Stereoselective synthesis of procyanidin B3-3-O-gallate and 3,3″-di-O-gallate, and their abilities as antioxidant and DNA polymerase inhibitor

A simple method for the synthesis of procyanidin B3 substituted with a galloyl group at the 3 and 3″ position is described. Condensation of a benzylated catechin-3-O-gallate electrophile with a nucleophile, catechin and catechin-3-O-gallate, proceeded smoothly and stereoselectively to afford the corresponding dimer gallates, procyanidin B3-3-O-gallate and procyanidin B3-3,3″-di-O-gallate, in good yields. Further, their antioxidant activities on UV-induced lipid peroxide formation, DPPH radical scavenging activity and inhibitory activity of DNA polymerase were also investigated. Among three procyanidin B3 congeners (procyanidin B3, 3-O-gallate and 3,3″-di-O-gallate), the 3,3″-di-O-gallate derivative showed the strongest antioxidant and radical scavenging activity. Interestingly, the 3-O-gallate derivative was the strongest inhibitor of mammalian DNA polymerase α with IC50 value of 0.26 μM, although it showed the weakest antioxidant and radical scavenging activity. It became apparent that the presence of a galloyl group at the C-3 position in the proanthocyanidin oligomer was very important for biological activity, however, the antioxidant activity of these compounds was not parallel to the DNA polymerase inhibitory activity.