3, 4-Di (2, 3, 4, 6-tetra-O-acetyl-β-d-mannopyranosyl)-1, 2, 5-oxadiazole 2-oxide (7) has been synthesised from d-mannose by a route involving as the key step dimerisation of mannopyranosyl nitrile oxide 2. Three methods were used for the generation of the nitrile oxide: isocyanate-mediated dehydration of nitromethylmannose derivative 4, treatment of aldoxime 5 with aq. hypochlorite, and base-induced dehydrochlorination of hydroximoyl ...