, 2242–2245. 18. W. Sucrow, G. Radecker, Chem. Ber., 1988, 121, 219–224. 19. T. Sunazuka, M. Handa, T. Hirose, T. Matsumaru, Y. Togashi, K. Nakamura, Y. Iwai, S. Omura, Tetrahedron Lett., 2007, 48, 5297–5300. 20. S. Kuwahara, S. Hamade, W. S. Leal, J. Ishikawa, O. Kodama, Tetrahedron, 2000, 56, 8111–8117. 118 Chapter 5

21. N. Toyooka, M. Okumura, H. Nemoto, J. Org. Chem., 2002, 67, 6078–6081. 22. F. A. Carey, R. J. Sundberg, Advanced Organic Chemistry: Part B: Reaction and Synthesis, Springer, 2007. 23. (a) P. Weyerstahl, H. Marschall, M. Weirauch, K. Thefeld, H. Surburg, Flavour Fragr. J., 1998, 13, 295–318. (b) M. McNally, R. J. Capon, J. Nat. Prod., 2001, 64, 645–647. 24. S. E. de Sousa, P. O’Brien, C. D. Pilgram, Tetrahedron, 2002, 58, 4643–4654. 25. L. M. Murray , P. O’Brien , R. J. K. Taylor, Org. Lett., 2003, 5, 1943–1946. 26. M. E. Jung, J. J. Chang, Org. Lett., 2012, 14, 4898–4901. 27. L. R. Cox, G. A. DeBoos, J. J. Fullbrook, J. M. Percy, N. Spencer, Tetrahedron: Asymmetry, 2005, 16, 347–359. 28. S. Bernard, D. Defoy, Y. L. Dory, K. Klarskov, Bioorg. Med. Chem. Lett., 2009, 19, 6127–6130. 29. D. Hermeling, H. J. Schäfer, Chem. Ber., 1988, 121, 1151–8. 30. K. Narasaka, I. Yamamoto, Tetrahedron, 1992, 48, 5743–54.

119 Chapter 5

120 Chapter 6

Chapter 6

Total Synthesis of (S)‐(–)‐zearalenone

In this chapter the catalytic asymmetric synthesis of (S)-(–)-zearalenone is described. The copper-catalyzed asymmetric allylic alkylation is the key strategic element in this synthesis.

Parts of this chapter have been published: M. P. Baggelaar, Y. Huang, B. L. Feringa, F. J. Dekker, A. J. Minnaard, Bioorg. Med. Chem. 2013, In Press.

121 Chapter 6

6.1 Introduction

Resorcylic acid lactones are mycotoxins produced by various strains of fungi via polyketide biosynthesis. These medium-sized macrocyclic lactones exhibit a wide variety of interesting biological activities,1 among which selective kinase inhibition has been characterized very well.2 Zearalenone (Figure 1), probably the best known member of the resorcylic acid lactones, was first isolated from Gibberella zeae in 19623 and four years later its structure was elucidated.4 It shows estrogen agonistic properties most likely because its macrocycle can adopt a conformation that is similar to that of steroids.1e Also of interest, is the fact that related 6-alkyl salicylates inhibit histone acetyl transferase activity.5 In addition, zearalenone has been shown to exhibit antibacterial, uterotropic and anabolic activity.3,6 We envisioned that the salicylate core structure in zearalenone could provide lipoxygenase inhibitory activity for this compound,7 which has been indeed observed in this study.

Figure 1. Structure of 17‐Estradiol and Zearalenone.

6.2 Biosynthesis of zearalenone

Zearalenone is synthesized via a polyketide pathway by several fungi.8 In the biosynthesis (Figure 2), two different proteins (ZEA 1 and ZEA 2) are involved. Condensation of one acetyl-CoA with 5 malonyl-CoA catalyzed by enzyme ZEA 2 gave the intermediate 4. Intermediate 4 was condensed with 3 malonyl-CoA catalyzed by enzyme ZEA 1 to form intermediate 5. After aldol reaction, aromatization and lactone formation, zearalenone 1 was formed.

122 Chapter 6

Figure 2. Biosynthesis of zearalenone 1.

6.3 Previous synthesis of zearalenone.

Many of the synthetic routes to zearalenone 1 either lead to the racemate9 or are based on natural chiral starting materials10 or on chiral auxiliaries.11 Some groups used kinetic resolution or an enzymatic approach to obtain the chiral building blocks.12 In 1998 the group of Nicolaou reported a solid phase synthesis of zearalenone 110d using a novel cyclorelease mechanism.13

Figure 3. Release of substrate from polymer.

Most solid-phase methods employ a heteroatom that links the substrate to the polymer support.14 The substrate is released from the polymer by deprotecting the heteroatom, however, the heteroatom remains in the substrate (Figure 3, A). Recently this method was

123 Chapter 6 replaced by ring closing metathesis and Stille coupling as shown by the group of

Nicolaou (Figure 3, B and C). Oxidation of Merrifield resin 2 (Figure 4) using K2CO3 and DMSO gave the corresponding aldehyde. Olefination of the above aldehyde gave vinyl resin 3. Radical reaction of vinyl resin 3 with nBu2SnHCl (prepared from nBu2SnH2 and nBu2SnCl2) afforded polystyrene-di-n-butyltin chloride (PBTC) 4 in 90% overall yield.

Figure 4. Synthesis of polymer‐supported PBTC.

nBu Li nBu nBu OTBS nBu 1. TBAF Sn 5 Sn Cl 2. NCS, Me2S 4 6 OTBS

1. nBu MgBr nBu TBSO 8 TBSO Sn nBu 1. TBAF nBu 2. NCS, Me S Sn 2 MEM O O O 7 9 O 2. OH 10 MEM MEMO I O O PPh3, DEAD OH O O MEMO 1. Pd(PPh ) 3 4 O I 2. HCl HO nBu nBu Sn O O 11 1 Figure 5. Solid‐phase total synthesis of (S)‐zearalenone 1 by a cyclorelease mechanism that uses the Stille coupling strategy.

Displacement of the chloride from PHTC 4 by vinyl lithium 5 gave product 6 in 87% yield. Removal of the TBS group to the corresponding free alcohol followed by oxidation using NCS and Me2S gave the aldehyde 7 in 92% overall yield. Addition of Grignard reagent 8 to aldehyde 7 gave the corresponding secondary alcohol which was followed by Corey-Kim oxidation to give ketone 9 in 97% overall yield. Removal of the TBS group to the free alcohol followed by Mitsunobu reaction of carboxylic acid 10 gave the

124 Chapter 6 desired product 11 in 76% overall yield. Palladium catalyzed cyclorelease, after deprotection, gave (S)-zearalenone 1 in 40% overall yield. In order to develop a short catalytic route with precise control over the absolute configuration in this important class of compounds, a catalytic approach is reported here for the synthesis of (S)-zearalenone using highly enantioselective copper-catalyzed asymmetric hetero allylic alkylation15 as the key step.

6.4 Retrosynthetic analysis

Figure 6. Retrosynthesis of zearalenone 1.

In our retrosynthetic analysis (Figure 6), the macrocycle 1 was planned to be formed not by macrolactonization, but by ring-closing metathesis to access the E-double bond. In this way, the starting material for this ring-closing reaction, 12, can be prepared from acid fluoride 13 and alcohol 14 which could be obtained from ester 15 by copper-catalyzed asymmetric allylic alkylation recently developed in our group as described in chapter 2.

