Plant proanthocyanidins. Part II. Proanthocyanidin-A2 and its derivatives
The 2,7″-epoxy-4,8″-biflavan structure (1) has been deduced for the natural proanthocyanidin-A2 on the basis of spectroscopic (13C and 1H n.m.r.) and chemical evidence. Spectroscopic data are also outlined which lead, in association with biogenetic arguments, to a proposal (1a) for the absolute stereochemistry of the natural product. Hydrogenolysis of proanthocyanidin-A2 leads to fission of the C–O and C–C inter-flavan bonds and formation of the phenols (20) and (22). The same technique also leads to rupture of the inter-flavan linkage of dimers of the B group and hence to a novel micro-procedure for their identification. The isolation and characterisation of three trimeric proanthocyanidins based on A2 is also discussed.