Synthesis and NMR and conformational studies of the four anomeric methyl glycosides of the trisaccharide D-Glcp-(1→2)-D-Glcp-(1→3)-α-D-Glcp

Abstract

The four anomeric methyl glycosides of the trisaccharide D-Glcp-(1→2)-D-Glcp-(1→3)-α-D-Glcp have been synthesized and used for ¹H and ¹³C NMR studies. All ¹H and ¹³C NMR resonances were assigned and comparison was made between the observed glycosylation shifts, i.e. the differences between chemical shifts for signals from the trisaccharides and those of the respective monomers, and those derived by addition of the glycosylation shifts for each disaccharide element. With a few exceptions, only minor deviations were found and the differences were mostly confined to signals from linkage carbons and the attached protons. Conformational analysis was performed using the HSEA and GESA approaches and measurements of the nuclear Overhauser enhancements. The results indicate that minimum energy conformations are similar to those of the disaccharides but that rotational freedom around some of the glycosidic bonds is restricted.