13C-NMR spectroscopy of biflavanoids

The 13C-NMR spectra of nine naturally occurring CC linked biflavanoids have been assigned. The signals for the carbon atoms I-6, I-8, II-6, and II-8 appear in the region 90.0 ppm to 105.0 ppm. On the basis of the chemical shifts of these signals and their multiplicities in the off-resonance spectra, it is possible to determine the interflavonoid linkage in biflavanoids, provided that the A ring is involved. The level of oxidation of the ring C can be readily determined by a consideration of the chemical shift value of the carbonyl resonances. The position of the methoxyl substitution can also be inferred.