B type proanthocyanidins are a specific type of proanthocyanidin, which are a class of flavanoids. They are oligomers of flavan-3-ols.
These molecules have the molecular formula C<sub>30</sub>H<sub>26</sub>O<sub>12</sub> (molar mass : 578.52 g/mol, exact mass : 578.142426).
The 4-8 bond can be in the alpha or in the beta position.
B-type procyanidin (catechin dimer) 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.
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 AgBF<sub>4</sub>. The coupled product can be transformed into procyanidin C2 by a known procedure.
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-4ñâÂÂ8-(+)-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 ñ-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.