Oligomeric flavanoids. Part 19. Reductive cleavage of the interflavanyl bond in proanthocyanidins
The interflavanyl bond in profisetinidins 1, 4 and 6, and methyl ethers 3, 5, 28 and 29 and procyanidins 24 and 26, and their methyl ethers 25 and 27 is readily subject to cleavage with sodium cyanoboranuide in trifluoroacetic acid at 0 °C. This method will contribute significantly to the structure elucidation of the 5-deoxy (A-ring) proanthocyanidins from important commercial sources. Boltzmann-averaged heterocyclic ring coupling constants as determined by a conformational global search routine (GMMX) and NOE difference spectroscopy were used to assign unequivocally the diastereotopic methylene protons in the 1H NMR spectra of flavan-3-ols, a prerequisite for corroboration of the cleavage mechanism.