Structures and Biogenesis of Rhoipteleanins, Ellagitannins Formed by Stereospecific Intermolecular C-C Oxidative Coupling, Isolated from Rhoiptelea chiliantha

Abstract

Seven novel ellagitannins named rhoipteleanin A-G were isolated from the fruits of Rhoiptelea chiliantha and their structures were unequivocally established on the basis of spectroscopic and chemical evidence. In the molecules of rhoipteleanin A-F, (S, S)-flavogallonyl esters spanned two glucopyranose moieties; hence, these tannins represent the first dimeric ellagitannins generated by stereospecific intermolecular C-C oxidative coupling between the galloyl and hexahydroxydiphenoyl groups. The change in the ¹H-NMR chemical shifts of specific proton signals of 1(β)-O-galloylpedunculagin, the biogenetic precursor of rhoipteleanin A, in dueterium oxide at various concentrations suggested that the stereochemically regulated hydrophobic interaction between two molecules of the precursor restricts the intermolecular C-C coupling to S-biphenyl bond formation.