Reactions of flavanoids and condensed tannins with sulphur nucleophiles

Abstract

Reactions of benzenethiol and arenesulphinic acids with flavan-4-ols under acidic conditions are facilitated by 5-, 7-, and 4′-methoxy-groups, and 5,7-dimethoxyflavan-4β-ol (V) reacts quickly with benzenethiol in the absence of added acid. All products are 4α-thio-substituted flavans. 2,3-trans-Flavan-3,4-diols react less quickly than the corresponding 4-ols and the resulting 4-thio-substituted flavan-3-ols have mixed stereochemistry (3,4-cis-and trans-). Flavan-4-yl sulphones are less labile than 4-thioethers under acidic conditions but their lability is increased by a 7-methoxy-group; the products of the acid-catalysed exchange of thio-groups at C-4 have 4α-stereochemistry.

Calluna vulgaris tannin reacts with benzenethiol in the absence of acid to give, after methylation, (+)-2,3-cis-3,4-trans-3′,4′,5,7-tetramethoxy-4-phenylthioflavan-3-ol (XXVI). On the other hand, Picea pungens tannin gives the same product with benzenethiol only in the presence of acid, and (–)-tetra-O-methylcatechin (XXVII) is produced as well.