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The Decarboxylative Ireland-Claisen Rearrangement: Methodology Studies and Approaches to the Total Synthesis of (-)-Suaveoline

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The Decarboxylative Ireland-Claisen Rearrangement: Methodology Studies and Approaches to the Total Synthesis of (-)-Suaveoline THE DECARBOXYLATIVE IRELAND-CLAISEN REARRANGEMENT: METHODOLOGY STUDIES AND APPROACHES TO THE TOTAL SYNTHESIS OF (-)-SUAVEOLINE A Thesis Presented by Simon Eliot Lewis In Partial Fulfilment of the Requirements For the Award of the Degree of DOCTOR OF PHILOSOPHY OF THE UNIVERSITY OF LONDON Thorpe Laboratory, Centre for Chemical Synthesis, Department of Chemistry, Imperial College London, South Kensington Campus, London SW7 2AZ United Kingdom January 2006 Page 1 of 314 Abstract This thesis is divided into three sections. Section one is a review of recent progress in the synthesis of macroline, sarpagine and ajmaline-related indole alkaloids. The review covers approximately the last ten years of published literature. Section two is divided into two parts and discusses the results of research into the decarboxylative Ireland-Claisen rearrangement. Part one gives the background to the project and discusses the mechanism of this rearrangement. The development of methodology for the synthesis of bifunctional rearrangement substrates is detailed. The competitive rearrangement of these bifunctional substrates is outlined and trends in reactivity are discussed. An account is given of the application of the decarboxylative Ireland-Claisen rearrangement to a cyclic malonate, which gave rise to a cyclopropane. Efforts towards a cyclic malonate substrate are detailed, including the use of carbon suboxide. Part two concerns studies towards the total synthesis of (-)-suaveoline. The retrosynthetic analysis is explained and pertinent methodology introduced. The initial construction of a synthetically relevant rearrangement substrate is outlined. The reasons for the failure of this substrate to rearrange are discussed, as is the modified protecting group strategy that was adopted. Subsequent successful rearrangement and the synthesis of a key cyclopentenyl intermediate are described. Unsuccessful attempts to alkylate this cyclopentene are detailed and an alternative strategy is put forward. Novel methodology for the formation of pyridine-A/-oxides is disclosed and attempts to apply this to the synthesis of suaveoline are discussed. Section three is the experimental section, which gives detailed descriptions of the synthesis and spectroscopic characteristics of the compounds discussed in section two. Page 2 of 314 Declaration I certify that all work in this thesis is solely my own, except where explicitly stated and appropriately referenced. Simon Eliot Lewis Page 3 of 314 Contents 0.1 Abstract Page 2 0.2 Declaration Page 3 0.1 Acknowledgement Page 7 0.2 Abbreviations Page 8 0.3 Stereochemical notation and tryptophan nitrogen designation Page 10 1 Review 1.1 Introduction and scope Page 11 1.2 Cook’s syntheses 1.2.1 The tetracyclic ketone Page 13 1.2.2 g.B-Unsaturated aldehyde Formation and Claisen Rearrangement Page 17 1.2.3 Aimaline and alkaloid G 1.2.3.1 1.4-Addition. oxvanion-Cooe rearrangement and selective oxidations Page 19 1.2.3.2 Oraanobarium chemistry and kinetic enolate quenching Page 21 1.2.4 Selenium chemistry and a pyrolytic rearrangement Page 23 1.2.5 Pvridine formation Page 26 1.2.6 Palladium methodology Page 27 1.2.7 Selective hvdroboration Page 31 1.2.8 Indole oxygenation Page 33 1.2.8.1 C10 oxygenation Page 34 1.2.8.2 C11 oxygenation Page 35 1.2.8.3 C12 oxygenation Page 37 1.2.9 Hofmann elimination Page 38 1.2.10 Oxindole formation Page 40 1.2.11 Tollens reaction Page 41 1.2.12 Modified Wacker oxidation Page 43 1.3 Martin's biomimeticsynthesis ofvellosimine Page 45 1.4 Martin’s olefin metathesis studies Page 47 1.5 Rassat’s synthesis of the tetracyclic ketone Page 49 1.6 Kwon’s formal syntheses Page 50 1.7 Kuethe’s aza-Diels-Alder approach page 52 1.8 Bailey’s synthesis of raumacline page 55 1.9 Ohba’s oxazole Diels-Alder approach page 58 Page 4 of 314 2 Results and discussion 2.1 Methodology studies on the decarboxvlative Claisen rearrangement 2.1.1 Backaround 2.1.1.1 Historical backaround Page 60 2.1.1.2 Prior work within the Craia arouD Page 61 2.1.1.3 Rationale for the current work Page 63 2.1.2 Svnthesis of bis(allvl) 2-(toluene-4-sulfonvl)malonates 2.1.2.1 Svnthesis of allvl alcohols Page 64 2.1.2.2 Svnthesis of2-(toluene-4-sulfonvl)malonates Bv direct sulfonvlation Page 65 2.1.2.3 Attemoted svnthesis of (toluene-4-sulfonvi) Meldrum’s a d d Page 68 2.1.2.4 Svnthesis of2-(toluene-4-sulfdnvl)malonates bv carboxvlation Page 70 2.1.3 Decarboxvlative Claisen rearranaements of bis!allvl) 2-(toluene-4-sulfon vl) malonates 2.1.3.1 Svmmetrical 2-(toluene-4-sulfonvl)malonates Page 77 2.1.3.2 Unsvmmetrical 2-(toluene-4-sulfonvDmalonates Page 79 2.1.4 CvdoDroDane-formina