Regioselective Synthesis of Procyanidin B6, A 4-6-Condensed (+)-Catechin Dimer, by Intramolecular Condensation.

Proanthocyanidins, also known as condensed tannins or oligomeric flavonoids, are found in many edible plants and exhibit interesting biological activities. Herein, we report a new, simple method for the stereoselective synthesis of procyanidin B6, a (+)-catechin-(4-6)-(+)-catechin dimer, by Lewis acid-catalyzed intramolecular condensation. The 5-O-t-butyldimethylsilyl (TBDMS) group of 5,7,3'4'-tetra-O-TBDMS-(+)-catechin was regioselectively removed using trifluoroacetic acid, leading to the "regio-controlled" synthesis of procyanidin B6. The 5-hydroxyl group of the 7,3',4'-tri-O-TBDMS-(+)-catechin nucleophile and the 3-hydroxyl group of 5,7,3',4'-tetra-O-benzylated-(+)-catechin electrophile were connected with an azelaic acid. The subsequent SnCl₄-catalyzed intramolecular condensation proceeded smoothly to give the 4-6-condensed catechin dimer. This is the first report on the complete regioselective synthesis of a 4-6-connected oligomer without modifying the 8-position.