Deoxy‐nitrosugars. 5 th Communication . The anomeric effect of the nitro group

The 1‐deoxy‐1‐nitro‐ D ‐ manno ‐pyranose 4 was transformed into the nitroolefin 5 and hence into the anomeric 1,2‐dideoxy‐1‐nitro‐3, 4, 6‐tri‐ O ‐benzyl‐ D ‐ arabino ‐hexopyranoses ( 3a and 3b ; cf . the Scheme ). Conformational analysis of 1‐benzyloxy‐2‐nitroethane ( 6 ) by 1 H‐NMR spectroscopy (Fig. 2) showed the synclinal conformation to be more stable than the antiperiplanar one by about 1.4 kcal/mol (attractive gauche ‐effect). This gauche ‐effect favours the 1‐deoxy‐1‐nitro‐2, 3, 4, 6‐tetra‐ O ‐benzyl‐β‐ D ‐ manno ‐hexopyranose ( 1b ) possessing an equatorial nitro group, which is, however, qualitatively the less stable anomer. The relative concentrations of the anomers of 1 and 3 , respectively, were determined by 1 H‐NMR spectroscopy after base catalyzed equilibration at 37° in CHCl 3 ‐solution (Table) . Anomeric effects for the nitro group of approximately 2.4 kcal/mol in 3 and of approximately 3.4 kcal/mol in 1 were calculated.