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

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