6.5 Results and discussion

The synthesis started with a Vilsmeier-Haack reaction16 of commercially available bromide 16 to afford aldehyde 17 in 82% yield (Figure 7). Initial Stille cross coupling17 using Pd2dba3, CuI in NMP resulted in no conversion. Employing THF as the solvent, product 18 could be obtained in 50% yield. The best yield was provided by Pd(PPh3)4

125 Chapter 6 without any copper additive in toluene. Subsequent Pinnick oxidation16 of aldehyde 18 gave acid 19 in 80% yield followed by fluorination using cyanuric fluoride18 to acid fluoride 13.

Figure 7. Synthesis of acid fluoride 13.

For the second building block 24, the synthesis started with the preparation of ketone 22. Alkylation19 (Figure 8) of ketoester 20 using 1-bromo-3-butene gave product 21 in 69% yield and the product was still contaminated with di-alkylated product. Hydrolysis and subsequent decarboxylation resulted in 22. Due to the impurities in the final product another route was tested.

Figure 8. First route towards the synthesis of ketone 22.

Figure 9. Second route towards the synthesis of ketone 22.

Copper catalyzed addition of the Grignard reagent20 to acid chloride 23 resulted in ketone 22 which was transferred into hydrazone21 24 for the subsequent C-C bond formation.

126 Chapter 6

Figure 10. Synthesis of building block 31.

The stereogenic center was installed by copper-catalyzed asymmetric allylic alkylation of 25 recently developed in our group; ester 15 was obtained in high yield and with excellent enantioselectivity. Hydrolysis followed by protection gave alkene 26 which was used for hydroboration to alcohol 27 using 9-BBN in high yield and regioselectivity. Subsequent iodination gave iodide 28 in 88% yield followed by coupling22 with hydrazone 24, after hydrolysis, gave 29 in 81% yield. Before removal of the TBDPS group the ketone had to be protected, as without this modification the compound would suffer loss of enantiopurity by a reversible intramolecular hydride shift as already reported in 1968 by Wendler et al.9f Protection of the ketone provided 30 followed by removal of the silane group using TBAF finished the synthesis of the other part.

127 Chapter 6

O O O O O O O O F KHMDS + HO O THF, 0oC O 13 82% O 31 32

O O O O AlI3, TBAI 2nd phloroglucinol Grubbs O pTsOH, acetone/H2O O toluene, 80 oC 5 oC, benzene o O 40 C, 83% O 63% 88% O 33 34 O O

OH O O OH HO O (S)-(-)-Zearalenone 1 HO OH phloroglucinol Figure 11. Final synthesis of zearalenone 1.

Ester formation23 using acid fluoride 13 and alcohol 31 gave alkene 32. Compound 32 was then subjected to alkene metathesis12b catalyzed by the Grubbs second generation catalyst to E-alkene 33. Removal of the acetal using p-TsOH and demethylation using

AlI3, TBAI and phloroglucinol (iodine scavenger) completed the total synthesis and gave zearalenone 1 as the final product. The spectroscopic data were in agreement with the reported data.12d

6.6 Biological studies

Figure 12. Activity of LOX‐5 at different concentrations of zearalenone.

128 Chapter 6

Lipoxygenases inhibition by (S)-(-)-Zearalenone was investigated using a spectrophotometric assay for the conversion of linoleic acid into hydroperoxy eicosatetraenoic acid (HPETE) by the group of Dekker at the faculty of mathematics and natural sciences (FWN). An inhibitory concentration 50% (IC50) of 51 +/- 2 μM was observed for inhibition of soybean lipoxygenase-1 (SLO-1), which indicates a modest inhibitory potency. Figure 12 depicts the activity at several different concentrations. To the best of our knowledge, no LOX-5 inhibition for this type of molecules has been reported.

6.7 Conclusion

In summary, we have completed the total synthesis of (S)-(-)-zearalenone 1 using catalytic asymmetric allylic substitution, Stille cross coupling and RCM as key steps. Asymmetric allylic alkylation was the step in the synthesis used to obtain the chiral allylic alcohol building block. The new synthetic route is currently explored in the preparation of other biologically active resorcylic acid lactones.

6.8 Experimental section

Starting materials were purchased from Aldrich, Alpha Aesar or Acros and used as received unless stated otherwise. All solvents were reagent grade and, if necessary, dried and distilled prior to use. Column chromatography was performed on silica gel ® (SiliaFlash 60, 230-400 mesh). TLC was performed on silica gel 60/Kieselguhr F254. 1H and 13C NMR spectra were recorded on a Varian VXR300 (299.97 MHz for 1H, 75.48 MHz for 13C) or a Varian AMX400 (399.93 MHz for 1H, 100.59 MHz for 13C) spectrometer in CDCl3 unless stated otherwise. Chemical shifts are reported in  values 1 13 (ppm) relative to the residual solvent peak (CHCl3, H = 7.24, C = 77.0). Carbon assignments are based on 13C and APT 13C experiments. Splitting patterns are indicated as follows: s (singlet), d (doublet), t (triplet), q (quartet), m (multiplet), br (broad). High resolution mass spectra (HRMS) were recorded on an AEI-MS-902 and a FTMS orbitrap (Thermo Fisher Scientific) mass spectrometer. Optical rotations were measured on a Schmidt+ Haensch polarimeter (Polartronic MH8) with a 10 cm cell (c given in g/100 mL).

129 Chapter 6

16 2-Bromo-4,6-dimethoxybenzaldehyde (17): POCl3 (3.26 mL, 34.8 mmol) was carefully added to a solution of 16 (3.00 g, 13,9 mmol) in DMF (7.2 mL) O O at 0 oC. The mixture was heated to 100 oC and stirred for 4 h. The reaction mixture was cooled to rt. The brownish oil was poured on ice O Br and left overnight. The precipitate was filtered and then dissolved in toluene. The solvent was removed under reduced pressure and the crude product was purified by flash chromatography (eluent pentane/ether) to give 17 as yellow solid (2.83 1 g, 83%). H NMR (400 MHz, CDCl3) δ 10.27 (s, 1 H), 6.74 (d, J = 2.2 Hz, 1 H), 6.40 (d, 13 J = 2.2 Hz, 1 H), 3.86 (s, 3 H), 3.84 (s, 3 H). C NMR (100 MHz, CDCl3) δ 189.2, 164.5, 163.7, 127.4, 116.9, 111.7, 98.2, 56.2, 56.0.

4,6-Dimethoxy-2-vinyl-benzaldehyde (18): To a stirred solution of 17 (450 mg, 1.8

O O mmol) in toluene (25 mL) was added Pd(PPh3)4 (21 mg, 0.018 mmol, 1 mol%) and tributyl(vinyl)tin (0.86 g, 2.7 mmol) at rt. The mixture was heated to 100 oC and stirred for 8 h followed by the addition of 25 mL of O aq. KF solution (4 M). The layers were separated and the organic layer was filtered over celite. The solvent was removed under reduced pressure and purified by flash chromatography (eluent pentane/ether) to give 18 as yellow solid (295 mg, 84%). 1 H NMR (400 MHz, CDCl3) δ 10.47 (s, 1 H), 7.58 (dd, J = 17.4, 10.9 Hz, 1 H), 6.61 (d, J = 2.2 Hz, 1 H), 6.41 (d, J = 2.3 Hz, 1 H), 5.63 (dd, J = 17.4, 1.4 Hz, 1 H), 5.38 (dd, J = 13 10.9, 1.4 Hz, 1 H), 3.89 (s, 6 H). C NMR (100 MHz, CDCl3) δ 190.5, 164.8, 164.8, 143.6, 136.6, 117.5, 116.4, 104.5, 97.5, 56.0, 55.7. HRMS (ESI+): m/z [M+H]+ calc. for

C11H13O3: 193.0859; found: 193.0857.

