A new straightforward formation of aminopyrazoles from isocyanides

The reaction of α-halogenoketone hydrazones with isocyanides and sodium carbonate leads to a new formation of 5-aminopyrazole derivatives. This reaction probably involves in situ azoalkene formation followed by a [4+1] cycloaddition with isocyanide. Good yields are obtained for electron deficient ketone hydrazones such as ethyl bromopyruvate hydrazones; this reaction was successfully applied to the one pot synthesis of an antihistaminic pyrazole.