Experimental and DFT 1H NMR study of conformational equilibria in trans-4',7-dihydroxyisoflavan-4-ol and trans-isoflavan-4-ol

The solution-state conformational equilibria of trans-4',7-dihydroxyisoflavan-4-ol (1) and trans-isoflavan-4-ol (2) were assessed based on the temperature dependence of their vicinal coupling constants J(H)(-)(2)(alpha)(,H)(-)(3) and J(H-3,H)(-)(4) in comparison to values calculated with density functional theory (DFT) methods at the B3LYP/cc-pVTZ//B3LYP/6-31G(d,p) level of theory. For each half-chair conformer, several rotamers with respect to the C-4 hydroxyl and C-3 phenyl were calculated and the overall diequatorial-to-diaxial ratio at 298 K was assessed as 66:34 for 1 and 73:27 for 2. The syntheses of 1 and 2 are described.