Total Synthesis of the Putative Structure of Tridachiahydropyrone

Author: David William Jeffery

Jeffery, David William, 2005 Total Synthesis of the Putative Structure of Tridachiahydropyrone, Flinders University, School of Chemical and Physical Sciences

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Polypropionate marine natural products have emerged as a class of compounds that display a high degree of structural diversity. Specifically, metabolites such as that reported as tridachiahydropyrone (7), isolated from sacoglossan molluscs, display novel ring systems. The introductory chapter gives some background on tridachione marine natural products and outlines the isolation of metabolites from several species of sacoglossan mollusc. Chapter One also gives examples of the utility of the tandem conjugate addition-Dieckmann condensation approach being applied to the synthesis of these compounds. Chapter Two describes the development of the tandem conjugate addition-Dieckmann condensation and subsequent trans methylation approach to cyclohexenone rings. The synthetic strategy utilised chiral, functionalised cyclohexenone rings as synthons in the formation of bicyclic ring systems, so development of the carbocyclic ring formation was of vital importance to the overall strategy. Examples are given which confirm the viability of the proposed synthetic route to cyclohexenones such as 91, 92 and 104 from the reaction of [alpha,beta]-unsaturated carbonyl compounds 39 and 59 with dialkyl and dialkenyl Gilman cuprates. Chapter Three describes the incorporation of chiral cyclohexenone 117 into the bicyclic framework of model compound 105, analogous to the marine natural product reported as tridachiahydropyrone (7). The chapter explores the use of cyclohexenone precursor 43 that contained the total carbon framework of the bicyclic core of the desired pyrone. Once again, a tandem conjugate addition-cyclisation reaction was employed using a dialkyl Gilman cuprate, followed by trans methylation to give the requisite cyclohexenone synthon 117. A novel Eaton’s reagent-promoted intramolecular cyclisation of acid 122 to pyrone 123 was then effected. Subsequent O-methylation afforded [alpha]-methoxy-[beta]-methyl-[gamma]-pyrone 105 as a single enantiomer, which had the identical core structure to the natural product. The structure, including relative stereochemistry of 105, was confirmed by single crystal X-ray analysis. Chapter Four builds on the previous two chapters and describes the conjugate addition-cyclisation with a higher order Gilman cuprate derived from vinyl bromide 44, which would deliver the vinyl side-chain required for the synthesis of reported natural product 7. The same acyclic precursor 43 as used in Chapter Three was cyclised and methylated to yield yet another cyclohexenone synthon 41. A single crystal X-ray analysis of related alcohol 162 confirmed the relative stereochemistry and structure. Another novel P2O5-mediated intramolecular cyclisation was achieved to give pyrone 168 and O-methylation provided a compound with the reported structure of natural product 7 as a single enantiomer. The structure of synthetic 7 was established unequivocally through extensive NMR studies. Comparisons of spectral data confirmed that natural tridachiahydropyrone was not the same as synthetic compound 7, so revision of the assigned natural product structure is warranted. Several other analogues were also synthesised using this methodology, highlighting the versatility of the method under development.

Keywords: marine polypropionates,tridachiahydropyrone,conjugate addition,Dieckmann condensation,Eaton's reagent,P2O5-mediated cyclisation

Subject: Chemistry thesis

Thesis type: Doctor of Philosophy
Completed: 2005
School: School of Chemical and Physical Sciences
Supervisor: Dr Michael Perkins