The diastereoisomers of 1,2- O-isopropylidene-3,5- O-(methoxymethylidene)-6- O- p-tolylsulfonyl-α- d-glucofuranose have been prepared by treatment of 1,2- O-isopropylidene-6- O- p-tolylsulfonyl-α- d-glucofuranose with trimethyl orthoformate in acidified N,N-dimethylformamide. Treatment of an equilibrated mixture of these diastereoisomers with acetone gave 1,2:3,5-di- O-isopropylidene-6- O- p-tolylsulfonyl-α- d-glucofuranose and methyl formate. Equilibration reactions of the diastereo-isomers in various solvents, at various temperatures, have been studied quantitatively by n.m.r. spectroscopy. The resulting thermodynamic data allow a decision as to the diastereoisomer that is stabilized by intramolecular dipole—dipole interactions, which, when used in conjunction with conformational evidence from vicinal coupling-constants obtained by a computed-spectrum analysis indicated the configuration of the new asymmetric carbon atom. The configurational assignments made for these diastereoisomers were supported by chemical-shift data and by the observation of a spin-coupling over five bonds. Evidence for a favored orientation of the methoxyl group in one of the diastereoisomers was obtained from nuclear Overhauser experiments in which a 25% enhancement of the signal intensity of the neighboring methylidene proton was observed. Van der Waals interaction energies have been calculated for the conformations assigned to the diastereoisomers and for the possible conformations of the closely related 1,2:3,5-di- O-benzylidene-α- d-glucofuranose derivatives. The nonbonded interactions in these conformations are discussed and compared.