4,6-Dimethoxy-2-vinyl-benzoic acid (19): To a stirred solution of 18 (600 mg, 3.12 O O mmol) in t-BuOH/THF=1:1 (30 mL) was added 2-methyl-2-butene (2.20 g, 31.2 mmol), NaClO (850 mg, 9.4 mmol) and NaH PO OH 2 2 4 (1.13 g, 9.4 mmol) dissolved in water (7 mL) at rt. The mixture was O stirred for 3 h. The solvents were removed under reduced pressure.

The residue was dissolved in H2O and acidified with aq. HCl. The water layer was extracted with CH2Cl2 (3x 20 mL). The combined organic layers were collected, dried over anhydrous Na2SO4, concentrated and purified by flash chromatography (eluent pentane/ethyl acetate) to give 19 as white solid (550 mg, 85%). 1H NMR (400 MHz, DMSO) δ 6.77 (d, J = 2.0 Hz, 1 H), 6.65 (dd, J = 17.4, 11.0 Hz, 1 H), 6.56 (d, J = 2.0 Hz, 1 H), 5.87 (d, J = 17.4 Hz, 1 H), 5.35 (d, J = 11.1 Hz, 1 H), 3.82 (s, 3H), 3.76 (s, 3 H). 13 C NMR (100 MHz, CD2Cl2) δ 172.5, 167.0, 163.5, 146.5, 140.3, 120.8, 116.3, 108.7, + + 102.5, 60.9, 60.0. HRMS (ESI ): m/z [M+H] calc. for C11H13O4: 209.0808; found:

130 Chapter 6

209.0805.

2,4-Dimethoxy-6-vinylbenzoyl fluoride(13): To a stirred solution of 19 (40 mg, 0.19

O O mmol) in CH2Cl2 (3 mL) was added pyridine (45 mg, 0.57 mmol) and cyanuric fluoride (39 mg, 0.29 mmol) at 0 oC. The mixture was stirred F for 1 h. A few drops of water were added and the mixture was diluted O with 3 mL of CH2Cl2. 2 mL water was added and the water layer was extracted with CH2Cl2 (3x 5 mL). The combined organic layers were collected, dried over anhydrous Na2SO4, concentrated and purified by flash chromatography (eluent pentane/ethyl acetate) to give 13 as white solid (34.1 mg, 85%). 1H NMR (400 MHz,

CDCl3) δ 6.99 (ddd, J = 17.2, 10.9, 3.0 Hz, 1 H), 6.66 (s, 1 H), 6.42 (d, J = 2.1 Hz, 1 H), 5.70 (d, J = 17.2 Hz, 1 H), 5.40 (d, J = 10.9 Hz, 1 H), 3.87 (s, 3 H), 3.86 (s, 3 H). 13C

NMR (100 MHz, CDCl3) δ 163.9, 161.1, 158.5, 155.0, 142.6, 134.5, 118.5, 103.6 C-F 19 + (J =2.2 Hz, 98.1, 56.3, 55.7. F NMR (400 MHz, CDCl3) δ –147.52. HRMS (ESI ): + m/z [M+H] calc. for C11H11O3: 191.0703; found: 191.0702.

Hept-6-en-2-one (22):20 To a stirred solution of acetyl chloride (0.96 mL, 13.5 mmol) in o O dry THF (17.5 mL) at 0 C under nitrogen was added copper(I) iodide (129 mg, 0.68 mmol) and pent-4-en-1-ylmagnesium bromide (22.0 mL, 0.59 mM) dropwise over 1 h, prepared from 5-bromopent-1-ene (2.10 g, 13.5 mmol) and magnesium turnings (400 mg, 16.5 mg). The mixture was subsequently stirred for another 1h after which the ice/salt bath was removed. The reaction mixture was quenched with a saturated aq. NH4Cl and extracted with DCM (3x 30 mL). The combined organic layers were collected, dried over anhydrous Na2SO4, concentrated and purified by flash chromatography (eluent pentane/ether) to give 22 as colorless oil (1.2 g, 78%). 1H NMR

(400 MHz, CDCl3) δ 5.84 – 5.68 (m, 1 H), 5.00 (m, 2 H), 2.43 (t, J = 7.4 Hz, 2 H), 2.13 13 (s, 3 H), 2.06 (q, J = 7.3 Hz, 2 H), 1.68 (p, J = 7.4 Hz, 2 H). C NMR (100 MHz, CDCl3) δ 208.9, 137.9, 115.2, 42.8, 33.0, 29.9, 22.7.

2-(Hept-6-en-2-ylidene)-1,1-dimethylhydrazone (24):20 To a stirred solution of N,N-dimethylhydrazine (1.15 g, 18.6 mmol) in EtOH (35 mL) was N N added a catalytic amount of acetic acid (18 mg, 0.31 mmol, 5 mol%) and hept-6-en-2-one 9 (0.700 g, 6.23 mmol) at rt. The mixture was heated at reflux for 3 h. The solvent was removed under reduced pressure, and the residue was dissolved in EtOAc. The organic solution was washed with saturated

NaHCO3 and dried over anhydrous Na2SO4. The solvent was removed under reduced pressure. Vacuum distillation of the residue gave 24 (807 mg, 84%) as a mixture of

131 Chapter 6

1 isomers (Z/E= 1/4) according to NMR. H NMR (400 MHz, CDCl 3) δ 5.84–5.68 (m, 1 H), 5.08 – 4.86 (m, 2 H), 2.40 (s, 6H major), 2.37 (s, 6H minor), 2.20–2.14 (m, 2 H), 2.08–2.00 (m, 2 H), 1.91 (s, 3H major), 1.89 (s, 3H minor), 1.66 – 1.50 (m, 2 H).

(S)-(But-3-en-2-yloxy)(tert-butyl)diphenylsilane (26): KOH (4.77 g, 85 mmol) was dissolved in 25 mL of water followed by the addition of 15 (1.5 g, 8.5 TBDPSO mmol). The mixture was stirred for 65 h at room temperature and extracted with DCM (3x 30 mL). The organic layers were collected and dried over anhydrous Na2SO4. To the above solution was added imidazole (2.3 g, 34 mmol) and TBDPSCl (4.5 g, 17 mmol) at 0 oC. The ice bath was removed and the reaction mixture was stirred at rt for 12 h. The reaction mixture was quenched with aq. NaHCO3 and extracted with DCM (3x 30 mL). The combined organic layers were collected, dried over anhydrous Na2SO4, concentrated and purified by flash chromatography (eluent pentane/DCM) to give 26 as 1 colorless oil (2.01 g, 77%). [α]D= + 0.8 (c=1.2, CHCl3). H NMR (400 MHz, CDCl3) δ 7.79 – 7.61 (m, 4 H), 7.49 – 7.33 (m, 6 H), 5.88 (ddd, J = 17.1, 10.4, 5.4 Hz, 1 H), 5.12 (dt, J = 17.2, 1.6 Hz, 1 H), 4.97 (dt, J = 10.4, 1.5 Hz, 1 H), 4.37 – 4.28 (m, 1 H), 1.15 (d, 13 J = 6.3 Hz, 3 H), 1.13 – 1.06 (s, 9 H). C NMR (100 MHz, CDCl3) δ 142.5, 135.9, 135.9, 134.6, 134.2, 129.5, 129.5, 127.5, 127.4, 112.7, 70.4, 27.0, 24.0, 19.3. HRMS (ESI+): m/z + [M+H] calc. for C20H26OSi: 310.1753; found: 310.2357.

