Oxidation and Rearrangement Reactions of Condensed Tannins

The chemistry of condensed tannins at alkaline pH is regulated by the formation of A- and/or B-ring quinone-methides as highly reactive intermediates. The former group of electrophilic species is of special significance in analogues with 5,7-dihydroxy A-ring functionality and may be successfully trapped by external nucleophiles such as phenolic nuclei and thiols. B-ring quinone-methides may similarly be trapped but are preferentially susceptible to intramolecular trapping to give analogues with rearranged pyran heterocycles. A mechanistic rationale to explain the base-catalyzed rearrangement reactions and the oxidative conversion of B- to A-type proanthocyanidins is proposed. These reactions are of relevance to the industrial application of condensed tannins.