Flavonoid epoxides. Part 22. Establishment of the configuration of the diastereomeric solvolysis products of 2-arylmethylenebenzo[b]furan-3(2H)-one (aurone) epoxides

Abstract

Reaction of aurone epoxides 4 with a variety of hydroxylic solvents gives rise to a number of diastereomeric solvolysis products. The stereochemistry of these compounds was initially tentatively assigned by comparison of their 1H nmr spectra with that of the erythro-acetal 15a resulting from acid catalysed methanolysis of the ( Z)-aurone epoxide 2. In this paper we establish the configurations of these solvolysis products on a firmer basis, by obtaining the threo acetal 15b by acid catalysed methanolysis of the corresponding ( E)-aurone epoxide 23. Both the erythro- 15a and the threo-acetal 15b were independently synthesised by an aldol condensation (using LDA as base) of the acetal 27 with benzaldehyde, and surprisingly a high level of diastereoselectivity was obtained, which was attributed to secondary orbital interactions between the electrophile and the enolate molecule. When bromomagnesium diisopropylamide (BMDA) was used as the base for this purpose, the product of the self-condensation of 27 was obtained as a single diastereomer along with the erythro-acetal 15a and starting material.