Efficient Stereoselective Synthesis of Proanthocyanidin Trimers with TMSOTf-Catalyzed Intermolecular Condensation

A stereoselective synthesis of seven benzylated proanthocyanidin trimers with TMSOTf-catalyzed condensation reaction is described. In particular, epicatechin-(4β-8)-epicatechin-(4β-8)-epicatechin trimer (procyanidin C1), catechin-(4α-8)-catechin-(4α-8)-catechin trimer (procyanidin C2), epicatechin-(4β-8)-epicatechin-(4β-8)-catechin trimer and epicatechin-(4β-8)-catechin-(4α-8)-epicatechin trimer derivatives were obtained in excellent yields. The structure of benzylated procyanidin C2 was confirmed by comparing the 1H NMR spectra of protected procyanidin C2 that was synthesized by two different condensation approaches. Finally, deprotection of (+)-catechin and (-)-epicatechin trimers derivatives gave four natural procyanidin trimers in good yields.