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The Structure Elucidation and Synthesis of Selected Natural Products

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The Structure Elucidation and Synthesis of Selected Natural Products THE STRUCTURE ELUCIDATION AND SYNTHESIS OF SELECTED NATURAL PRODUCTS by WILHELMINA MARAIS Thesis presented in fulfillment of the requirements for the degree PHILOSOPHIAE DOCTOR in CHEMISTRY in the FACULTY OF SCIENCE of the RAND AFRIKAANS UNIVERSITY Supervisor: Professor C.W. Holzapfel December 2000 LIST OF ABBREVIATIONS Ac acetyl acac acetylacetonate AHR asymmetric Heck reaction Bn benzyl CO carbon monoxide dba dibenzylidene acetone dipp 1,3-bis(diisopropylphosphine)propane DME 1,2-dimethoxyethane DoM directed ortho metallation EI-MS electron impact mass spectromety ES-MS electronspray mass spectrometry EtOAc ethyl acetate FAB-MS fast atom bombardment mass spectrometry GC-MS gas chromatography-mass spectrometry HPLC high pressure liquid chromatography m-CPBA meta-chloroperbenzoic acid Me methyl NMR nuclear magnetic resonance nOe nuclear Overhauser effect OTf triflate PHMS poly(methylhydrosiloxane) Piv pivaloyl SQHC sesquiterpene hydrocarbons TBAB tetrabutylammonium bromide TBAI tetrabutylammonium iodide THE tetrahydrofuran TLC thin layer chromatography TONs total catalyst turnover numbers triflates trifluoromethanesulfonates VNS vicarious nucleophilic substitution SYNOPSIS The objective of the research described in the first part of this thesis was to develop a general method utilising palladium catalysed reactions for the synthesis of the anti-cancer compound, lavendamycin and analogues thereof. Therefore, the development of a general route to synthetic equivalents of the lavendamycin AB quinoline system, 2 -hydroxyquinolines, with potential for coupling to the CDE or CD moiety, was addressed. The first protocol for the synthesis of 2-hydroxyquinolines involved the use of appropriately substituted o- nitrophenyltriflates (readily prepared from phenols) in a Heck reaction under neutral conditions followed by a one-pot reduction and cyclisation step. The synthetic potential of such an approach was demonstrated by the preparation of a suitably substituted lavendamycin AB synthon from commercially available guiacol. A second general strategy towards the synthesis of the AB synthon utilising a preformed ring system such as commercially available 8- hydroxyquinoline has been successfully developed. This approach requiring the introduction of a suitable leaving group in the 2-position involved the following sequence of reactions: protection of the 8-hydroxyl group, N-oxidation, and a rearrangement step. This methodology yielded five different key intermediates all possessing suitable functionality in the 2 position which would allow further cross-coupling to an appropriate CDE ring equivalent. The next part of this research revolved around approaches to the preparation of a suitable CDE ring system. However, it was realised that although constituting a convergent approach and employing modern catalytic processes the cross-coupling strategy would be very long. Therefore, an alternative approach to lavendamycin via a one-pot palladium catalysed carbonylation of a suitable 2-chloroquinoline derivative with tryptophan as nucleophile to yield an amide containing all the skeletal atoms, was developed. This was followed by a Bischler- Napieralski cyclisation to give the targeted pentacyclic system. Completion of the synthesis will entail a sequence of nitration, reduction and oxidation to complete the left hand side while hydrolysis, lithiation and introduction of a suitable electrophile will complete the right hand side. Although the synthetic approach for lavendamycin described in this thesis is not shorter than any of the published methods, it is unique in that the same methodology will allow access to a range of lavendamycin analogues required for the study of physiological structure-activity relationships. The next part of the thesis involved the development of methods for cyclic enamide synthesis. One of the methods for the preparation of cyclic enamides involved dehydration of the corresponding N-acyl-carbinolamines which was easily prepared from readily available pyrrolidone. The successful application of these compounds in the Fischer indole synthesis on route to the neurohormone, melatonin and other derivatives e.g. a tryptophan analogue was demonstrated. In the final part of the thesis, the isolation and characterisation of some natural products are described. This research forms part of an ongoing collaboration with chemotaxonomists of the Department of Botany. The chemotaxonomic survey resulted not only in the isolation of several new compounds, one of them the unique compound plicatoloside 4.1 from Aloe plicatilis, but it also successfully demonstrated the application of HPLC coupled to ES-MS in identifying