Linkage isomerism in trimeric and polymeric 2,3-cis-procyanidins

Abstract

Four procyanidin trimers have been isolated and their structures unequivocally established as: epicatechin-(4β→8)-epicatechin-(4β→8)-catechin (5); epicatechin-(4β→6)-epicatechin-(4β→8)-catechin (6); epicatechin-(4β→8)-epicatechin-(4β→6)-catechin (7); and epicatechin-(4β-8)-epicatechin-(4β→8)-epicatechin (13). The presence of the first three trimers in Pinus taeda phloem, and the isolation of both epicatechin-(4β→8)-4β-benzylthioepicatechin (9) and epicatechin-(4β→6)-4β-benzylthioepicatechin (10) from partial acid-catalyzed cleavage of the tannins from Pinus palustris phloem and Photinia glabrescens leaves, shows that linkage isomerism exists in natural oligomeric and polymeric procyanidins. A new system of nomenclature for proanthocyanidins is proposed.