STRUCTURE AND SYNTHESIS OF PHLOBATANNINS RELATED TO (-)-FISETINIDOL-(-)-EPICATECHIN PROFISETINIDINS

Article abstract of DOI:10.1016/0031-9422(90)87119-F

Several members of the class of natural 'phlobaphene' condensed tannins, representing the products of C-ring isomerization of (-)-fisetinidol-(4α,8)-(-)-epicatechin profisetinidins have been characterized.These comprise four functionalized tetrahydropyrano<2,3-h>chromenes, two <2,3-g>-analogues and four <2,3-f>-regioisomers.A novel protocol of effecting differentiation between the regio-isomers based on homonuclear NOE difference spectroscopy (1H NMR) is proposed.The structures of some of the natural products were confirmed by synthesis via base-catalysed rearrangement of their presumed precursors.Under these mild alkaline conditions, the biflavanoids are susceptible to epimerization at C-2 (F-ring) hence leading to conversion of a (-)-epicatechin to a (-)-catechin DEF moiety.The natural occurence of pyran rearranged analogues related to both these units presumably reflects similar mechanisms for the in vivo and in vitro processes.