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Synthetic studies in indole alkaloids Gletsos, Constantie

Abstract

The total synthesis of a variety of indole and dihydroindole alkaloids, is described. Mere specifically, the d1-epimers of naturally occurring vincadine, vincaminoreine, vincaminorine, minovine, vincadifformine, N-methyl-quebrachamine and vincaminoridirie have been obtained by appropriate modifications in the general synthetic scheme. This work also illustrates that the transannular cyclization reaction previously developed in our laboratories is of great versatility in the synthesis of alkaloids in the Vinca and Aspidosperma families. In essence, the synthetic sequence involves the reaction of the aldehydo-ester (118) with either tryptamine or 6-methoxytryptamine to provide in high yield, the tetracyclic lactams (119 or 166). Lithium aluminum hydride reduction of the latter, followed by hydrogenolysis of the benzyl group provided the corresponding alcohols (94 or 165). These compounds were transformed via their mesylate derivatives to the quaternary salts (95 or 187) which served as the crucial intermediates for the preparation of the nine-membered-ring alkaloids. Finally transannular cyclization of the latter substances leads to the pentacyclic Aspidosperma and Vinca alkaloids.

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