NMR-SPECTRAL STUDIES OF 2-LINKED GLYCOSIDES - 2-O-GLYCOSYLATION SHIFTS OF 2-O-GLYCOSYLATED ALPHA-L-ARABINOPYRANOSIDES AND BETA-L-ARABINOPYRANOSIDES

Anomalous glycosylation shift values of the signals for C-1 (the anomeric carbon atom) and C-2 (the glycosyloxylated carbon atom) were sometimes observed for 2-O-β-d-glycopyranosyl (or -β-d-xylopyranosyl)-β-d-glucopyranosides, whereas no remarkable displacements of the other sugar-carbon signals were observed in these cases. This can be explained in terms of change of the orientation of the glycosyl linkages, owing to strong, steric interaction between the 2-O-glycosyl group and the 1-O-aglycon (or -sugar) group. Various 2-O-glycosylated α- and β-l-arabinopyranosides were synthesized. In the case of α-l-arabinopyranosides, the processes of 2-O-β-d-glucosylation, -β-d-xylosylation, and -α-l-arabinosylation resulted in unexpected, upfield shifts of the C-3, -4, and -5 signals, together with displacement of the C-1 and -2 resonances. Furthermore, significant alteration of the values of 3JH1,H2 and 1JC1,H1 was also observed for the 2-O-glycosylated α-l-arabinopyranoside moiety, indicating an increase in the contribution of the 1C4 conformation of the glycosylated α-l-arabinopyranoside in these cases. On the other hand, no remarkable variation in the signals of C-3, -4, and -5, or in the 3JH1,H2 and 1JC1,H1 values, was found for 2-O-α-l-rhamnosylation, except for 4-epihederagenin-3-yl 2-O-α-l-rhamnopyranosyl-α-l-arabinopyranoside. In the present study, such unusual 2-O-glycosylation shifts were not encountered for the 2-O-glycosylation of β-l-arabinopyranosides. © 1984.