Marine natural products: synthetic aspects† Jonathan C. Morris * a and Andrew J. Phillips b Received 3rd March 2010, Accepted 7th June 2010 DOI: 10.1039/b919366a Covering: January to December 2008. Previous review: Nat. Prod. Rep., 2009, 26, 245 An overview of marine natural products synthesis during 2008 is provided. As with earlier installments in this series, the emphasis is on total syntheses of molecules of contemporary interest, new total syntheses, and syntheses that have resulted in structure confirmation or stereochemical assignments. 1 Introduction 2 Review articles 3 Pinnatoxin 4 Theopederin D 5 Polyketides: callystatin, aburatubolactam, 15G256 (and related compounds) and bryostatin 16 6 Terpenoids: vannusal B and cortistatin A 7 Largazole and the trichodermamides 8 Acknowledgements 9 References 1 Introduction This review is designed to provide an overview of key features of the 2008 literature covering the synthesis of marine natural products, and should act as a companion to the Marine Natural Products review published in this journal. 1 The emphasis is on total syntheses of molecules of contemporary interest. Tabulated data for other syntheses are also provided. While every effort has been made to be comprehensive within these boundary condi- tions, we apologize in advance for any oversights. 2 2 Review articles A number of reviews that cover various aspects of marine natural products synthesis have appeared: ‘‘Synthesis of marine alkaloids from the oroidin family’’, 3 ‘‘Analogues of marine pyrroloimino- quinone alkaloids: synthesis and antitumor properties’’, 4 ‘‘Synthetic studies of heterocyclic natural products’’, 5 ‘‘Amphi- dinolides and its related macrolides from marine dinoflagel- lates’’, 6 ‘‘The synthetic challenge of diazonamide A, a macrocyclic indole bis-oxazole marine natural product’’, 7 ‘‘Synthesis of marine natural products with antimalarial activity’’, 8 ‘‘The continuing saga of the marine polyether bio- toxins’’, 9 ‘‘Convergent strategies for the total synthesis of poly- cyclic ether marine metabolites’’, 10 ‘‘Natural marine antiviral products’’, 11 ‘‘The biology and chemistry of the zoanthamine alkaloids’’, 12 ‘‘A potential source of anticancer agents: natural products and their analogs. Extraction, characterization, bio- logical activity and synthesis’’, 13 ‘‘Synthetic efforts toward, and biological activity of, thyrsiferol and structrurally-related analogues’’, 14 ‘‘Review of cytotoxic cephalostatins and ritter- azines: isolation and synthesis’’, 15 ‘‘The chemistry of marine furanocembranoids, pseudopteranes, gersolanes, and related natural products’’, 16 and ‘‘The structure activity relationship of discodermolide analogues’’. 17 Other reviews of relevance are cited in the text. 3 Pinnatoxin Zakarian’s synthesis of pinnatoxin A 1 18,19 was based around the dissection of the target into two key domains, 2 and 3, and highlighted the power of the Ireland–Claisen rearrangement for the establishment of quaternary stereocenters (Scheme 1). 20 In this context, when ester 7 (readily prepared by EDCI-mediated coupling of 4 and 5) was subjected to deprotonation with chiral amide base 6, the desired (Z)-enolate was formed. Trapping as the silylketene acetal, followed by warming to room temperature, resulted in the desired [3,3]-rearrangment to give 9 in an excellent 94% yield. A sequence of 14 steps installed the cyclohexene ring, and advanced material to 3. Addition of the organolithium derived from 2 (with t-BuLi) to aldehyde 3 occurred in 75% yield, and three further steps (desilylation with TBAF, Dess–Martin oxidation, and vinylmagnesium bromide addition) gave 10. Ring-closing metathesis with the Hoveyda–Grubbs 2nd-genera- tion catalyst (25 mol% loading) followed by oxidation gave 11 in 57% yield for the two steps. Diastereoselective conjugate addi- tion of MeCu(CN)Li installed the final remaining methyl group (11 / 12, 81%) and spiroketalization with LiBF 4 in wet iPrOH led to 13 in 60% yield. This material was advanced to 14 by a sequence of 7 steps, and after Staudinger reduction of the azide with Me 3 P, the imine was installed using Kishi’s conditions (triethylammonium 2,4,6-triisopropylbenzoate in xylenes at 85 C) to yield 15 (70% over two steps). The synthesis was completed by ester hydrolysis to give (+)-pinnatoxin A. Nakamura and Hashimoto have also reported a synthesis of pinnatoxin A (Scheme 2). 