Synthesis, n.m.r., and conformational studies of some 3,4-di-O-glycopyranosyl- substituted methyl α-D-galactopyranosides

Abstract

Methyl α-D-galactopyranosides substituted in the 3- and 4-positions with one L-fucopyranosyl and one D-glucopyranosyl group have been synthesized, with all anomeric combinations. The trisaccharides were used for 1H and 13C n.m.r. studies. All 1H and 13C n.m.r, resonances were assigned and comparison was made between the observed glycosylation shifts of the trisaccharides and those calculated by using the glycosylation shifts for each disaccharide element. Large deviations, in most cases upfield shifts, were found in the 13C n.m.r. spectra for signals from some of the linkage carbons. Conformational analysis has been performed using the HSEA and GESA approaches. This analysis indicated restricted rotational freedom around the glycosidic bonds of the trisaccharides relative to those of the disaccharides. A number of proton-oxygen and proton-proton interactions in the trisaccharides was indicated which could be correlated to downfield and upfield shifts, respectively, of the proton signals.