Flavanyl units

Proanthocyanidins of the B-type are characterized by singly linked flavanyl units, usually between C-4 of the chain-extension unit and C-6 or C-8 of the chain-terminating moiety. They are classified according to the hydroxylation pattern of the chain-extension units (see T able 11.1). 

Prototypes (1) to (3) possess resorcinol-type flavanyl units attached to (-b)-catechin or (+)-gallocatechin, whereas, procyanidin prototypes (4a) are uniformly based on phloroglucinol-type flavanyl units. 

Scheme 1. Flavan-4-ols (1), flavan-3,4-diols (2) or 4-benzylsulfanyl analogues, and flavan-3-ols (5) as precursors to profisetinidin (R =H) or procyanidin (R =OH) oligomers with (2R,35 ) constituent flavanyl units...

In contrast to the aforementioned proanthocyanidins of the B-type where the constituent flavanyl units are linked by only one bond, analogues of the A-class possess an unusual second ether linkage to C(2) of the T-unit. Studies focussing on the oxidative formation of this bond are surprisingly limited and are hitherto restricted to the use of hydrogen peroxide (73) and dioxygen (74-76) as oxidants. 

The susceptibility of the constituent flavanyl units of proanthocya-nidins to intramolecular rearrangement via B-ring quinone methides... 

Hundt H K L, Roux D G 1978 Condensed tannins Determination of the point of linkage in terminal (-f-)-catechin units and degradative bromination of 4-flavanyl-flavan-3,4-diols. J Chem Soc Chem Commun 696-698... 

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