Epoxidation and hydroxylation of Flav-3-enes, and synthesis of 2,3-trans-Flavan-3-ols

Abstract

Flav-3-ene epoxides cannot be prepared by the reaction of peracetic acid with a flavene, which leads mainly to the 2,3-trans-3,4-cis-4-acetoxyflavan-3-ols. The epoxides of flav-3-enes lacking electron releasing substituents in ring A may be prepared under weakly basic conditions, and on reduction give 2,3-trans-flavan-3-ols. Flavenes with electron-releasing substituents in conjugated positions in the A ring do not give epoxides under acidic or basic conditions, but the 2,3-trans-flavan-3-ols can be prepared by hydrogenolysis of the products obtained by interaction with peracetic acid. Mechanisms are proposed to account for the products obtained in the peroxidation reactions, and N.M.R. was used to establish the configurations of all the products. Migration of the acetyl group from the 4- to the 3-oxygen atom was observed in two cases, and the resultant 2,3-trans-3,4-cis-3-acetoxyflavan-4-ols showed virtual coupling of the 2- and 4-protons because of the accidental coincidence in chemical shift of the 2- and 3-protons. 2,3-cis-3,4-trans-3-Bromo-4,4'-dimethoxyflavan showed similar virtual coupling owing to near equivalence in the chemical shift of the 3- and the 4-protons.