Synthesis of condensed tannins. Part 13. The first 2,3-trans-3,4-cis-procyanidins: sequence of units in a ‘trimer’ of mixed stereochemistry

Abstract

The condensation of (+)-leucocyanidin with (–)-epicatechin initiates a succession of substitutions leading mainly to the introduction of [4,8]-2,3-trans-3,4-trans-procyanidin units, but also to the incoporation of ‘terminal’ moieties which possess the hitherto unique 3,4-cis-procyanidin configuration. The bonding patterns in the products, and also the sequence of units in one of the ‘trimers’ of mixed stereochemistry, are resolved by ¹H n.m.r. spectroscopy through selective use of solvents at elevated temperatures.