B type proanthocyanidin

[1] Dimeric proanthocyanidins can also be synthesized with procyanidin-rich grape seed extracts reacted with flavan-3-ols under acid catalysis.

The coupled product can be transformed into procyanidin C2 by a known procedure.

[3] A coupling utilising a C8-boronic acid as a directing group was developed in the synthesis of natural procyanidin B3 (i.e., 3,4-trans-(+)-catechin-4α→8-(+)-catechin dimer).

The key interflavan bond is forged using a novel Lewis acid-promoted coupling of C4-ether 6 with C8-boronic acid 16 to provide the α-linked dimer with high diastereoselectivity.

Through the use of a boron protecting group, the new coupling procedure can be extended to the synthesis of a protected procyanidin trimer analogous to natural procyanidin C2.