Oligomeric flavanoids. Part 5. Base-catalyzed C-ring isomerization of (+)-fisetinidol-(+)-catechin profisetinidins

Abstract

The (+)-fisetinidol-(+)-catechin profisetinidins are subject to similar base-catalyzed C-ring isomerizations to the quasi-enantiomeric (–)-fisetinidol-(+)-catechins. Whereas ‘upper’ 2,3-trans-3,4-trans-flavan-3-ol units are susceptible to stereospecific transformations, those species with a 2,3-trans-3,4-cis-configuration react more rapidly in a stereospecific manner and are furthermore subject to isomerization with concomitant interchange of resorcinol A- and pyrocatechol B-rings. These differences are explicable in terms of the effect of configuration at C-4 on both the rate of formation of intermediate B-ring quinone-methides and the recyclization step. This comparative study not only confirms the mechanism of A/B-ring interchange but also reveals serious shortcomings in the c.d. method for defining absolute configuration of phlobatannins with cis-trans- and all-trans configuration of their C-rings.