Conformational preferences in glycosylamines. Implications for the exo-anomeric effect

The conformational preferences about the CN bond in N-(4-methoxyphenyl)-2,3,4,6-tetra-O-acetyl-α (1) and β-d-glucopyranosylamine (2), in the solid state and in solution, have been investigated. The crystal structure of the axially substituted α anomer (1) indicates a conformational preference about the C-1N bond in which nN→σ*CO exo-anomeric interactions may be expressed, although this conformational preference is not displayed in solution. The solution conformation relieves steric interactions that result from expression of the exo-anomeric effect in the solid-state conformation. The conformational preference in the equatorially substituted β anomer (2) both in solution and in the solid state is similar and permits expression of nN→σ*CO exo-anomeric interactions. The structural data for 1 and 2 indicate significant differences in O-5C-1N-1 bond angles but insignificant differences in each of the O-5C-1 or C-1N-1 bond lengths. The JC-1H-1 coupling constants in 1 and 2 indicate a greater coupling constant for the α anomer that is consistent with a dominant nO→σ*CH orbital interaction in the β anomer that weakens the C-1H-1 bond.