A new straightforward formation of aminopyrazoles from isocyanides

The reaction of alpha-halogenoketone hydrazones with isocyanides and sodium carbonate leads to a new formation of 5-aminopyrazole derivatives. This re action probably involves in situ azoalkene formation followed by a [4+1] cy cloaddition with isocyanide. Good yields are obtained for electron deficien t ketone hydrazones such as ethyl bromopyruvate hydrazones; this reaction w as successfully applied to the one pot synthesis of an antihistaminic pyraz ole.