Synthesis of condensed tannins. Part 16. Stereochemical differentiation of the first ‘angular’(2S,3R)-profisetinidin tetraflavanoids from Rhus lancea(karree) and the varying dynamic behaviour of their derivatives

Abstract

Four novel ‘angular’ diastereoisomeric profisetinidin tetraflavanoids, shown to be constituted of (2S,3R)-(+)-fisetinidol and [4,6]-bi-[(2S,3R)-(+)-fisetinidol] substituents at C-6 and C-8 on the enantiomeric (2R,3S)-(+)-catechin moiety, occur in the heartwood of Rhus lancea. Structural and stereochemical differentiation was provided by synthesis of their tridecamethyl ether tetra-acetates. Our earlier allocation of a ‘linear’ arrangement of flavanyl units for one of the isomers is accordingly revised to an ‘angular’ assignment comprising [4,6: 4,8]-bi-[2,3-trans-3,4-cis-(+)-fisetinidol] and [4,6]-23 trans-3,4-trans-(+)-fisetinidol substituents on (+)-catechin. Its derivative [the revised structure (II)] exhibits ‘static’ and the remainder varying degrees of ‘dynamic’ behaviour in solution, relative con-figurations determining their relative rotational stabilities. The first ‘branched’ profisetinidin penta-flavanoid homologue resulted as synthetic byproduct.