Condensed tannins: Quinone methide intermediates in procyanidin synthesis

Abstract

Proanthocyanidins (condensed tanruns) are widely distributed in plants and are found in sufficiently high concentration in some tree barks to encourage their industrial utilization. These polymers consist of flavanoid units linked through the C-4 of the pyran ring to the C-6 or C-8 carbons of the aromatic A-ring. Recent advances in the chemistry of condensed tannins are due largely to the successful synthesis of many proanthocyanidin isomers by acid-catalysed condensations. These facile acid-catalysed condensations have prompted the use of the term 'biomimetic synthesis' and hypotheses on polymer biosynthesis that involve protonation of a flavan-3,4-diol or a flav-3-en-3-ol to produce dimers and oligomers. While proanthocyarudins can be readily synthesized in vitro by protonation of flavan-3,4-diols, it seems equally plausible that the route to their biosynthesis could be through a quinone methide intermediate. Acid- and base-catalysed reactions of flavan derivatives containing good leaving groups at C-4 were compared to examine this hypothesis.

Citation

Hemingway, Richard W.; Foo, L. Y. 1983. Condensed tannins: Quinone methide intermediates in procyanidin synthesis. Journal of The Chemical Society Chemical Communications, p. 1035-1036