Stereochemical revision of brevipetins E and F: ellagitannin dimers with opposite hexahydroxydiphenoyl atropisomerism

Abstract

Brevipetins E and F, ellagitannin dimers from Cleidion brevipetiolatum, were originally reported to bear both a 3,4-(R_a)-hexahydroxydiphenoyl (HHDP)-glucopyranose and a 3,4-(R_a)-HHDP-rhamnopyranose moiety. However, a detailed reevaluation of the reported electronic circular dichroism (ECD) data, corroborated by DFT-based ¹H and ¹³C NMR chemical shift and ECD calculations, supports a stereochemical reassignment of these ellagitannins. Accordingly, the 3,4-HHDP-rhamnopyranose moiety in these compounds is reassigned to the (S_a)-atropisomeric configuration, in line with the observed reduction in Δε magnitudes relative to the constituent monomers.