SYNTHESES AND NMR-STUDIES OF PYRUVIC-ACID 4,6-ACETALS OF SOME METHYL HEXOPYRANOSIDES

The pyruvic acid 4,6-acetals of the methyl glycosides of alpha- and beta-D-glucose, alpha- and beta-D-mannose, and alpha- and beta-D-galactose which correspond to naturally occurring residues in polysaccharides, have been synthesized and their H-1 and C-13 NMR spectra fully assigned. The acetalation was accomplished by reacting the 4,6-di-0-trimethylsilyl-derivative with ethyl pyruvate under trimethylsilyl-trifluoromethanesulfonate promotion. In most cases a mixture of the R- and the S-isomer of the acetal was obtained, which could be separated by silica gel chromatography. Spectra from some compounds with the unnatural acetal, i.e., the one with an axial methyl group, were also assigned. For compounds corresponding to the natural residues significant H-1 NMR chemical shift changes (>0.05 ppm) were obtained for signals from protons at different positions of the ring depending on the compound in question. The largest changes were obtained for the linkage positions, up to approximately 0.5 ppm. Large differences between glucose and mannose on the one hand and galactose on the other, were observed. For C-13 NMR signals large downfield shifts were observed for signals from C-4 and C-6, up to 6 ppm, and large upfield shifts were observed for signals from C-3 and C-5 with shifts of up to 10 ppm for the latter. The values will be used to extend the monosaccharide database in the computer program CASPER, with which computer-assisted structural analysis of oligo- and poly-saccharides can be accomplished.