Structural Variations in Proanthocyanidins and Their Derivatives

TLDR: The structure complexity of condensed tannins (polymeric proanthocyanidins) is centered principally on variations in hydroxylation patterns of the flavan chain extender units, the stereochemistry at the three chiral centers of the heterocyclic ring, the location and type of interflavanoid bond, and the structure of the terminal unit as discussed by the authors.

Abstract: Structural complexity in the condensed tannins (polymeric proanthocyanidins) is centered principally on variations in hydroxylation patterns of the flavan chain extender units, the stereochemistry at the three chiral centers of the heterocyclic ring, the location and type of interflavanoid bond, and the structure of the terminal unit. Superimposed on these basic structural features are derivatizations such as O-methylation, C- and O-glycosylation, and O-galloylation. Because of the strong nucleophilicity of the resorcinolic or phloroglucinolic A-rings, some unusual conjugates linked through the C-6 or C-8 positions have also been found. A further level of structural diversity results from facile rearrangements of these compounds. Although great progress has been made in elucidation of the structure of oligomeric proanthocyanidins, our knowledge of the structure of higher polymers rests primarily on interpretations of their 13C-NMR spectra.