(S)-3-((tert-Butyldiphenylsilyl)oxy)butan-1-ol (27): To a stirred solution of 26 (800 mg, 2.6 mmol) in dry THF (8 mL) was added 9-BBN (10 mL, 0.5 M TBDPSO OH in THF) at 0 oC under nitrogen. The ice bath was removed, and the mixture was stirred for 4 h at rt. The reaction mixture was cooled to 0 oC, and quenched with ethanol (10 mL) followed by the addition of aq. NaOH (6.5 mL, 4 M) and

H2O2 (6.5 mL). The mixture was warmed to rt and stirred for 12 h. The reaction mixture was subsequently heated to 60 oC for 2 h and cooled to rt, diluted with ether (100 mL).

The organic layer was washed with aq. NH4Cl, dried over anhydrous Na2SO4, concentrated and purified by flash chromatography (eluent pentane/ether) to give 27 as 1 colorless oil (740 mg, 87%). [α]D = +10.0 (c= 1.0, CHCl3). H NMR (400 MHz, CDCl3) δ 7.77 – 7.67 (m, 4 H), 7.49 – 7.36 (m, 6 H), 4.12 (td, J = 6.2, 4.4 Hz, 1 H), 3.84 (ddd, J = 12.7, 8.4, 4.6 Hz, 1 H), 3.70 (dt, J = 10.7, 5.3 Hz, 1 H), 2.32 – 2.06 (m, 1 H), 1.89 – 1.76 (m, 1 H), 1.66 (dtd, J = 14.2, 5.8, 4.7 Hz, 1 H), 1.10 (d, J = 6.2 Hz, 3 H), 1.09 – 1.04 (m, 13 9 H). C NMR (100 MHz, CDCl3) δ 135.9, 135.8, 134.2, 133.7, 129.8, 129.7, 127.7, 127.5, 68.7, 59.9, 40.7, 27.0, 23.0, 19.1. HRMS (ESI+): m/z [M+Na]+ calc. for

C20H28O2SiNa: 351.1751; found: 351.1754.

132 Chapter 6

(S)-tert-Butyl((4-iodobutan-2-yl)oxy)diphenylsilane (28): To a stirred solution of

imidazole (296 mg, 4.36 mmol) and PPh3 (674 mg, 2.57 mmol) in o TBDPSO I dry DCM (15 mL) was added iodine (653 mg, 2.57 mmol) at 0 C.

The mixture was stirred for 30 min followed by the addition of 27 dissolved in dry DCM (10 mL). The mixture was warmed to rt and stirred for 2h. The reaction mixture was quenched with a saturated aq. NH4Cl and extracted with DCM (3x 30 mL). The combined organic layers were collected, dried over anhydrous Na2SO4, concentrated and purified by flash chromatography (eluent pentane/ether) to give 28 as colorless oil (760 mg, 88%). 1 [α]D = – 7.3 (c=1.3 CHCl3). H NMR (400 MHz, CDCl3) δ 7.76 – 7.64 (m, 4 H), 7.49 – 7.33 (m, 6 H), 3.99 – 3.86 (m, 1 H), 3.21 (t, J = 7.4 Hz, 2 H), 2.07 (td, J = 14.1, 6.9 Hz, 1 H), 1.94 (dtd, J = 14.1, 7.7, 4.5 Hz, 1 H), 1.10 – 1.00 (m, 12 H). 13C NMR (100 MHz,

CDCl3) δ 135.9, 134.5, 133.8, 129.7, 129.5, 127.6, 127.5, 69.8, 43.5, 27.0, 22.9, 19.3, 2.4. + + HRMS (ESI ): m/z [M+Na] calc. for C20H28IOSiNa: 439.0982; found: 439.0952.

(S)-10-((tert-Butyldiphenylsilyl)oxy)undec-1-en-6-one (29): To a stirred solution of 14 (320 mg, 2.05 mmol) in dry THF (40 mL) was O added dropwise n-BuLi (1.3 mL, 2.05 mmol) at 0 TBDPSO oC. The mixture was stirred for 1 h and warmed to rt followed by the addition of 28 dissolved in 20 mL of dry THF. After stirring about 4h the reaction mixture was quenched with 2 M HCl and stirred overnight. The mixture was extracted with DCM (3x 30 mL). The combined organic layers were collected, dried over anhydrous Na2SO4, concentrated and purified by flash chromatography (eluent 1 pentane/ether) to give 29 as colorless oil (465 mg, 81 %. [α]D= –13.2 (c= 1.3, CHCl3). H

NMR (400 MHz, CDCl3) δ 7.71-7.64 (m, 4 H), 7.47 – 7.31 (m, 6 H), 5.77 (ddt, J = 17.0, 10.2, 6.7 Hz, 1 H), 5.08 – 4.93 (m, 2 H), 3.93 – 3.76 (m, 1 H), 2.34 (t, J = 7.4 Hz, 2H), 2.26 (t, J = 7.3 Hz, 2 H), 2.04 (dd, J = 14.3, 7.1 Hz, 2 H), 1.72 – 1.62 (m, 2 H), 1.61 – 1.50 (m, 2 H), 1.50 – 1.32 (m, 2 H), 1.07 (d, J = 8.1 Hz, 12 H). 13C NMR (100 MHz,

CDCl3) δ 210.9, 138.0, 135.9, 135.9, 134.8, 134.4, 129.5, 129.4, 127.5, 127.4, 115.2, 69.2, 42.8, 41.7, 38.8, 33.1, 27.0, 23.1, 22.8, 19.6, 19.3. HRMS (ESI+): m/z [M+Na]+ calc. for C27H38O2SiNa: 445.2533 found: 445.2534.

(S)-tert-Butyl ((5-(2-(pent-4-en-1-yl)-1, 3-dioxolan-2-yl) pentan-2-yl)oxy)diphenyl silane (30): To a stirred solution of 29 (275 mg, 0.65 mmol) in benzene (6 mL) was

added pTsOH•H2O (5.0 mg, 0.026 mmol), ethylene O O glycol (1.20 g, 19.5 mmol) and a few molecular TBDPSO sieves at rt. The mixture was warmed to 80 oC and

133 Chapter 6 stirred for 48 h. The reaction mixture was cooled to rt followed by the addition of EtOAc, a drop of triethylamine and aq. NaHCO3. The mixture was extracted with DCM (3x 30 mL). The combined organic layers were collected, dried over anhydrous Na2SO4, concentrated and purified by flash chromatography (eluent pentane/ether) to give 30 as 1 colorless oil (227 mg, 75%. [α]D = –6.5 (c= 2.8, CHCl3). H NMR (400 MHz, CDCl3) δ 7.68 (d, J = 7.9 Hz, 4 H), 7.46 – 7.29 (m, 6H), 5.86 – 5.71 (m, 1 H), 4.98 (ddd, J = 13.7, 11.2, 1.1 Hz, 2 H), 3.95 – 3.76 (m, 5 H), 2.04 (q, J = 6.9 Hz, 2 H), 1.57– 1.23 (m, 10 H), 13 1.13 – 0.99 (m, 12 H). C NMR (100 MHz, CDCl3) δ 138.8, 136.0, 135.1, 134.7, 129.6, 129.5, 127.6, 127.5, 114.7, 111.8, 69.7, 65.0, 39.8, 37.3, 36.8, 34.1, 27.2, 23.4, 23.3, 19.8, + + 19.4. HRMS (ESI ): m/z [M+H] calc. for C29H43O3Si: 467.2976; found: 467.2964.

