Kinetics of Acid-Catalyzed Cleavage of Procyanidins

Abstract

Comparisons of the rates of cleavage of isomeric procyanidin dimers in the presence of excess phenylmethane thiol and acetic acid showed that compounds with a C(4)–C(8) interflavanoid bond were cleaved more rapidly than their C(4)–C(6) linked isomers, that 2,3-cis isomers with an axial flavan substituent were cleaved more rapidly than a 2,3-trans isomer with an equatorial substituent, and that cleavage rate was independent of the stereochemistry in the terminal unit. Cleavage rate constants followed Arrhenius temperature dependence. The C(4)–C(8) interflavanoid bond in 2,3-cls procyanidins should be labile to cleavage at 20°C over a pH range of 3.6 to 5.4. Reaction of loblolly pine bark tannins in the presence of excess (+)-catechin with acetic acid rapidly produced oligomeric procyanidins. The ratio of the procyanidin Bl to B7 decreased from 2.9:1 after 4 hours to 1.3:1 after 50–100 hours at 90°C. The structure and reactions of condensed tannins are interpreted in light of these results.