Conformation and Complexation of Tannins: NMR Spectra and Molecular Search Modeling of Flavan-3-ols

Studies of flavan-3-ols in their biologically significant phenolic form show that both H-6 and C-6 resonances are downfield from H-8 and C-8. Therefore, assignments for the H atoms of the A-ring are inverse to those commonly reported. By contrast, in the methyl ether and methyl ether acetate derivatives, both H-8 and C-8 are downfield from H-6 and C-6 and assignments commonly reported for C-6 and C-8 are incorrect. The assignments commonly reported for the peracetate derivatives are correct. In contrast to results observed for dimeric flavans, solvent effects on chemical shifts and coupling constants are small in monomeric flavan derivatives. The small heterocyclic ring J2,3 coupling constants of 2,3-cis-flavans can be defined by lineshape analysis of H-3. These results provide the first evidence for the conformations of the common 2,3-cis-flavans in solution. With the exception of compounds carrying a bulky acetate at C-3, a GMMX global search protocol provides reasonable predictions of heterocyclic ring coupling constants.