Nitrile oxides from nitrolic acids

Nitrile oxides are generated from nitrolic acids [RC(N02)=N0H] by elimination of the elements of nitrous acid. This can occur spontaneously, on heating, or by the action of sodium bicarbonate, and it thereby provides access to symmetrically substituted dialkyl-, diaryl-, and diacyl-furoxans. 

The first examples of furazan and furoxan nitrile oxides have been reported in the early 1990s. 4-Aminofurazan-3-carbonitrile oxide (65) was generated from the hydroximoyl chloride with base and its cycloaddition reactions investigated <92KGS687>, and the 4-phenyl analogue (66) is formed via the nitrolic acid derivative by treatment of the aldoxime with dinitrogen tetroxide <93LA44i>. Furazan-3-amidoximes react in the usual way with nitriles to yield 3-(furazan-3-yl)-1,2,4-oxadiazoles <9013941 >. 

Direct dehydrogenation of oximes is also possible, particularly for sterically hindered nitrile oxides. The most successful oxidizing agents are alkaline hypobromite and NBS in the presence of base, the latter being valuable when basic groups are present. Lead tetraacetate and alkaline hypochlorite have also been employed, but the yields are lower. The formation of furoxans from the reaction of alkoximes with nitrogen oxides is believed to proceed via the corresponding nitrolic acids. 

---