Anomeric sugar configuration of anthocyanin O‐pyranosides determined from heteronuclear one‐bond coupling constants

Using the nuclear magnetic resonance technique, two-dimensional inverse heteronuclear shift correlation through multiple quantum coherence (HMQC), the one-bond coupling constants between the carbon atoms and protons (1J(C, H)) have been measured for a number of anthocyanins represented by 6-hydroxydelphinidin and all the common aglycones (except peonidin) and monosaccharides found as building blocks of anthocyanins. The large anomeric 1J(C, H) values of rhamnopyranosyls (168-170 Hz) show that these protons are in equatorial positions while the smaller 1J(C, H) values of the other anomeric protons (162-165 Hz) correspond to axial positions, thus assigning the anomeric configurations of the examined anthocyanins to be β for the gluco-, galacto- and xylopyranosyls and α for the rhamno- and arabinopyranosyls. In the pentoses, xylo- and arabinopyranosyls, it was found that the 1J(C, H) couplings for the equatorial H-5 protons (149-150 Hz) are larger than for the corresponding axial H-5 protons (140-143 Hz).