Rotameric behaviour of methoxy groups in some aldopyranoses

Abstract

Through the determination of the chemical shift increments for the ring protons in pyranoses at the geminal and vicinal positions of a methoxy grouping it is possible to obtain qualitative insight about the rotameric states of the latter. This is exemplified on methyl α- and β-D-glucopyranoside, 2-O-methyl-α- and -β-D-glucopyranose, 3-O-methyl-β-D-gulopyranose, 4-O-methyl-α- and β-L-arabinopyranose and methyl 2-O-methyl-α- and -β-glucopyranoside. It is stated that an H-5 axial proton is much less deshielded by an axial methyxo group at the anomeric position than by the hydroxyl function. a small shielding for protons involved in H(g+, g-,g-)CH3 or H(g-,g+,a)CH3 conformational fragments (δ-effect) seems to occur.