A reversal in the order of H-6_<i>R</i> and H-6_<i>S</i> chemical shifts of some aldohexopyranose derivatives, associated with the acetylation of OH-4 and OH-6 groups. A distinction between 3- and 4-linked <scp>D</scp>-glucose residues in disaccharides

Abstract

On peracetylation of methyl α- or β-D-glucopyranoside, there is a reversal in the order of the chemical shifts of the 6,6′-methylene protons, i.e., whereas the H-6 S signal appears downfield of H-6 R in the spectra of the glucosides, their relative locations in the spectra of the tetraacetates are interchanged. Evidence is presented from comparative data for a wide range of regioselectively acetylated D-glucose derivatives, showing that the reversal in chemical shifts of the methylene protons occurs only when both O-4 and O-6 are acetylated. Consequently, in the disaccharide series, shift reversal is not observed with glucose residues that are bonded glycosidically through O-4, whereas the reversal occurs when the linkage is through O-3. A conformational model offered to account for these effects suggests that the 4- and 6-O-acetyl substituents are oriented by means of a weak, mutual, interaction in such a way that the magnetic anisotropy of the carbonyl group of the 6-O-acetyl substituent can induce selective deshielding of H-6 R . The comparable influences of O-benzoyl and O-(4-nitro)benzoyl substituents on chemical shift are consistent with this proposal. D-Mannopyranosides exhibit characteristics analogous to those of their D-gluco epimers, whereas D-galactopyranosides give a different, more complex, chemical shift pattern.