Conformationally flexible ellagitannins: conformational analysis of davidiin and punicafolin by DFT-based 1H–1H coupling constant calculations

Abstract

Many ellagitannins with various conformations of their glucose moiety have been isolated from natural plant sources. Herein, we performed a conformational analysis using density functional theory (DFT) calculations of ¹H–¹H coupling constants. In solution, davidiin was found to exist as an equilibrium mixture of the B_O,3 (boat) and ¹C₄ (chair) conformations, whereas punicafolin is an equilibrium mixture of the ³S₁ (skew-boat) and ¹C₄ conformations. Their conformational equilibria vary depending on solvent and temperature. Such conformational flexibility may be important for the biosynthesis of ellagitannins with diverse structures.