(S)-5-(2-Pent-4-enyl-[1,3]dioxolan-2-yl)-pentan-2-ol (31): To a stirred solution of 30 (98 mg, 0.21 mmol) in dry THF (4 mL) was added TBAF O O (0.42 mL, 0.42 mmol, 1 M in THF) at rt. After stirring for HO 48 h the reaction mixture was quenched with a saturated aq. NH4Cl and extracted with DCM (3x 30 mL). The combined organic layers were collected, dried over anhydrous Na2SO4, concentrated and purified by flash chromatography (eluent pentane/ether) to give 31 as colorless oil (42.4 mg, 88%). [α]D = 1 + 5.4 (c = 1.7, CHCl3). H NMR (400 MHz, CDCl3) δ 5.79 (ddt, J = 16.9, 10.2, 6.7 Hz, 1 H), 5.07 – 4.85 (m, 2 H), 3.99 – 3.87 (m, 4 H), 3.79 (m, 1 H), 2.04 (q, J = 7.1 Hz, 2 H), 1.68 – 1.54 (m, 4 H), 1.52 – 1.32 (m, 7 H), 1.18 (d, J = 6.2 Hz, 3 H). 13C NMR (100

MHz, CDCl3) δ 138.8, 114.8, 111.8, 68.1, 65.1, 39.6, 37.1, 36.7, 34.0, 23.6, 23.3, 20.1. + + HRMS (ESI ): m/z [M+H] calc. for C13H25O3: 229.1798; found: 229.1795.

(S)-5-(2- (Pent-4-en-1-yl)- 1, 3-dioxolan-2-yl) pentan-2-yl-2, 4-dimethoxy-6- vinyl benzoate (32): To a stirred solution of 31 (0.22 mmol, 50.0 mg) in dry THF (3mL) was added KHMDS (44 µL, 0.5 M, 0.22 mmol) at O O O O 0oC under nitrogen. The mixture was stirred O for an additional 10 min followed by the O addition of acid fluoride 13 (0.13 mmol, 25 mg) dissolved in THF (1 mL). The mixture was stirred for 2h. The reaction mixture was quenched with a saturated aq. NH4Cl and extracted with DCM (3x 30 mL). The combined organic layers were collected, dried over anhydrous Na2SO4, concentrated and purified by flash chromatography (eluent pentane/ether) to give 32 as colorless oil (48 mg, 82%). 1 [α]D = + 11.2 (c = 1.6, CHCl3). H NMR (400 MHz, CDCl3) δ 6.73 (dd, J = 17.3 Hz, 10.9, 1 H), 6.63 (d, J = 2.1 Hz, 1 H), 6.38 (d, J = 2.1 Hz, 1 H), 5.83-5.73(m, 1 H), 5.70 (d, J = 17.3 Hz, 1 H), 5.31 (d, J = 11.0 Hz, 1 H), 5.21-5.12(m, 1 H), 5.04 – 4.88 (m, 2 H), 3.91 (s,

134 Chapter 6

4 H), 3.82 (s, 3 H), 3.79 (s, 3 H), 2.04 (q, J = 7.1 Hz, 2 H), 1.78 – 1.37 (m, 10 H), 1.32 (d, 13 J = 6.3 Hz, 3 H). C NMR (100 MHz, CDCl3) δ 167.5, 161.2, 157.9, 138.6, 137.3, 133.8, 116.9, 116.7, 114.6, 111.5, 101.3, 98.2, 71.9, 64.9, 55.9, 55.4, 36.9, 36.6, 36.1, 33.9, 23.1, + + 20.1, 19.8. HRMS (ESI ): m/z [M+Na] calc. for C24H34O6Na: 441.2248; found: 441.2247.

Zearalenone dimethyl ether ethylene glycol (33): To a stirred solution of 32 (67 mg, 0.16 mmol) in toluene (43 mL) was added Grubbs second O O generation catalyst {(1,3-bis(2,4,6-trimethyl O phenyl)-2-imidazolidiny lidene) dichloro (phenylmethylene) O O (tricyclohexyl phosphine)ruthenium (6.7 mg, 0.17 mmol) o O at rt. The resulting solution was heated to 80 C and stirred for 4 h. The solvent was removed under reduced pressure. Flash chromatography of the residue over silica gel using 1:1 pentane-Et2O yielded zearalenone dimethyl ether 1 ethylene glycol 33 as white solid (55 mg, 88%). [α]D = + 62.6 (c = 1.0, CHCl3). H NMR

(400 MHz, CDCl3) δ 6.58 (d, J = 2.0 Hz, 1 H), 6.43 (d, J = 16.2 Hz, 1 H), 6.35 (d, J = 1.9 Hz, 1 H), 6.29 (dt, J = 16.0, 5.6 Hz, 1 H), 5.29 – 5.14 (m, 1 H), 3.91 (s, J = 8.9 Hz, 4 H), 3.82 (s, 3 H), 3.80 (s, 3 H), 2.43-2.33 (m, 1 H), 2.19-2.07 (m, 1 H), 1.91 – 1.76 (m, 2 H), 1.76 – 1.59 (m, 4 H), 1.58 – 1.37 (m, 4 H), 1.34 (d, J = 6.3 Hz, 3 H). 13C NMR (100 MHz,

CDCl3) δ 168.2, 161.0, 157.4, 136.6, 133.0, 126.1, 116.9, 111.8, 101.0, 97.5, 70.8, 64.3, 64.1, 55.9, 55.4, 35.2, 34.8, 33.1, 30.1, 21.2, 20.2, 19.6. HRMS (ESI+): m/z [M+Na]+ calc. for C22H30O6Na: 413.1935; found: 413.1935.

(S)-(+)-Zearalenone dimethyl ether (34): To a stirred solution of 33 in an acetone/water

mixture (2 mL, 20:1) was added p-TsOH•H2O (1 mg, 0.005 O O mmol) at rt. The mixture was warmed to 40 oC and stirred O overnight. The reaction mixture was quenched with a O saturated aq. NH4Cl and extracted with DCM (3x 30 mL). O The combined organic layers were collected, dried over anhydrous Na2SO4, concentrated and purified by flash chromatography (eluent pentane/ethyl acetate) to give 34 as white solid (18.5 mg, 83%). [α]D = + 48.7 (c= 0.85, 10c 1 CHCl3) [Lit. + 47.8 (c= 0.5, CHCl3)]. H NMR (400 MHz, CDCl3) δ 6.59 (d, J = 1.8 Hz, 1 H), 6.45 – 6.32 (m, 2 H), 6.08 – 5.92 (m, 1 H), 5.38 – 5.24 (m, 1 H), 3.83 (s, 3 H), 3.80 (s, 3 H), 2.83 – 2.57 (m, 1 H), 2.46-2.24 (m, 3 H), 2.22 – 1.96 (m, 3 H), 1.90 – 1.45 13 (m, 5 H), 1.33 (d, J = 6.3 Hz, 3 H). C NMR (100 MHz, CDCl3) δ 211.5, 167.7, 161.5, 157.8, 136.9, 133.3, 129.2, 116.5, 101.4, 97.9, 71.3, 56.1, 55.6, 44.2, 37.7, 35.3, 31.4,

135 Chapter 6

+ + 21.9, 21.5, 20.2. HRMS (ESI ): m/z [M+Na] calc. for C20H26O5Na: 369.1673; found: 369.1674.

