Base-catalyzed addition of nitromethane to the 2-ethyl hemialdal 3, resulting from the reversible addition of ethanol to the dialdehyde 2 obtained from NaIO 4 oxidation of methyl 4,6- O-benzylidene-α- 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 catalytic proportion of the same base on 9 produced 5. The configuration at C-2C-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 transiion 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 1H-n.m.r. data, are discussed. Comparative NaIO 4 oxidations were performed on methyl 4,6,- O-benzylidene-β- D -glucopyranoside ( 43) resulting in the formation of the corresponding C-2C-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 13C-n.m.r. parameters with compounds obtained in the α- D series.