Synthesis of condensed tannins. Part 11. Intramolecular enantiomerism of the constituent units of tannins from the anacardiaceae: stoicheiometric control in direct synthesis: derivation of 1H nuclear magnetic resonance parameters applicable to higher oligomers

Abstract

Tannins from the heartwoods of Schinopsis spp. (quebracho) and Rhus spp. (karee) represent mutual condensation products of their associated precursors (2S,3R,4S)(–)-leucofisetinidin, (2R,3S)-(+)-catechin and, to a minor extent, (2R,3R)-(–)epicatechin. The unique enantiomeric relationship between the electrophile and two nucleophiles at C-2 is reflected both in the biflavanoid metabolites comprising four [4,6]- and [4,8]-(+)-fisetinidol-(+)-catechins and [4,8]-(+)-fisetinidol-(–)-epicatechin, and in the extension of the former group to four ‘angular’ triflavanoid [4,6:4,8]-bi-[(+)-fisetinidol]-(+)catechin diastereoisomers. Stoicheiometric control of in vitro condensation of the precursors provide similar oligomeric mixtures with selective or specific emphasis on either bi- or tri-flavanoids respectively. 1H N.m.r. coupling constants and chemical shift parameters derived from these compounds and their (–)-fisetinidol analogues are of potential diagnostic value at higher oligomeric levels.