21 Taking a page from Trost’s recent successes in complex molecule synthesis [see also the bryostatin 16 synthesis later in this review], the key step was a School of Chemistry, University of New South Wales, Sydney, Australia 2052. E-mail: [email protected]; Fax: +61 2 93856141; Tel: +61 2 93854733 b Department of Chemistry, Yale University, New Haven, Connecticut, 06520, USA. † Footnote: This paper is part of an NPR themed issue on Synthesis, guest-edited by Andreas Kirschning and Andy Phillips. 1186 | Nat. Prod. Rep., 2010, 27, 1186–1203 This journal is ª The Royal Society of Chemistry 2010 REVIEW www.rsc.org/npr | Natural Product Reports Downloaded by DUKE UNIVERSITY on 01 October 2010 Published on 22 June 2010 on http://pubs.rsc.org | doi:10.1039/B919366A View Online
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Marine natural products: synthetic aspects†
Jonathan C. Morris*a and Andrew J. Phillipsb
Received 3rd March 2010, Accepted 7th June 2010
DOI: 10.1039/b919366a
Covering: January to December 2008. Previous review: Nat. Prod. Rep., 2009, 26, 245
An overview of marine natural products synthesis during 2008 is provided. As with earlier installments
in this series, the emphasis is on total syntheses of molecules of contemporary interest, new total
syntheses, and syntheses that have resulted in structure confirmation or stereochemical assignments.
aSchool of Chemistry, University of New South Wales, Sydney, Australia2052. E-mail: [email protected]; Fax: +61 2 93856141; Tel:+61 2 93854733bDepartment of Chemistry, Yale University, New Haven, Connecticut,06520, USA.
† Footnote: This paper is part of an NPR themed issue on Synthesis,guest-edited by Andreas Kirschning and Andy Phillips.
1186 | Nat. Prod. Rep., 2010, 27, 1186–1203
products’’,11 ‘‘The biology and chemistry of the zoanthamine
alkaloids’’,12 ‘‘A potential source of anticancer agents: natural
products and their analogs. Extraction, characterization, bio-
logical activity and synthesis’’,13 ‘‘Synthetic efforts toward, and
biological activity of, thyrsiferol and structrurally-related
analogues’’,14 ‘‘Review of cytotoxic cephalostatins and ritter-
azines: isolation and synthesis’’,15 ‘‘The chemistry of marine
furanocembranoids, pseudopteranes, gersolanes, and related
natural products’’,16 and ‘‘The structure activity relationship of
discodermolide analogues’’.17 Other reviews of relevance are
cited in the text.
3 Pinnatoxin
Zakarian’s synthesis of pinnatoxin A 118,19 was based around the
dissection of the target into two key domains, 2 and 3, and
highlighted the power of the Ireland–Claisen rearrangement for
the establishment of quaternary stereocenters (Scheme 1).20 In
this context, when ester 7 (readily prepared by EDCI-mediated
coupling of 4 and 5) was subjected to deprotonation with chiral
amide base 6, the desired (Z)-enolate was formed. Trapping as
the silylketene acetal, followed by warming to room temperature,
resulted in the desired [3,3]-rearrangment to give 9 in an excellent
94% yield. A sequence of 14 steps installed the cyclohexene ring,
and advanced material to 3. Addition of the organolithium
derived from 2 (with t-BuLi) to aldehyde 3 occurred in 75% yield,
and three further steps (desilylation with TBAF, Dess–Martin
oxidation, and vinylmagnesium bromide addition) gave 10.
