Configurational Equilibria in Methyl 3,6-Dideoxy-3-nitro-α-L-hexopyranosides

Abstract

Epimerizations in alkaline medium, by which the methyl 3,6-dideoxy-3-nitrohexopyranosides having the α-L-gluco (1), α-L-galacto (2), α-L-manno (3), and α-L-talo (4) configurations are mutually interconverted, were investigated quantitatively by n.m.r. spectroscopy and by preparative separation of products. The epimeric equilibrium compositions obtained depend on whether an equimolar or a catalytic amount of alkali is supplied. In the first case they are governed by the thermodynamic stabilities of the nitronate anions [Formula: see text], and in the second case, by those of the free nitro glycosides [Formula: see text]. The results are discussed in terms of conformational analysis, and a value of 2 kcal/mol is found for the contribution of the A(1,3) interaction to the conformational free energy in the nitronates. The A(1,3) effect strongly influences not only the product composition at the nitronate equilibrium but also the pathway of epimerization.