such compounds e.g. from A. africana, A. speciosa and A. broomii. The types of compounds isolated in this study can be regarded as typical of the genus Aloe and may prove to have some chemotaxonomical significance. In addition, the major constituent of Siphonochilus aethiopicus (Zingiberaceae), commonly known as wild ginger, was characterised. It is the only indigenous member of the family in South Africa and has a unique and distinctive morphology reflected in the obvious chemical and thus, biogenetic differences in the structure of the terpenoid constituents of S. aethiopicus and Zingiber officinale Roscoe. SAMEVATTING Die doel van die ondersoek, soos bespreek in die eerste gedeelte van hierdie proefskrif, behels die ontwikkeling van 'n algemene sintese vir die anti-kanker verbinding, lavendamisien, deur gebruik te maak van 'n reeks palladium gekataliseerde reaksies. Aangesien 2-hidroksikinoliene as 'n geskikte sintetiese ekwivalent vir die lavendamisien AB kinolien ringstruktuur beskou is en ook geredelik aan CDE of CD gedeelte gekoppel kan word, is die ontwikkeling van 'n algemene roete na sulke verbindings, geloods. Die eerste protokol vir die bereiding van 2- hidroksikinoliene benut o-nitrofenieltriflate (maklik bereibaar vanaf fenole) as uitgangstof in 'n Heck reaksie (neutrale kondisies) gevolg deur 'n 'eenpot' reduksie en silklisering. Die suksesvolle bereiding van 'n geskikte lavendamisien AB ringstruktuur vanaf die kommersieel beskikbare, guiacol bevestig die sintetiese potensiaal van so 'n benadering. Die daaropvolgende strategie vir die sintese van 'n AB ringstruktuur maak gebruik van die modifikasie van 'n bestaande kinolien, die kommersieel beskikbare 8-hidroksikinolien. Die vereiste invoeging van 'n geskikte verlatende groep in C-2 van 8-hidroksikinolien is verkry deur die volgende transformasies, nl.: beskerming van die 8-hidroksigroep, N-oksidasie en herrangskikking. Hierdie proses het vyf verskillende sleutelverbindings met die verlangde funksionaliteit in die 2- posisie en gereed vir verdere kruiskoppeling, opgelewer. In die volgende gedeelte van die ondersoek is verskeie benaderings vir die bereiding van 'n geskikte CDE ringstruktuur oorweeg. Hieruit was dit egter duidelik dat die konvergerende strategie wat op 'n moderne katalitiese proses soos kruiskoppeling gebaseer is, te veel stappe sou behels. Dus is die ontwikkeling van 'n alternatiewe benadering vir die bereiding van lavendamisien deur karbonilering in te span, ontwikkel. 'Eenpot' palladiumgekataliseerde karbonileringsreaksie van 'n geskikte 2-chlorokinolien met triptofaan as nukleofiel het 'n amied gelewer wat deur 'n Bischler-Napieralski siklisering gevolg is, om sodoende die verlangde pentasikliese lavendamisien voorloper te verskaf. Die voltooiing van die sintese behels opeenvolgende reaksies bestaande uit nitrering, reduksie en oksidasie om die linkerkant te voltooi terwyl die regterkant afgehandel sal word deur hidrolise, litiering en invoeging van 'n geskikte elektrofiel. Alhoewel hierdie benadering vir lavendamisien nie veel korter is as die bestaande metodes nie, is dit uniek in die opsig dat dit toegang verleen tot 'n reeks lavendamisien analoe wat noodsaaklik is vir die bestudering van struktuur aktiwiteits verwantskappe. In die volgende gedeelte van die tesis is die ontwikkeling van verskeie metodes vir die bereiding van sikliese eenamiede beskryf. Een van die metodes vir die bereiding van sikliese eenamiede maak gebruik van die dehidrasie van die ooreenstemmende N-asiel-karbinolamiene wat op hul beurt geredelik berei kan word vanaf pirrolidiene. Die suksesvolle toepassing van die verbindings in die Fischer indool sintese vir die bereiding van die neurohormoon, melatonien en ander derivate, o.a. 'n triptofaan analoog, is ook bespreek. In die finale gedeelte van die tesis word die isolasie en struktuurbepaling van natuurprodukte in samewerking met chemotaksonome van die departement Plantkunde, beskryf. Die resultate van hierdie chemotaksonomiese ondersoek sluit die isolasie van verskeie nuwe verbindings, by. die unieke verbinding plikatalosied uit Aloe plicatilis in, maar dit vestig ook die aandag op die besondere rol wat die gebruik van HPLC gekoppel met ES-MS in die identifisering van sulke verbindings speel, by. die uit A. Africana, A. speciosa en A. broomii. Hierdie verbindings wat tydens die ondersoek gelsoleer is, kan as verteenwoordigend van die genus Aloe beskou word en mag dalk van bepaalde chemotaksonomiese belang wees. In aansluting by bogenoemde is die hoofverbinding van Siphonochilus aethiopicus (Zingiberaceae), algemeen bekend as wilde gemmer, gekarakteriseer. Hierdie spesie is die enigste inheemse lid van die bepaalde familie in Suid-Afrika en word gekenmerk deur 'n unieke en karakteristieke morfologie wat waarskynlik weerspieel word deur die klaarblyklike chemiese en dus, biogenetiese verskille
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