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

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Abstract
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 1H‐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 1H‐NMR spectroscopy after base catalyzed equilibration at 37° in CHCl3‐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.