Ring-closing metathesis with the Hoveyda–Grubbs 2nd-genera-
tion catalyst (25 mol% loading) followed by oxidation gave 11 in
57% yield for the two steps. Diastereoselective conjugate addi-
tion of MeCu(CN)Li installed the final remaining methyl group
(11 / 12, 81%) and spiroketalization with LiBF4 in wet iPrOH
led to 13 in 60% yield. This material was advanced to 14 by
a sequence of 7 steps, and after Staudinger reduction of the azide
with Me3P, the imine was installed using Kishi’s conditions
(triethylammonium 2,4,6-triisopropylbenzoate in xylenes at
85 �C) to yield 15 (70% over two steps). The synthesis was
completed by ester hydrolysis to give (+)-pinnatoxin A.
Nakamura and Hashimoto have also reported a synthesis of
pinnatoxin A (Scheme 2).21 Taking a page from Trost’s recent
successes in complex molecule synthesis [see also the bryostatin
16 synthesis later in this review], the key step was
This journal is ª The Royal Society of Chemistry 2010
Table 1 First total syntheses of marine natural products reported in 2008
Compound Reference Notes
Gung and Omollo81 � 5 steps from known compound� Resolution� Absolute configuration determined
Giddens et al.82 � Pseudopyrinone A: 3 steps from known compound� Pseudopyrinone B: 5 steps from known compound� Biological activity: good potency and selectivity
against parasitic protozoa
Greshock et al.83 � 18 steps from commercially available 6-hydroxyindole
� Biomimetic synthesis� Racemic synthesis
Kumar and Shaw84 � 16 steps from commercially available materials� Non-racemic synthesis� Biological activity: potent cytotoxicity
Ghosh, Kumar and Shashidhar85 � 21 steps from known compound� Non-racemic synthesis� Determined absolute configuration� Biological activity: cytotoxic against 3YI rat normal
fibroblast cells
Skepper et al.86 � 5 steps from pentadecyne� Non-racemic synthesis� Biological activity: significant antifungal
Cordes et al.87 � 7 steps from 2,6-dimethoxybenzaldehyde
Crimmins and Ellis88 � 8 steps from known intermediate used to prepare11-acetoxy-4-deoxtasbestinin D
� Non-racemic synthesis
Sofiyev, Navarro and Trauner89 � 13 steps from known compounds� Biomimetic synthesis� No protecting groups used
1198 | Nat. Prod. Rep., 2010, 27, 1186–1203 This journal is ª The Royal Society of Chemistry 2010
Reduction of the lactone in two steps was accompanied by
migration of the TBDPS group to the primary alcohol, which
facilitated the protecting group and redox reorganizations
required to arrive at ketone 195. Upon reaction with hydroxyl-
amine, an intermediate oxime was presumably formed that
underwent intramolecular O-alkylation with the epoxide to
provide oxazine 196 as a single diastereomer in 65% yield. A
three-step sequence of TBDPS protection, acetonide cleavage
and Corey–Winter olefination gave 197. Oxidation-state
manipulations over five steps gave 198, and at this juncture the
synthesis could be completed by a sequence that was analogous
to that described by Zakarian.
A large number of other total syntheses of marine natural
products were reported in the review period, and papers
describing first total syntheses are presented in Table 1. New
total syntheses of compounds previously prepared are summa-
rized in Table 2.
8 Acknowledgements
We would like to thank Professor John Blunt and Professor
Murray Munro (University of Canterbury, Christchurch, New
Zealand) for a copy of the 2008 version of the MarinLit data-
base80 which facilitated data collection for this review. Members
of the Morris group are thanked for their assistance in drawing
structures.
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