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UBC Theses and Dissertations

Oxidative coupling of dibenzylbutanolides catalyzed by plant cell culture extracts Palaty, Jan


This thesis aims to develop a new and inexpensive synthetic route to the anti-cancer drug etoposide (6) via 4'-demethylpodophyllotoxin (4) or 4'-demethylepipodophyllotoxin (5) involving the oxidative coupling of a dibenzylbutanolide catalyzed by a cell-free extract (CFE) from plant cell culture. This step was studied in depth using the Catharanthus roseus CFE-catalyzed biotransformation of frans-2-(3,5-dimethoxy-4-hydroxybenzyl)-3-(3-hydroxy-4-methoxybenzyl)butanolide (58) to 1-(3,5-dimethoxy-4-hydroxyphenyl)-6-hydroxy-3-hydroxymethyl-7-methoxy-1,2,3,4-tetrahydro-2-naphthoic acid γ lactone (59) as a model. The optimum values of reaction pH, enzyme:substrate ratio and co-factonsubstrate ratio were determined. The butanolide 58 was synthesized by a route involving the Stobbe condensation of 3-benzyloxy-4-methoxybenzaldehyde with dimethylsuccinate to yield 2-(3-benzyloxy-4-methoxybenzylidene)butanedioic acid 1-methyl ester (69). Hydrogenation of 69 to 2-(3-benzyloxy-4-methoxybenzyl)butanedioic acid 1-methyl ester (70) followed by reductive lactonization afforded 3-(3-benzytoxy-4-methoxybenzyl)butanolide (71). Alkylation of 71 with 4-benzyloxy-a-bromo-3,5-dimethoxytoluene (72) gave frans-2-(4-benzyloxy-3,5-dimethoxybenzyl)-3-(3-benzyloxy-4-methoxybenzyl)butanolide (73) which was then converted to the butanolide 58 by catalytic hydrogenolysis. In order to investigate the effect of different aromatic substituents on the oxidative coupling of butanolides, C. roseus CFE-catalyzed biotransformations of frans-2-(3,5-dimethoxy-4-hydroxybenzyl)-3-(3,4-methylenedioxybenzyl)butanolide (74) and frans-2-(3,5-dimethoxy-4-hydroxybenzyl)-3-(3,4-dihydroxy-a-hydroxybenzyl)butanolide (94) were also performed. The biotransformation of 74 gave 2-(3,5-dimethoxy-4-hydroxybenzylidene)-3-(3,4-methylenedioxybenzyl)butanoiide (76) as the sole isolated product. A pathway involving oxidative demethylatton is proposed to account for the balance of the unrecovered material. The butanolide 94, a potential precursor to etoposide, was prepared from piperonal. The lithium anion of 1-bis(phenylthio)methyl-3,4-methylenedioxybenzene (97) and the bromide 72 were added consecutively to but-2-en-4-olide to afford frans-2-(4-benzyloxy-3,5-dimethoxybenzyl)-3-(3,4-methylenedioxy-α,α-bis(phenylthio)benzyl)butanolide (96). A synthetic sequence involving the oxidation of 96 to frans-2-(4-benzyloxy-3,5-dimethoxybenzyl)-3-(3,4-methylenedioxybenzoyl)-butanolide (100), reduction to frans-2-(4-benzyloxy-3,5-dimethoxybenzyl)-3-(α-hydroxy-3,4-methylenedioxybenzyl)butanolide (109) and cleavage of the methylenedioxy and benzyl protecting groups gave the catechol 94. Unfortunately, the CFE-catalyzed oxidation of 94, following treatment with sodium borohydride, yielded 4-(3,4-dihydroxyphenyl)-5,7-dimethoxy-6-hydroxy-2-hydroxymethyl-1,2,3,4-tetrahydro-2-naphthoic acid γ lactone (103) as the sole isolated product. [Formulas omitted]

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