Plant proanthocyanidins. Part 4. Biosynthesis of procyanidins and observations on the metabolism of cyanidin in plants

Evidence based on the results of feeding experiments with labelled (3H,14C) cinnamic acids is presented to show that the various procyanidin dimers are biosynthesised from two metabolically distinct units. One is the flavan-3-ol [(+)-catechin (10) or (–)-epicatechin (9)] and the other is the C-4 carbocation (7) or (8). It is postulated that the carbocations are derived by protonation of the flav-3-en-3-ol (6) and are intermediates in the reduction to the flavan-3-ols (9) and (10). The relationship of procyanidin biosynthesis to anthocyanidin formation in certain plants is briefly discussed.