While isoxazol-5(2H)-ones substituted with heterocycles at C2 but unsubstituted at C3 react with amines to give either amidines or malonamides, their reaction at low temperatures with lithium dialkylamides is a preparatively useful procedure for obtaining the amidines in most cases. Longer reaction times may lead to formation of pyrimidin-4-ones when ester groups are present at C4 of the isoxazolone.
Reaction of 2-heterocyclisoxazol-5(2H)-ones with bases leads to the formation of ketenimines, which react with nucleophiles in competition with intramolecular reactions. Such reactions in the presence of enamines, enamine anions or enolates are reported. Enamines undergo addition through carbon and nitrogen to the ketenimine in competition with direct addition-elimination to the isoxazolone. Enolates of imines or ketones add to the ketenimine to give a mixture of products: only the reaction with the enolate of cyclohexanone is sufficiently specific to provide a useful new synthetic procedure.