Condensed tannins. Base-catalysed reactions of polymeric procyanidins with phloroglucinol: intramolecular rearrangements

Abstract

Reactions of polymeric procyanidins with phloroglucinol at pH 12.0 and temperatures of 23 or 50 °C gave epicatechin-(4β)-phloroglucinol (7), by cleavage of the interflavanoid bond between procyanidin units with subsequent addition of phloroglucinol, and (+)-catechin from the terminal unit. The phloroglucinol adduct (7) rearranged to an enolic form of 8-(3,4-dihydroxyphenyl)-7-hydroxy-6(2,4,6-trihydroxyphenyl)bicyclo[3.3.1]nonane-2,4,9-trione (9). Rearrangement of a dimeric procyanidin phloroglucinol adduct resulted in the formation of 3′-{8-(3,4-dihydroxyphenyl)-7-hydroxy-2,4,9-trioxobicyclo[3.3.1 ]nonan-6-yl}-4-(3,4-dihydroxyphenyl)-2′,3,4′,5,6′,7-hexahydroxyflavan (10), also in an enolic form. (+)-Catechin, from the terminal unit, gave catechinic acid, an enolic form of 6-(3,4-dihydroxyphenyl)-7-hydroxybicyclo[3.3.1]nonane-2,4,9-trione (4).