Oxidative dimerizations of (E)- and (Z)-isoeugenol (2-methoxy-4-pro-penylphenol) and (E)- and (Z)-2,6-dimethoxy-4-propenylphenol

Abstract

Reaction of (E)-isoeugenol (3) with 1 equiv. of hydrogen peroxide catalysed by peroxidase gives a mixture of dehydrodi-isoeugenol {4-[2,3-dihydro-7-methoxy-3-methyl-5-(E)-propenylbenzofuran-2-yl]-2-methoxyphenol}(7a)(65%). threo- and erythro-1 -(4-hydroxy-3-methoxyphenyl)-2-[2-methoxy-4-(E)-propenylphenoxy]propan-1-ol (9a)(17%) and (10a)(5%). and isomers of 2,5-bis-(4-hydroxy-3-methoxyphenyl)-3,4-dimethyltetrahydro-furan (13a)(4%) and (14a)(9%) which result from β–5. β–O, and β–β coupling, respectively. Oxidation of (Z)-isoeugenol (4) under identical conditions yields products differing only in the propenyl side-chain configurations of compounds (7), (9), and (10): viz. (7b)(22%), (9b)(40%), (10b)(13%), (13a)(8%), and (14a)(17%). The tetrahydrofuran dimethyl ethers (13b) and (14b) were identified as the (±)-forms of the lignans galbelgin and veraguensin, respectively. Oxidation of (E)-2,6-dimethoxy-4-propenylphenol (5) with 1 equiv. of hydrogen peroxide-peroxidase or potassium ferricyanide affords a 1 : 2 mixture of isomers of 2,5-bis-(4-hydroxy-3,5-di-methoxyphenyl)-3,4-dimethyltetrahydrofuran (13c) and (14c). In identical oxidative treatments, the (Z)-phenol (6) gives isomers (13c), (14c), (15c), and (16c) in the ratio 1:2:2:1. The tetrahydrofuran configurations were assigned by reductive degradation experiments involving ring scission and by n.m.r. spectral comparisons with analogous lignans. The β–β coupling of phenols (3)–(5) is remarkably stereospecific and produces exclusively threo-compounds. whereas the (Z)-phenol (6) gives threo- and erythro-coupling products in equal amounts, A mechanism is proposed for this coupling which involves the intermediacy of a dimeric ‘tail-to-tail’ charge-transfer complex, formed by association of phenoxyl radicals. The feasibility of the alternative ‘head-to-tail’ complexes is also discussed. The differences in the probabilities of coupling modes in the oxidations of (E)- and (Z)-isoeugenol is considered to be due to the characteristics of these intermediate complexes rather than to the differences in free-electron densities.