STRUCTURE AND REACTIONS OF AMINO-HEPTOSEPTANOSIDES AND NITRO-HEPTOSEPTANOSIDES OBTAINED BY CYCLIZATION OF DIALDEHYDES WITH NITROMETHANE

Base-catalyzed addition of nitromethane to the 2-ethyl hemialdal 3, resulting from the reversible addition of ethanol to the dialdehyde 2 obtained from NaIO4 oxidation of methyl 4,6-O-benzylidene-alpha-D-glucopyranoside (1) in aqueous solution, yields four main isomeric 3-nitroheptoseptanosides having respectively the D-glycero-D-manno (7.42%), D-glycero-D-ido (5, 36%), D-glycero-D-galacto (6, 12%) and D-glycero-D-talo (9, < 10%) configurations. Compound 7 equilibrated slowly in pyridine with the D-glycero-D-altro isomer 11, whereas, under similar conditions, 6 underwent a fast epimerization to the D-glycero-D-gulo isomer (10). Equilibration of 5 in the presence of an excess of sodium methoxide afforded a 3:2 mixture of 9 and 7, whereas conversely action of a catalytic proportion of the same base on 9 produced 5. The configuration at C-2-C-4 for the series of nitroheptoseptanosides have been assigned by conversion into the corresponding methyl 3-acetamido-3-deoxyheptopyranosides and then into known 3-amino-3-deoxyhexose derivatives. Tentative correlations have been drawn between the stereochemistry in the transition state for the addition of the nitroalkyl carbanion, the conformation of the resulting seven-membered ring and its nitronate, and the relative distribution and stability of epimeric nitroheptoseptanosides obtained in the reaction. Conformational preferences for nitroheptoseptanosides, as inferred from H-1-n.m.r. data, are discussed. Comparative NaIO4 oxidations were performed on methyl 4,6-O-benzylidene-beta-D-glucopyranoside (43) resulting in the formation of the corresponding C-2-C-3 dialdehyde 44 isolated as its 2-ethyl hemialdal 45. Base-catalyzed nitromethane addition to 45 resulted in the formation of at least three isomeric nitroheptoseptanosides with respective configurations D-glycero-D-altro, D-glycero-D-gluco, and D-glycero-D-galacto which were identified mostly by comparison of their C-13-n.m.r. parameters with compounds obtained in the alpha-D series.