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

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.