Reactions of Oligomeric Proanthocyanidins

The discussions of the reactions of flavan-3-ols (Sect. 7.6.3.1.3) and of the flavan-3,4-diols (Sect. 7.6.3.2.3) are, for the most part, directly applicable to those of the oligomeric proanthocyanidins and condensed tannins, the critical difference, of course, being reactions at the interflavanoid bond. Although this difference is obvious, the interflavanoid bond, particularly its lability to either acid- or base-catalyzed solvolysis (107, 144, 148, 152, 225), has not been given adequate consideration in many instances. Reactions of condensed tannins are sometimes incorrectly postulated to parallel those of the flavan-3-ols. It seems far more appropriate now to use dimeric proanthocyanidins as model compounds for study of the reactions of condensed tannins, because these compounds are easily synthesized and their spectral properties are well known. 

Brown and Shaw (37) propose an ether linkage for the condensed tannins of Calluna vulgaris on the basis that epicatechin-(4)ff)-phenyl sulfides can be obtained in good yield by refluxing the tannin with benzene thiol in ethanol-water with no added acid. This result can possibly be explained on the basis of the extreme lability of the l. i-cis- iA-trans procyanidins with a linkage (148), but 

Until recently, little attention has been given to the reactions of polymeric proanthocyanidins under alkaline conditions. It was known that solution of polymeric procyanidins in base caused a marked decrease in the reactivity of these polymers with aldehyde and that both carbonyl and relatively acidic functions were generated analogous to the rearrangement of catechin to catechinic acid (152, 335). 

Acknowledgements. I am indebted to Linda Korb who assisted in gathering reference materials, Cleo Lane who typed a number of drafts, and Drs. L.Y. Foo, J.J. Karchesy, G. W. McGraw, L. J. Porter, and D.G. Roux who have made helpful suggestions. 

1 Agrawal S, Misra K 1977 Leucoanthocyanidins from Ficus racemosa bark. Chem Scripta 12 37-39 

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