B type proanthocyanidin

B type proanthocyanidins are a specific type of proanthocyanidin, which are a class of flavanoids. They are oligomers of flavan-3-ols.

Dimeric B type proanthocyanidins

These molecules have the molecular formula C₃₀H₂₆O₁₂.

Molecules with 4→8 bonds

The 4-8 bond can be in the alpha or in the beta position.

Procyanidin B1 or epicatechin--catechin
Procyanidin B2 or -epicatechin---epicatechin
Procyanidin B3 or catechin--catechin
Procyanidin B4 or catechin--epicatechin

Molecules with 4→6 bonds

Procyanidin B5 or epicatechin--epicatechin
Procyanidin B6 or catechin--catechin
Procyanidin B8 or catechin--epicatechin

Chemistry

B-type procyanidin can be converted to A-type procyanidin by radical oxidation.
Dimeric proanthocyanidins can also be synthesized with procyanidin-rich grape seed extracts reacted with flavan-3-ols under acid catalysis.

Trimeric B type proanthocyanidins

Chemical synthesis

A stereoselective synthesis of benzylated catechin trimer under intermolecular condensation is achieved using equimolar amount of dimeric catechin nucleophile and monomeric catechin electrophile catalyzed by AgOTf or AgBF₄. The coupled product can be transformed into procyanidin C2 by a known procedure.

Iterative oligomer chemical synthesis

A coupling utilising a C8-boronic acid as a directing group was developed in the synthesis of natural procyanidin B3 -catechin-4α→8-. 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.