Preparation of anthocyanidins and their glygosides from related flavonoids

Abstract

Improvements in the method of reductive acetylation of flavonols provide pure samples of anthocyanidins, the examples studied being cyanidin and its O-pentamethyl and O-5,7,3′,4′-tetramethyl derivatives. The method can also be used for the conversion of flavonol glycosides into anthocyanins; the preparation of cyanidin and pelargonidin-3-rhamnoglucosides are described. 3-Hydroxyflavanones, e.g. taxifolin, aromadendrin and dihydrorobinetin undergo isomeric change into ψ-base acetates of anthocyanidins on heating at 150–155° with acetic anhydride solution and a base catalyst and the ψ-base acetates undergo conversion into anthocyanidins in very good yields. The mechanism of this transformation is made clear by the isolation of intermediate stages in the case of taxifolin tetramethyl ether. This result suggests the possibility of a direct route for the biogenesis of 3-hydroxy anthocyanidins from the 3-hydroxy flavanones. But this change has not so far been successfully carried out with flavanones not having the 3-hydroxyl. Flavones, e.g. apigenin and its trimethyl ether, yield on reductive acetylation, a mixture of products, in which flavenes seem to be predominant. The intermediates undergo conversion into apigeninidin chloride and its O-trimethyl derivative in good yields. 2′-Hydroxy-4,4′,6′-trimethoxy dibenzoyl methane on reduction with borohydride gives the diol which undergoes conversion in acids satisfactorily to apigeninidin trimethy] ether. Substituted flavan-4-ols, prepared by reduction of a flavanone, give the corresponding anthocyanidins (yield ca. 10 per cent) on heating in alcoholic hydrochloric acid.