Herbert Baumann

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

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.

NMR spectroscopy of hydroxy protons of 3,4-disubstituted methyl α-D-galactopyranosides in aqueous solution

The 1H NMR chemical shifts, coupling constants, temperature coefficients, exchange rates, inter-residue NOEs, and the deuterium isotope effects on 13C chemical shifts have been measured, in aqueous solution, for the hydroxy protons of two 3,4-disubstituted galactopyranosides, β-L-Fucp-(1→3)-[α-L-Fucp-(1→4)]-α-D-Galp-OMe and α-D-Glcp-(1→3)-[β-D-Glcp-(1→4)]-α-D-Galp-OMe, and their constituent monomeric methyl glycosides. All the hydroxy proton resonances could be assigned and for both trisaccharides the data indicated hydrogen bonding interactions between the hydroxy groups at the C-2 position of the two non-reducing sugars. Using the hydroxy protons, the number of inter-residue NOEs for the two trisaccharides was increased from 4 to 6 and from 4 to 7, respectively, relative to those obtained in D2O solutions. These results show that the NMR data obtained from hydroxy protons can provide important information in terms of hydrogen bonding interactions and inter-residue NOEs, which could be further used in structural and conformational analysis to improve the characterisation of oligosaccharides in aqueous solution.

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

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.

N.m.r. and conformational studies on some 1,3-linked disaccharides

13 C and 1H N.m.r. data have been obtained from 10 methyl 6-deoxyhexosylhexosides in which the 6-deoxysugar is the L- or D-enantiomer of fucose or rhamnose. A large variation in the magnitude of displacement shifts of signals for α- and β-carbons is observed. For all compounds Hard Sphere Exo Anomeric effect (HSEA) calculations have been performed to estimate favoured conformations. From the derived minimum energy conformations, a number of 1H n.m.r. chemical shifts can be rationalised.

Synthesis and nuclear magnetic resonance studies of some L-fucosyl-containing disaccharides

Eleven disaccharides containing an L-fucosyl group have been synthesized and their 1H and 13C NMR spectra assigned. HSEA analysis has indicated which inter-residue atomic interactions are present in the disaccharides, some of which could be correlated with observed glycosylation shifts. The results give further knowledge about glycosylation shifts, valuable for the interpretation of NMR spectra of larger saccharides.

Synthesis of some 3-O, 4-O, and 3,4-di-O-glycosyl-substituted methyl α-D-galactopyranosides

Seven disaccharide and eight trisaccharide methyl glycosides required for n.m.r. and conformational studies have been synthesized. They are all derivatives of methyl α-D-galactopyranoside which has been 3-O-, 4-O-, or 3,4-di-O-glycosylated with either L-fuco- or D-gluco-pyranosyl groups. All anomeric forms were synthesized, either via thioglycosides and methyl triflate promotion or via glycosyl bromides and silver triflate promotion.