NMR AND CONFORMATIONAL-ANALYSIS OF SOME 2,3-DISUBSTITUTED METHYL ALPHA-L-RHAMNOPYRANOSIDES

Conformational studies of the branched trisaccharide glycosides X-(1 --> 2)[Y-(1 --> 3)]-alpha-L-Rha-OMe (where X and Y are residues of alpha-L-, beta-L-, alpha-D-, and beta-D-hexopyranoses) were based on H-1- and C-13-n.m.r. data (n.O.e.'s, C-13 chemical shifts) and theoretical calculations. In the majority of the trisaccharide glycosides, there is insignificant restriction of rotation around the glycosidic linkages in the disaccharide units as compared to the corresponding disaccharide glycosides X-(1 --> 2)-alpha-L-Rha-OMe and Y-(1 --> 3)-alpha-L-Rha-OMe. Differences in the conformations observed for several compounds resulted in changes of the n.O.e. patterns and in deviations from additivity of glycosylation effects in the C-13-n.m.r. spectra.