Abstract The 1, 3-dipolar cycloaddition of nitrile oxides to methylenecyclopropanes substituted on the exocyclic double bond gives prevalently or exclusively 4- spirocyclopropane isoxazolines when the substituent is arylic or alkylic group, 5- spirocyclopropane isoxazolines when the substituent is an electron-withdrawing group. Adducts 16 and 17 selectively rearrange photochemically to the enaminoenone 19.