Condensed tannins: Base-catalysed reactions of polymeric procyanidins with toluene-α-thiol. Lability of the interflavanoid bond and pyran ring.

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Reaction of polymeric procyanidins (condensed tannins) with toluene-α-thiol at pH 12.0 and 23°C gave predominantly one stereoisomer of 1,3-bisbenzylthio-1-(3,4-dihydroxyphenyl)-3-(2,4,6- trihydroxyphenyl)propan-2-ol (10) by stereoselective reaction at C-4 and C-2 of the quinone methide derived from the upper 2,3-cis procyanidin units. Smaller amounts of two isomers of 1-benzylthio-1-(3,4-dihydroxyphenyl)-3-(2,4,6- trihydroxyphenyl)propan-2-ol (4) were obtained by reaction at the C-2 of catechin obtained from the terminal units. At higher temperatures, (10) loses toluene-α-thiol preferentially from C-1 to give 1-benzylthio-1-(3,4- dihydroxyphenyl)-3-(2,4,6-trihydroxyphenyl)-propan-2-one (11) by a tautomeric rearrangement of the quinone methide via an enol to the ketone. Loss of toluene-α-thiol from (4) gave 1-(3,4-dihydroxyphenyl)-3-(2,4,6- trihydroxyphenyl)propan-2-one (5) by a similar rearrangement. This compound further rearranges to 1-(3,4-dihydroxyphenyl)-4,6-dihydroxyindan-2-one (6).

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Journal of the Chemical Society, Perkin Transactions 1