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

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Abstract
Both 1H and 13C n.m.r. studies and conformational analyses have been performed on methyl α-D-galactopyranosides substituted in the 3-, 4-, and 3,4-positions with different anomeric forms of either L-fucopyranosyl or D-glucopyranosyl groups. Conformational analysis using the HSEA and GESA approaches indicated restricted rotational freedom around the glycosidic bonds of the trisaccharides compared to those of the disaccharides. It also indicated a number of proton-oxygen and proton-proton interactions which could be correlated to downfield and upfield glycosylation shifts, respectively, of the proton signals in both the di- and the tri-saccharides. Similar inter-residue atomic interactions could also be correlated to the 13C n.m.r. glycosylation shifts observed for the disaccharides and to some extent for the trisaccharides. 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 for the signals for many linkage carbons.

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