Synthesis of condensed tannins. Part 15. Structure of natural ‘angular’ profisetinidin tetraflavanoids: asymmetric induction during oligomeric synthesis

Abstract

Synthetic sequencing combined with the application of diagnostic chemical shifts of heterocyclic protons available from high-temperature n.m.r. spectroscopy permit the structural elucidation of the tridecamethyl ether tetra-acetates of four [4,6:4,8]-bi-(–)-fisetinidol-[4,6]-(–)-fisetinidol-(+)-catechin diastereoisomers derived from a fraction of the heartwood extract of Acacia mearnsii. In terms of the synthetic analogy these ‘angular’ profisetinidin tetraflavanoids arise from regiospecific 6-substitution on the ‘upper’[4,8]-(–)-fisetinidol substituent on the (+)-catechin moiety of the triflavanoid analogue. Asymmetric induction by the nucleophilic substrate apparently directs the final step in each biomimetic condensation.