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Title:	Kinetics of acid-catalyzed cleavage of procyanindins
Author(s):	Hemingway, Richard W.; McGraw, Gerald W.
Date:	1983
Source:	Journal of Wood Chemistry and Technology 3(4):421-435
Description:	Comparison 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-cis procyanidina 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 B1 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 condemed tannins are interpreted in light of these results.
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