Conformationally Flexible Ellagitannins: Conformational Analysis of Davidiin and Punicafolin via DFT Calculation of 1H NMR Coupling Constants

Many ellagitannins with various conformations of their glucose moiety have been isolated from natural sources. Here, a conformational analysis was performed via the density functional theory calculation of 1H NMR coupling constants. It was observed that, in the solution state, davidiin exists as an equilibrium mixture of the BO,3 (boat) and 1C4 (chair) conformational states, while punicafolin is an equilibrium mixture of the 3S1 (skew-boat) and 1C4 conformational states. Their equilibrium states changed depending on the solvent and temperature. Such conformational flexibility may be important for the biosynthesis of ellagitannins with diverse structures.