Preparation of α and β anomers of various isomeric methyl O- d-galactopyranosyl- d-galactopyranosides. Standards for interpretation of 13C-n.m.r. spectra of d-galactopyranans

Abstract

The four isomers of methyl O-β- d-galactopyranosyl-β- d-galactopyranoside were prepared by condensation of 2,3,4,6-tetra- O-acetyl-α- d-galactopyranosyl bromide with appropriate, partially O-substituted derivatives of methyl β- d-galactopyranoside. Reaction of 3,4,6-tri- O-acetyl-1,2- O-(1-ethoxyethylidene)-α- d-galactopyranose with the same acceptors, in the presence of mercuric bromide, led to the formation of α and β linkages. Thus, it was possible to assign 13C-n.m.r. resonances of α and β anomers of methyl O- d-galactopyranosyl-β- d-galactopyranosides. In terms of application of these shift values and those of related d-galactobioses to the structural analysis of d-galactopyranans by shift comparisons, some generalizations can be made. For β- d-galactopyranans, the resonances of glycosyloxylated carbon atoms of methyl O-β- d-galactopyranosyl-β- d-galactopyranosides are sensitive to structure and appear to have typical values, whereas limited variation was observed with shifts of C-1′ signals. On the other hand, for assigning structures to d-galactopyranans containing α linkages, the C-1′ shifts (at higher field) of methyl O-α- d-galactopyranosyl-β- d-galactopyranosides are sensitive to linkage position, whereas those of glycosyloxylated carbon atoms vary only a little.