Optimization for the Preparation of Procyanidin B3: Strategy for Gram-Scale and Stereoselective Formation of 4,8-Interflavan Bonds

Oligomeric proanthocyanidins (OPCs) have a variety of biological functions, but the formation of 4,8-interflavan bonds faces scaling-up difficulties due to the challenging control of stereoselectivity and the degree of polymerization. Here we report a process to produce procyanidin B3 (1) by mainly optimizing the condensation reaction and improving benzylation, C4 activation, and one-pot hydrogenolysis reactions. In an optimized seven-step process, the product 1 was achieved by only one-step chromatography in the case of poor crystallinity of polyphenols. This strategy provided effective access to the stereoselective synthesis of the title compound and other C4–C8 connected OPCs.