(S)-(-)-Zearalenone (1):12d To a stirred solution of aluminum powder (37.0 mg, 1.37

OH O mmol) in benzene (2.2 mL) was added iodine (130 mg, 0.51 mmol) at rt. The mixture was heated at reflux until it was O colorless. After cooling to 7 oC a few crystals of TBAI were HO added followed by the addition of phloroglucinol (19.5 mg, O 0.16 mmol) and 34 (11 mg, 0.03 mmol). The suspension was stirred for 10 min before 40 mL of Na2S2O4 (aq.) was added. The water layer was extracted with EtOAc (3x 10 mL). The combined organic layers were collected, dried over anhydrous Na2SO4, concentrated and purified by flash chromatography (eluent pentane/ethyl acetate) to give (S)-(–)-zearalenone 1 (6.4 mg, 63%) as a white solid. 1H

NMR (400 MHz, CDCl3) δ 7.01 (dd, J = 15.4 Hz, 1.2Hz, 1 H), 6.41 (d, J = 2.6 Hz, 1 H), 6.35 (d, J = 2.5 Hz, 1 H), 5.74 (s, 1 H), 5.72 – 5.62 (ddd, J = 10.4, 5.4 Hz, 1 H), 5.14 – 4.81 (m, 1 H), 2.96 – 2.76 (ddd, J = 12.4, 6.4, 2.8 Hz, 1 H), 2.69 – 2.54 (m, 1 H), 2.42 – 2.32 (m, 1 H), 2.26 – 2.08 (m, 4 H), 1.87 – 1.70 (m, 2 H), 1.70 – 1.56 (m, 3 H), 1.55 – 13 1.43 (m, 1 H), 1.38 (d, J = 6.1 Hz, 3 H). C NMR (100 MHz, CDCl3) δ 211.6, 171.5, 165.6, 160.7, 144.2, 133.3, 132.7, 108.5, 102.6, 73.6, 43.1, 36.8, 34.9, 31.2, 22.4, 21.2, + + 21.0. HRMS (ESI ): m/z [M+H] calc. for C18H23O5: 319.1540; found: 319.1538.

Biological assay The enzyme soybean lipoxygenase-1 (SLO-1) was obtained from Cayman Chemicals and linoleic acid was obtained from Sigma. Sodium borate buffer (H3BO3 0.2 M) pH 9.0 was used as an assay buffer for all SLO-1 inhibition experiments. The enzyme SLO-1 was diluted 1:4000 using the SLO-1 assay buffer and the inhibitor (S)-(-)-zearalenone (100 mM in DMSO) was diluted using the same buffer to 200 µM. Linoleic acid (20 mM in EtOH) was diluted with SLO-1 assay buffer to 200 µM. The enzyme (400 µL, 1:4000) was mixed with the inhibitor (400 µL, 200 µM) and the mixture was incubated for 10 min. Subsequently, linoleic acid (800 µL, 200 µM) was added to give a mixture with 50 µM inhibitor and 100 µM linoleic acid. Enzyme inhibition was measured by the residual enzyme activity after 10 min incubation with the inhibitor at room temperature. The enzyme activity was determined by conversion of lipoxygenase substrate linoleic acid into hydroperoxy eicosatetraenoic acid (HPETE). The conversion rate was followed by UV absorbance of the conjugated diene at 234 nm (ε = 25000 M-1cm-1) over a period of 20 min and started 10 sec after the addition of the substrate linoleic acid. The UV absorbance increase over time was used to

136 Chapter 6

determine the enzyme activity. Inhibitory concentration 50% (IC50) was determined by measuring the enzyme activity using various concentrations of (S)-(-)-zearalenone (12.5, 25, 50 and 100 μM, Figure 12). The average and standard deviation of three experiments were plotted. Calculations were performed with Excel 2010 and the non-linear curve fitting (Figure 12) was performed with the Origin 8 software. Fitting a sigmoidal curve provided for (S)-(-)-zearalenone an IC50 of 51 +/- 2.0 μM.

6.8 References and notes

1. (a) T. W. Schulte, S. Akinaga, S. Soga, W. Sullivan, B. Stensgard, D. Toft, L. M. Neckers, Cell Stress Chaperon. 1998, 3, 100. (b) S. V. Sharma, T. Agatsuma, H. Nakano, Oncogene, 1998, 16, 2639. (c) V. Hellwig, A. Mayer-Bartschmid, H. Müller, G. Greif, G. Kleymann, W. Zitzmann, H. V. Tichy, M. Stadler, J. Nat. Prod. 2003, 66, 829. (d) M. Isaka, C. Suyarnsestakorn, M. Tanticharoen, P. Kongsaeree, Y. Thebtaranonth, J. Org. Chem. 2002, 67, 1561. (e) N. Winssinger, S. Barluenga, Chem. Commun. 2007, 22–36. 2. (a) A. Zhao, S. H. Lee, M. Mojena, R. G. Jenkins, D. R. Patrick, H. E. Huber, M. A. Goetz, O. D. Hensens, D. L. Zink, D. Vilella, A. W. Dombrowski, R. B. Lingham, L. Huang, J. Antibiot. 1999, 52, 1086. (b) J. Ninomiya-Tsuji, T. Kajino, K. Ono, T. Ohtomo, M. Matsumoto, M. Shiina, M. Mihara, M. Tsuchiya, K. Matsumoto, J. Biol. Chem. 2003, 278, 18485. (c) T. Hofmann, K. Altmann, C. R. Chim. 2008, 11, 1318. 3. M. Stob, R. S. Baldwin, J. Tuite, F. N. Andrews, K. G. Gillette, Nature, 1962, 196, 1318. 4. W. H. Urry, H. L. Wehrmeister, E. B. Hodge, P. H. Hidy, Tetrahedron Lett. 1966, 7, 3109. 5. (a) M. Ghizzoni, A. Boltjes, C. de Graaf, H. J. Haisma, F. J. Dekker, Bioorg. Med. Chem. 2010, 18, 5826. (b) M. Ghizzoni, J. Wu, T. Gao, H. J. Haisma, F. J. Dekker, Y. G. Zheng, Eur. J. Med. Chem. 2012, 47, 337. 6. (a) T. Kuiper-Goodman, P. M. Scott, H. Watanabe, Regul. Toxicol. Pharm. 1987, 7, 253. (b) R. J. Miksicek, J. Steroid Biochem. 1994, 49, 153. 7. I. Ivanov, D. Heydeck, K. Hofheinz, J. Roffeis, V. B. O’Donnell, H. Kuhn, M. Walther, Arch. Biochem. Biophys. 2010, 503, 161. 8. F. Trail, I. Gaffoor, Appl. Environ. Microbiol., 2006, 72, 3, 1793. 9. (a) R. N. Hurd, D. H. Shah, J. Org. Chem., 1973, 38, 390. (b) R. N. Hurd, D. H. Shah, J. Med. Chem., 1973, 16, 5, 543. (c) E. J. Corey, K. C. Nicolaou, J. Am. Chem. Soc., 1974, 96, 5614. (d) T. Takahashi, K. Kasuga, M. Takahashi, J. Tsuji, J. Am. Chem. Soc., 1979, 101, 5072. (e) T. Takahashi, H. Ikeda, J. Tsuji, Tetrahedron Lett., 1981, 22, 1363. (f) T. Takahashi, T. Nagashima, H. Ikeda, J. Tsuji, Tetrahedron Lett., 1982, 23, 4361. (g) A. V. Rama Rao, M. N. Deshmukh, G. V. M. Sharma, Tetrahedron, 1987, 43, 779. (f) D. Taub, N. N. Girotra, R. D. Hoffsommer, C. H. Kuo, H. L. Slates, S. Weber, N. L. Wendler. Tetrahedron, 1968, 24,

