1,3-Dicarbonyl sugar derivatives from sugar nitro-olefins

The sugar nitro-olefins 5–9, easily available from the aldehydo-sugars 1–4 and simple nitroalkanes, undergo a slow transformation, by treatment in DMF with a mixture of tetrabutylammonium hydrogensulfate (2 mol) and potassium fluoride (4 mol) at room temperature, to give the corresponding enol ethers 10–14, in 10–71% yields, as E/Z mixtures, with the exception of 12, which is configurationally homogeneous. The spectroscopic properties of the enol ethers 10–14 are in agreement with their structures. A proof of the potential usefulness of these compounds as equivalent of sugar 1,3-dicarbonyl compounds is the reaction of 12 with hydrazine hydrate, which affords the pyrazole derivative 15 (53%). The formation of 10–14 from 5–9 may be rationalised in terms of a vinylogous Nef-type reaction. Jacobson conditions, as applied to the nitroalkene 7, led to the dimethyl acetal 16 (47%) as a E/Z mixture, which was transformed into the pyrazole derivative 15 in poor yield (14%). This method is therefore less convenient for 7 than the one described here, which seems to be of wider applicability in the carbohydrate field.