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 J(5,6pro-R) and J(5,6pro-S) values indicate that, for the 6-substituted D-gluco- and D-galacto-pyranosid es, the rotamer distribution around the C-5-C-6 bond deviates somewhat from that observed for the respective unsubstituted monosaccharide gl ycosides. 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 glycosid es on the other, with somewhat larger values for J(5,6pro-R) far the l atter 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 irra diation of the anomeric proton indicated that sugars 6-substituted wit h alpha-D- or P-L-glycosyl groups have highly populated conformations in which H-l and H-6 pro-S are proximal, and for beta-D- and alpha-L-g lycosyl groups conformations in which H-1 and H-6 pro-R are proximal.