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2443. (g) I. Vlattas, I. T. Harrison, L. Tökés, J. H. Fried, A. D. Cross, J. Org. Chem. 1968, 33, 4176. 10. (a) S. A. Hitchcock, G. Pattenden, Tetrahedron Lett., 1990, 31, 3641. (b) A. Kalivretenos, J. K. Stille, L. S. Hegedus, J. Org. Chem., 1991, 56, 2883. (c) S. A. Hitchcock, G. Pattenden, J. Chem. Soc., Perkin Trans. 1, 1992, 1323. (d) K. C. Nicolaou, N. Winssinger, J. Pastor, F. Murphy, Angew. Chem. Int. Ed., 1998, 37, 2534. 11. G. Solladié, M. S. Maestro, A. Rubio, C. Pedregal, M. C. Carreno, J. L. G. Ruano, J. Org. Chem., 1991, 56, 2317. 12. (a) E. Keinan, S. C. Sinha, A. Sinha-Bagchi, J. Chem. Soc., Perkin Trans. 1, 1991, 3333. (b) A. Fürstner, O. R. Thiel, N. Kindler, B. Bartkowska, J. Org. Chem., 2000, 65, 7990. (c) I. Navarro, J. F. Basset, S. Hebbe, S. M. Major, T. Werner, C. Howsham, J. Bärckow, A. G. M. Barrett, J. Am. Chem. Soc.,2008, 130, 10293. (d) H. Miyatake-Ondozabal, A. G. M. Barrett, Tetrahedron, 2010, 66, 6331. (e) J. S. Yadav, P. V. Murthy, Synthesis, 2011, 13, 2117. 13. (a) M. Peterseim, W. P. Neumann, React. Polym., 1993, 20, 189 – 205. (b) U. Gerigk, M. Gerlach, W. P. Neumann, R. Vieler, V. Weintritt, Synthesis, 1990, 448 – 452. 14. B. J. Backes, J. A. Ellman, Curr. Opin. Chem. Biol., 1997, 1, 86 – 93. 15. K. Geurts, S. P. Fletcher, B. L. Feringa, J. Am. Chem. Soc., 2006, 128, 15572–15573. 16. K. Koch, J. Podlech, E. Pfeiffer, M. Metzler, J. Org. Chem., 2005, 70, 3275. 17. Y. Cho, C. Cho, Tetrahedron, 2008, 64, 2172. 18. G. A. Olah, M. Nojima, I. Kerekes, Synthesis, 1973, 487. 19. B. Kongkathip, R. Sookkho, N. Kongkathip, Chem. Lett., 1985, 14, 1849. 20. S. T. Kemme, T. Smejkal, B. Breit, Adv. Synth. Catal., 2008, 350, 989 – 994. 21. M. Rosillo, E. Arnáiz, D. Abdi, J. Blanco-Urgoiti, G. Domínguez, J. Pérez-Castells, Eur. J. Org. Chem., 2008, 3917-3927. 22. D. Enders, T. Schüßeler, New J. Chem., 2000, 24, 973. 23. J. Pospišil, C. Müller, A. Fürstner, Chem. Eur. J., 2009, 15, 5956.

138

English Summary

English Summary

139

English Summary

Copper-catalyzed asymmetric allylic alkylation and asymmetric conjugate addition in natural product synthesis

This thesis described the copper-catalyzed hetero AAA and ACA applied in the total synthesis of several biologically active molecules. A second part of this thesis aims at the development of a novel catalytic asymmetric route towards skipped dienes (1,4-dienes) with a methyl substituted central stereogenic carbon by copper-catalyzed AAA and its application in the total synthesis of natural product, Phorbasin B.

Figure 1. Structure of Lasiodiplodin.

In chapter 2, the catalytic asymmetric formal synthesis of Lasiodiplodin is described (Figure 1). The present approach takes maximum benefit of asymmetric catalysis-copper catalyzed hetero AAA of Grignard reagent which led to excellent enantioselectivity. sp3-sp2 Suzuki coupling and RCM are applied for the construction of the macrocycle.

O O

O OH O

OH Rasfonin Figure 2. Structure of Rasfonin.

In chapter 3, a very efficient total synthesis of the apoptosis inducer (–)-rasfonin has been described (Figure 2). CuBr/JosiPhos catalyzed iterative ACA of MeMgBr has been employed to install the stereogenic centers in the upper half side chain with excellent yield and stereoselectivity. The hydroxy-lactone core could be prepared by a subsequent stereospecific hydroxy-directed Achmatowicz rearrangement followed by an oxidation-reduction sequence. The synthesis of the lower makes use of the perfect

140

English Summary transfer of chirality in the conjugate addition to butenolide followed by selective construction of the E,E-diene-ester part. The availability of an effective route to Rasfonin now allows to study its role in inhibiting the Ras signaling pathway, provides access to functional analogs and might lead to the identification of its target protein.

Figure 3. Copper‐catalyzed AAA of diene allylic bromides.

In chapter 4, copper-catalyzed AAA of methylmagnesium bromide as nucleophile employing prochiral diene allylic bromides as substrates was described (Figure 3). The reaction leads to important chiral 1,4-diene building blocks with excellent regio- and enantioselectivity (ee values up to >99%; SN2’/SN2 ratio up to 97:3) in nearly all cases.

Figure 4. Structure of Phorbasin B.

In chapter 5, several attempts towards the synthesis of Phorbasin B have been described (Figure 4). So far we have achieved the synthesis of the right part (the 1,4-diene unit) in very high enantioselective manner described in chapter 4. For the left part, we have achieved the synthesis of the cyclohexyl ring. Further functionalization of the ring is required in the future.

Figure 5. Structure of Zearalenone.

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English Summary

In chapter 6, the catalytic asymmetric total synthesis of Zearalenone is described (Figure 5). Copper-catalyzed AAA is the key step in the synthesis to obtain the chiral allylic alcohol building block. Stille cross coupling and RCM are applied for the construction of the macrolactone. Biological test of Zearalenone led to a moderate Lipoxygenases inhibitor with IC50 of 51 +/- 2 μM.

142 Nederlandse Samenvatting

Nederlandse Samenvatting

143 Nederlandse Samenvatting

De Toepassing van Koper-gekatalyseerde Asymmetrische Allylische Alkylatie en Asymmetrische Geconjugeerde Additie in de Totaalsynthese van Natuurproducten

In dit proefschrift wordt de koper-gekatalyseerde hetero AAA en ACA toegepast in de totaalsynthese van een aantal biologisch actieve moleculen. In het tweede deel van dit proefschrift ligt de focus op een nieuwe katalytische asymmetrische route naar 1,4-dieenen met een methyl gesubstitueerd chiraal koolstofatoom door middel van koper-gekatalyseerde AAA. Deze nieuwe strategie wordt toegepast in de totaalsynthese naar het natuurproduct, Phorbasin B.

Figuur 1. Structuur van Lasiodiplodin.

