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

1989; Issue: 4 Linguagem: Inglês

10.1039/p19890000671

2050-8255

Johann F. W. Burger, Jan P. Steynberg, Desmond A. Young, E. Vincent Brandt, Daneel Ferreira,

Carbohydrate Chemistry and Synthesis

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.