The four anomeric methyl glycosides of the trisaccharide D -G l c p -( l ~2 ) -D -G l c p -( l ~3 ) -a -D -G l c p have been synthesized and used for l H and 13C N M R studies. All "H and 13C N M R 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.NMR and conformational studies have previously been performed on a number of 2,3-' and 3,4-branched2*, trisaccharides with the purpose of studying models for vicinally disubstituted branchpoint regions in oligo-and poly-saccharides. The glycosylation shifts in the trisaccharides were shown to deviate from the sum of the glycosylation shifts of the constituent disaccharides, and for many of the signals from atoms at or near a linkage large deviations were observed. This was assumed to be due to atomic interactions between sugar residues, causing displaced energy minima with changed values for torsional angles, and restricted rotational freedom around the glycosidic bonds. As deviations from additivity of glycosylation shifts were observed for one type of vicinally disubstituted residues, and an understanding of the phenomenon was desired, studies on other related trisaccharides were implied. We now report synthesis of, and NMR and conformational studies on, the four anomers of the trisaccharide D-GIcP-(1-+2)-~-Glcp-( 1-3)-a-~-Glcp-OMe. The aim of the study is to investigate whether additivity of NMR glycosylation shifts holds at a 1,2-linked residue, which is not a branched residue but nevertheless vicinally disubstituted. This would help in the prediction of NMR spectra of 1,2-linked residues, and of neighbouring residues, as performed by the computer program CASPER4*' used for structural studies of oligo-and polysaccharides. The glycosylation shifts are needed in the computer analysis.
ExperimentalMaterials.-The structure of trisaccharides 1-4 are given in Table 1. The syntheses of the intermediates 5,6 7,' 11,8 13 and 14,9 18," 22 and 2412 have been reported. General Methods.-Concentrations were performed under reduced pressure at temperatures < 40 "C (bath). Optical rotations were measured at 22 "C for solutions in chloroform or water (c = 1.0-1.7 mg cm3) with a Perkin-Elmer 241 polarimeter. Silver trifluoromethanesulphonate-mediated couplings were performed essentially as described.' 3-'6 Data on coupling reactions and deprotection procedures, together with physical con...
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.