Synthesis of condensed tannins. Part 14. Biflavanoid profisetinidins as synthons. The Acid-induced ‘phlobaphene’ reaction

Abstract

Free-phenolic [4,6]- and [4,8]-2,3-trans-(–)-fisetinidol-(+)-catechin diastereoisomers of both 3,4-trans and 3,4-cis configuration, which serve as synthons for higher oligomers, are available in improved yields from direct condensation, and also via novel 6-iodo-(+)-catechin as an intermediate substrate. Acid-induced transformations of the predominant [4,8]-all-trans isomer, illustrative of the well-known ‘phlobaphene reaction’ of condensed tannins, is shown to include ring-isomerization and fission of the inter-flavanoid bond, followed in the latter instance by the alternatives of anthocyanidin formation, positional rearrangement and self-condensation.