In hoofdstuk 2 van dit proefschrift wordt de formele totaalsynthese van Lasiodiplodin (Figuur 1) beschreven. De strategie die wordt toegepast maakt optimaal gebruik van de asymmetrische koper-gekatalyseerde hetero AAA met methylmagnesium bromide. Met deze methode werd een zeer hoge enantioselectiviteit bereikt. Sp3-sp2 Suzuki koppeling en ring sluiting-metathese werden gebruikt voor de constructie van de macrocyclische ring.

Figuur 2. Structuur van (─)‐Rasfonin.

In hoofdstuk 3 wordt een efficiënte synthese naar (─)-Rasfonin (Figuur 2) beschreven. Dit natuurproduct staat bekend om zijn apoptose inducerende werking. CuBr/JosiPhos gekatalyseerde iteratieve ACA met methylmagnesium bromide is gebruikt om de stereo centra met hoge stereoselectiviteit te introduceren. Het hydroxy-lacton kon worden geconstrueerd door middel van een stereospeciefieke hydroxy-gestuurde Achmatowics omlegging gevolgd door een oxidatie- en een reductiestap. De synthese van het onderste

144 Nederlandse Samenvatting deel van het molecuul maakt gebruik van geconjugeerde additie aan buteenolide, gevolgd door de selectieve constructie van het E,E-dieen-ester deel. Deze nieuwe effectieve route naar (─)-Rasfonin kan helpen bij de bepaling van de rol die het molecuul speelt bij de inhibitie van het RAS signaal transductie systeem. Ook geeft deze synthetische methode toegang tot functionele varianten en kan het leiden tot de identificatie van het eiwit dat wordt geïnhibeerd door het molecuul.

N PPh 3 3 Ph P 2 R CuBr•SMe2 5mol% R3 R 2 1 R1 Br TaniaPhos 6 mol% R1 R + Fe MeMgBr 1.2 equiv. 2 R2 R2 R DCM, -80 oC, o.n. SN2' SN2 (S,S)-TaniaPhos tot >99% ee 97/3 SN2'/SN2 Figuur 3. Koper‐gekatalyseerde AAA van dieenen met allylische bromides.

In hoofdstuk 4 wordt de koper-gekatalyseerde AAA met methylmagnesium bromide als nucleofiel op een prochiraal dieen met een allylisch bromide als substraat beschreven (Figuur 3). Deze reactie geeft toegang tot de belangrijke 1,4-dieen bouwstenen met hoge regio en stereoselectiviteit (ee waardes tot >99%; en een SN2’/SN2 verhouding tot 97:3) in bijna alle gevallen.

Figuur 4. Structuur van Phorbasin B.

In hoofdstuk 5 worden enkele pogingen voor de synthese van Phorbasin B (Figuur 4) beschreven. De synthese van het rechter deel (het 1,4-dieen deel) van het molecuul is voltooid met met hoge enantioselectiviteit, zoals beschreven in hoofdstuk 4. Ook de cyclohexyl is gesynthetiseerd, maar verdere functionalisering van de zesring is nog nodig om het volledige molecuul te kunnen synthetiseren.

Figure 5. Structuur van (S)‐(─)‐Zearalenone.

145 Nederlandse Samenvatting

Hoofdstuk 6 beschrijft de totaalsynthese van (S)-(─)-Zearalenone (Figuur 5). De koper-gekatalyseerde AAA die leidt naar het chirale allylische bouwsteen is een cruciale stap in deze totaalsynthese. Een Stille koppeling werd gebruikt voor de introductie van een allyl groep op de fenyl ring. De macrocyclysche ring werd gevormd door middel van ring-sluitende metathese. (S)-(─)-Zearalenone is getest op de inhibitie van lipoxygenase; het molecuul bleek een redelijke lipoxygenase inhibitor te zijn met een IC50 van 51 ± 2 μM.

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Acknowledgments

147 Six years! What a long time. I still remember the first day when I came to Netherlands. I took the train to Groningen at Schiphol, and accidently I arrived at Leeuwarden. I didn’t know I should change the cars at Zwolle. Anyway I arrived at Groningen around 1 am finally. The next thing which I was confused about is that I should reserve the taxi when I called the taxi service number. Fortunately Groningen is not very big and I finally arrived at my apartment by walking with two big luggages. What a first day!

Netherlands is really a beautiful country, and I am very happy to decide to come instead of going to England. The quality of higher level education is very good here. Here I would like to say “thank you” to many people who contributed to my great success in Netherlands.

First one, Michael Pollard, I remember the first time we met your English almost killed me, too fast for me at that time. As my first supervisor in Groningen, you taught me a lot about chemistry, especially synthesis. That’s the time I decided to set my career as a synthetic organic chemist. I am very glad we had one joint publication later for my master project. Thanks for your supervision again.

Adri, you are a very knowledged professor in synthesis. I am glad we worked together for a long period. And I learned a lot from you, especially on total synthesis and paper writing. I still remember the first manuscript I prepared for asymmetric hydrogenation. After reading my manuscript, you said to me“your writing is terrible”. And then you decided to write the paper by yourself. For the experimental part, I don’t remember how many times we did the correction. At least 5! Maybe around 10! Thanks again for that although these days you still need to correct my writing a lot.

Ben, as my PhD supervisor, I am always impressed by your ideas. I am really glad to work with you. I love your motors a lot although I am not working on it. I remember we had one disagreement about“do we need the second synthesis of …”after the publication of rasfonin. I know you are looking for novel synthetic routes and higher level publications. I totally understand that. However, I found one article from nature chemistry recently and the topic is “does the world really need another synthesis of …”. And the final answer according to the author is “yes” due to two reasons. One is for the development of novel methodologies, and the other is to train students. I remember once I talked with Steve Ley after his lecture. I asked “you spent almost 20 years finishing Rapamycin and later you published it only on Chem. Eur. J., is it worth?” He answered “yes”. Personal answer is also “yes” according to my job hunting experience both in China and States. The employers did not pay too much attention to my publications; 148 however, they cared about the reactions I did and I know and my ability of accomplishment of complex molecules. Anyway in academia higher level publications are way more important.

Martín Fañanás-Mastral, I am glad to work with you on the skipped diene and 1,6-addition projects. You are a very talented chemist. Thanks for your help for those two projects. I hope one day you can really start you own academic career.

I also want to thank Theodora for GC and HPLC before, then HRMS later. Wim for NMR. Monic for GC and HPLC now. Hans for elemental analysis.

I also want to thank my current lab mates (Maria, Derk Jan, Claudia, Wim, Wiktor, Anja, Mickel, Anniek), old ones (Bin, Stella, Peter, Celine) and my lovely colleagues, however, it’s too much to name them all here; I still want to mention several (Milon, Jiawen, Kuang-Yen, Massimo, Suresh, Valentin, Lili, Xiaoyan, Depeng, Jeffrey, Beatriz, Niek, Peter, Miriam, Tiziana, Leticia).

Marc, my dear master student and friend, I am so glad to work with you. You are a very smart guy. I am very happy when you finished the molecule of zearalenone and the paper is published recently. Excellent work! I hope you can keep being smart at Leiden. Wu zhongtao and Liu yun, I am glad to have you guys as friends here. I enjoyed a lot when having dinner together with you. And my friends in my apartment, Yinwang, Wufan, Zhangliang, Chen tianshu, Fujin, I enjoyed a lot when we were playing Majiang. I am sorry about the money I won from you although it’s not a lot.

yange

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