decarboxvlative Claisen rearrangement 2.1.4.1 Original proposal Page 87 2.1.4.2 Model svstem Page 87 2.1.4.3 Studies towards the 7-membered cvdic malonate Page 90 2.1.5 Conclusions from methodoloav studies Page 100 2.2 Studies towards the total svnthesis of (-)-suaveoline 2.2.1 Backaround and isolation Page 101 2.2.2 Other workers’ syntheses 2.2.2.1 Cook’s svntheses Page 102 2.2.2.2 Bailev’s svntheses Page 103 2.2.2.3 Ohba's svnthesis Page 105 2.2.3 Relevant methodoloav andretrosvnthesis Page 106 2.2.4 Svnthesis of rearranaement substrate Page 108 2.2.5 Attempts at decarboxvlative Claisen rearranaement Page 113 2.2.6 Alternative orotectina arouD strateaies Page 114 2.2.7 Ootimisation of route to decarboxvlative Claisen rearranaement substrate Page 119 2.2.8 Mosher’s esters Page 123 2.2.9 Improvements to decarboxvlative Claisen rearranaement Page 123 Page 5 of 314 2.2.10 Attempted transition-metal couplings Page 126 2.2.11 Second rearranaements Page 130 2.2.12 Cvclooentene formation Page 137 2.2.13 Attempts at masked aldehvde Introduction 2.2.13.1 Sulfonvl o-metailation Page 139 2.2.13.2 Reductive desulfonvlation Page 142 2.2.14 Pvridine N-oxide methodoloav Page 144 2.2.15 Oxidative cleavaae and indole protection Page 147 2.2.16 Final approaches to an advanced (-)-suaveoline intermediate Page 150 2.2.17 Concludina remarks and future prospects Page 153 3. Experimental 3.1 General laboratory procedures Page 155 3.2 General svnthetic procedures Page 156 3.3 Individual svnthetic procedures and compound data 3.3.1 Svmmetrical malonates Page 159 3.3.2 Malonvl monoesters Page 160 3.3.3 Unsvmmetrical malonates from malonvl monoesters Page 161 3.3.4 2-fToluene-4-sulfonvl)malonates from malonates Page 165 3.3.5 o-Nitroohenvl carbonates Page 170 3.3.6 (Toiuene-4-suifonvDacetates Page 173 3.3.7 2-(Toluene-4sulfonvl)malonates from (toluene-4-sulfbnvl)acetates Page 178 3.3.8 Decarboxvlative Claisen rearranaements Page 187 3.3.9 Rina-dosina metathesis Page 204 3.3.10 Compounds relevant to (toluene-4-sulfonvl)-Meldrum’s acid Page 208 3.3.11 Compounds relevant to allvl alcohol preparation Page 210 3.3.12 Compounds relevant to cvdoorooane formation Page 215 3.3.13 Compounds relevant to ovridine N-oxide formation Page 238 3.3.14 Compounds relevant to studies towards (-)-suaveoHne Page 242 4. Appendices 4.1 X-Rav Structures 4.1.1 Sinale rearranaement product 384 Page 302 4.1.2 Unexpected lactone 365 Page 305 Page 6 of 314 Acknowledgement First and foremost I would like to thank Professor Donald Craig for the opportunity to pursue my interest in synthetic chemistry in such a rewarding environment. His advice and boundless enthusiasm have been an inspiration. Thanks are due also to Mark Lansdell at Pfizer for being an ever-present source of helpful advice and second opinions. I would also like to thank my co-workers for providing intellectual stimulation and entertainment in equal measure. I can not imagine that any other lab could be quite the same. Special mention must be afforded to Santiago Carballares, John Caldwell, Tanya Matthie, Fabienne Grellepois, Paolo Innocenti, Damien Bourgeois, Henrik Jensen, Chi Ming Cheung, Volker Rahn, Chris Hyland, Alan Stewart, Alan Braunton, Gavin Henry, Samuel Beligny, Barry Dillon, Dave Mountford, Alice Fleming, Federica Paina, Steve Johns, Sophie Gore, Darryl Thomas, Pengfei Lu, Barry Cottam-Howarth, Cassim Ashraff, Yunas Bhonoah, Paula Rzepa and Marcus Medley. They all made the Craig group into something I will never forget. Several other colleagues at Pfizer also helped out in my hours of need, especially Peter Wilson and Torren Peakman. At Novartis in Vienna, Hubert Gstach and Manfred Auer provided great encouragement in the early days and played no small part in shaping my future. All of the faculty at Imperial College have influenced this work in one form or another; in particular Prof. Charles Rees, whose advice should be listened to! Finally I must extend my heartfelt thanks to my family and especially to my wife Alex, who has supported me so much throughout this endeavour. I could not have done it without her. Page 7 of 314 Abbreviations Ac acetyl AIBN azobis(/sobutyronitrile) app appears 9-BBN 9-borabicyclo[3.3.1 ]nonane Bn benzyl Boc fe/f-butoxycarbonyl bp boiling point br broad bu butyl BSA bis(trimethylsilyl)acetamide Bz benzoyl Cl chemical ionisation Cbz benzyloxy carbonyl d doublet dd doublet of doublets ddd doublet of doublet of doublets ddq doublet of doublet of quartets dqd doublet of quartets of doublets dt doublet of triplets DBU 1,8-diazobicyclo[5.4.0]undec-7-ene DCC A/./V'-dicyclohexyl carbodiimide dCr decarboxylative Claisen rearrangement DDQ 2,3,5,6-dichlorodicyanoquinone DIBAL-H Di/sobutylaluminium hydride DIC A/.A/'-di/sopropyl carbodiimide DMAP 4-(/V, /V-di methylam ino)pyridine DM DO dimethyldioxirane DMP Dess-Martin Periodinane DMSO dimethyl sulfoxide DMF /V,A/-dimethylformamide EDCI 1-ethyl-3-(3-dimethylaminopropyl)
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