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-alpha (1) and beta -D-glucopyranosylamine (2), in the solid state and in solution, have been investigated. The crystal structure of the axially substituted alpha anomer (1) indicates a conformational preference about the C-1-N bond in which n(N) --> sigma (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 beta anomer (2) both in solution and in the solid state is similar and permits expression of n(N) --> sigma (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 J(C-1-H-1) coupling constants in 1 and 2 indicate a greater coupling constant for the alpha anomer that is consistent with a dominant n(O) --> sigma (C-H)* orbital interaction in the beta anomer that weakens the C-1-H-1 bond. (C) 2001 Elsevier Science Ltd. All rights reserved.