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

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.