Reactions of Nitro Sugars. XXVI. Analysis and Separation of Stereoisomers by Nuclear Magnetic Resonance Spectroscopy and Column Chromatography of Benzylidene Derivatives
Stereoisomeric methyl 4,6-O-benzylidene-3-deoxy-3-nitrohexopyranosides proved to be distinguishable by the chemical shifts of their benzylidene methine proton signals, and ratios of such isomers in mixtures could be estimated by signal integration. The method was applied to a reinvestigation of the epimeric distributions in which methyl 3-deoxy-3-nitro-α-D-hexopyranosides arise, firstly, in the nitromethane cyclization of 2-O-[(S)-formyl(methoxy)methyl]-(R)-glyceraldehyde and, secondly, in their base-catalyzed configurational equilibration via nitronates. Previous results were essentially confirmed as far as the preferred configurations are concerned but require some revision as to the relative abundance of less favored configurations. By preparative chromatography of benzylidene acetals and subsequent debenzylidenation, some nitrohexopyranosides of the a-D series were isolated for the first time in crystalline condition. Separation of isomers in the β-D series was improved by use of chromatography and by a new method involving differential reactivities of compounds toward alkali.