Reactions at the A-ring of Proanthocyanidins

Abstract

The most common reactions at the A-ring of proanthocyanidins (condensed tannins) are examples of electrophilic aromatic substitution where an electrophile displaces a hydrogen (i.e., at C-6 and/or C-8 of procyanidins and profisetinidins). Other types of displacements include dehalogenation, procyanidin degradation, and oxidative free-radical coupling. The hydroxylation pattern of the A-ring controls both reactivity and orientation of substitution, partially by steric effects, but electronic factors have more influence. Examples of A-ring reactions include those of commercial interest (methylolation, condensation) and of diagnostic interest (halogenation, alkylation, hydrogen/deuterium exchange). C- and O-glycosylation are further types of A-ring reactions leading to natural products.