NMR studies of some (1-->6)-linked disaccharide methyl glycosides

NMR studies have been performed on the methyl glycosides of some (1-->6)-linked disaccharides. Observed J5,6pro-R and J5,6pro-S values indicate that, for the 6-substituted D-gluco- and D-galacto-pyranosides, the rotamer distribution around the C-5-C-6 bond deviates somewhat from that observed for the respective unsubstituted monosaccharide glycosides. There is also a difference between 6-O-alpha-D- or 6-O-beta-L- on the one hand and 6-O-beta-D- or 6-O-alpha-L-substituted glycosides on the other, with somewhat larger values for J5,6pro-R for the latter two indicating a higher proportion of the gauche-trans conformer. The glycosylation shifts observed for the signals from the 6-protons in the glycosidic linkage were dependent on the type of anomeric and absolute configuration of the glycosyl group. NOE measurements by irradiation of the anomeric proton indicated that sugars 6-substituted with alpha-D- or beta-L-glycosyl groups have highly populated conformations in which H-1 and H-6pro-S are proximal, and for beta-D- and alpha-L-glycosyl groups conformations in which H-1 and H-6pro-R are proximal.