Configurational and conformational influences on the carbon-13 chemical shifts of some carbohydrates

Abstract

Carbon-13 chemical shifts for sugars and methyl glycosides are reported. The following general effects have been observed: (a) a comparison of chemical shifts with expected electron densities suggests that 13C-2 and -4 experience relatively stronger shielding than do 13C-1, -3, and -5, possibly because they oppose the ring O; (b) inversion of an equatorial O to axial O is associated with increased shielding of the 13C nucleus to which it is bonded, of adjacent 13C nuclei, and of those in the β-position (i.e., having an opposing axial H or O); (c) removal of a gauche O,O interaction is associated with 13C deshielding. It appears that since these same steric relationships affect stability, the overall shielding states of 13C nuclei of the different isomers reflect variations in the magnitude of repulsive interactions in the molecules; on this basis, the anomeric effect does not arise through instability of the equatorial anomeric C—O bond. A destabilizing interaction appears to cause polarization of most C—H bonds in a molecule, 13C becoming more shielded, and 1H less shielded, suggestive of a concerted means for delocalizing the instability. In comparing anomers or epimers, this polarization is evident as a reordering of the shielding state of different nuclei, e.g., for several sugars and their glycosides increased shielding of 13C-1 is accompanied by deshielding of the bonded H and increased shielding of the appended O—H or methoxyl 13C nucleus.