Annulation Approach to Doubly Linked (A-type) Oligocatechins: Syntheses of (+)-Procyanidin A2 and (+)-Cinnamtannin B1

The first stereoselective syntheses of doubly linked (A-type) oligocatechins, (+)-procyanidin A(2) and (+)-cinnamtannin B-1, have been achieved. Ethylenedioxy-bridged flavans served as excellent platforms, thus allowing annulation with nucleophilic catechin units in a stereoselective manner. An additional key was the new synthetic approach to selectively protected nucleophilic catechin, thus enabling regioselective construction of the key dioxabicyclo skeleton of the A-type oligocatechins.