Conformational isomerism of phenolic procyanidins: Preferred conformations in organic solvents and water

NMR studies of catechin-(4α→8)-epicatechin (1) and catechin-(4α→8)-catechin (2) provided complete assignment of the proton and carbon resonances for both the more extended and compact conformers in the free phenolic form. When 1 is in organic solvents, the more extended rotamer is preferred over the more compact rotamer (10:7), but in water, the compact rotamer dominates (2:10). When 2 is in organic solvents, the more compact rotamer is slightly preferred (8:10), but in water, only trace amounts of the more extended rotamer are detected. NOE experiments show rotational conformation exchange despite the fact that two distinct and sharp sets of signals are seen for each rotamer. The upper unit heterocyclic ring exists in an approximate half-chair conformation in each rotamer for both dimers. However, coupling constants of the lower unit heterocycles show substantial axial orientation of the B-ring. Lineshape analysis of the terminal unit H-3_F excludes E- and A-conformational interchange. These results and NOE experiments show a skewed-boat conformation for the terminal unit in 1 and between a half-chair and skewed-boat conformation for the terminal unit in 2.