Regioselectivity Inversion in the Cycloaddition Reaction between Stable Nitrile Oxides and Acylalkyne Derivatives

1,3-Dipolar cycloaddition reaction of stable nitrile oxides with α,β-alkynyl hydrazones affords 5-iminoisoxazoles, whereas the reaction of nitrile oxides with α,β-alkynyl carbonyl compounds gives 4-acylisoxazoles. The regioselectivity of the cycloaddition reaction is reversed due to the electron-donating character of the hydrazone group. The 5-iminoisoxazoles are readily converted to the corresponding 4-acylisoxazoles by dehydrazonation.