Plant proanthocyanidins. Part 3. Conformational and configurational studies of natural procyanidins
Means to generate the C-4 carbocations (3) and (4) corresponding to (+)-catechin (1) and (–)-epicatechin (2), respectively, are outlined, and the use of these intermediates for the synthesis of model procyanidins and for the biogenetically patterned synthesis of natural procyanidins is discussed. 13C N.m.r. data for model flavan systems and natural procyanidins are reported and analysed and the information is used to assign the 4R-configuration to four natural procyanidin dimers. The phenomenon of conformational isomerism is demonstrated for the natural procyanidin dimers, and two different forms of restricted rotation about the interflavan bond are proposed. The information is used to clarify many earlier structural anomalies, to predict preferred conformations, and to specify a C(4)–C(8) link for the four principal dimers (B-1–4). The properties of some procyanidin polymers are noted, and structures of opposite helicity are proposed for two of the major types found in nature.