FLAVONOID EPOXIDES - PART 22 - ESTABLISHMENT OF THE CONFIGURATION OF THE DIASTEREOMERIC SOLVOLYSIS PRODUCTS OF 2-ARYLMETHYLENEBENZO[B]FURAN-3(2H)-ONE (AURONE) EPOXIDES

Reaction of aurone epoxides(4) with a variety of hydroxylic solvents g ives rise to a number of diastereomeric solvolysis products. The stere ochemistry of these compounds was initially tentatively assigned by co mparison of their H-1 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 pr oducts on a firmer basis, by obtaining the threo acetal 15b by acid ca talysed methanolysis of the corresponding (E)-aurone epoxide 23. Both the erythro-15a and the threo-acetal 15b were independently synthesise d by an aldol condensation (using LDA as base) of the acetal 27 with b enzaldehyde, and surprisingly a high level of diastereoselectivity was obtained, which was attributed to secondary orbital interactions betw een the electrophile and the enolate molecule. When bromomagnesium dii sopropylamide (BMDA) was used as the base for this purpose, the produc t of the self-condensation of 27 was obtained as a single diastereomer along with the erythro-acetal 15a and starting material. (C) 1998 Els evier Science Ltd. All rights reserved.