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Studies in natural products Part I. The biosynthesis of erythrina alkaloids Part II. An attempted in vitro demethylation of lanosterol Gervay, Joseph Edmund

Abstract

In Part I, hypotheses for the biogenesis of Erythrina alkaloids are discussed. Di-(β-3,4-dihydroxyphenyl)-ethylamine the theoretical precursor predicted by the biogenetic theory, was prepared and ring closure to the erythrinane ring system by oxidative coupling was attempted under various conditions. Consequently, the biogenesis of the Erythrina alkaloids was re-examined and a new proposal is advanced for the biosynthesis of these alkaloids. Synthetic routes to a hypothetical precursor, proposed here for the first time as a potential intermediate, are described. The biogenetic-type synthesis of the spiro-amine ring system present in the Erythrina alkaloids was achieved by oxidative coupling of the blocked diphenolic precursor, as predicted by the proposed biosynthetic scheme. Oxidation of di-(β-3-hydroxy-4-methoxyphenyl)-ethylamine by alkaline potassium ferricyanide afforded 3,15-dimethoxy-16-hydroxy-2-oxo-erythrina-1(6),3-diene in 15% yield. Reduction of the latter by sodium borohydride gave 3,15-dimethoxy-2,16-dihydroxyerythrina-l(6),3-diene. Acetylation of the dienone yielded 3,15-dimethoxy-16-acetoxy-2-oxoerythrina-1(6),3-diene. The total biogenetic-type synthesis of erysodine is therefore but two steps from completion. The results as a whole confirm the hypothesis that Erythrina alkaloids are produced in Nature by oxidative coupling of diphenols. They also demonstrate the directing role of the protective groups in the phenolic precursor. The evidence allows a biosynthetic pathway for the aromatic Erythrina alkaloids to be considered, and the mechanism for the ring closure process is discussed. The isotopically labelled precursor 3-hydroxy-4-methoxy-N-(3-hydroxy-4-methoxyphen[1-¹⁴C]ethyl)-phenethylamine was prepared to test the biosynthetic hypothesis in the plant. Feeding experiments are in progress. In Part II, the biogenesis of cholesterol and methods for functionalising inert methyl groups are reviewed, and a new theoretical approach to removal of the 14α-methyl group from lanosterol is described. The removal of this methyl group in vitro could not be achieved, but a series of interesting compounds was obtained. Evidence for the structures of these compounds is presented. Thus, photosensitized oxygenation of dihydrolanosteryl acetate in the presence of para-nitrobenzenesulphonyl chloride yielded 3β-acetoxylanosta-7,9(ll)-diene, 3β-acetoxylanost-8-ene-7-one and 3β-acetoxylanost-8-ene-7a-hydroperoxide. In addition a compound having an ambiguous structure and designated as IP1 was obtained. The dibromo-derivative of the latter is 33-acetoxy-7a,lla-dibromolanostane-8 a,9α-epoxide, the structure of which was determined by X-ray crystallographic study. A working structure for compound IP1 based on the physical and chemical evidence is discussed.

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