Ethyl 5-oxo-2,5-dihydroisoxazole-4-carboxylate has been treated with a number of chlorinated heterocycles to yield the corresponding substitution products: ethyl 2-(isoquinolin-1-yl)-5-oxo-2,5-dihydroisoxazole-4-carboxylate, ethyl 5-oxo-2-(quinolin-2-yl)-2,5- dihydroisoxazole-4-carboxylate, ethyl 5-oxo-2-(purin-2-yl)-2,5- dihydroisoxazole-4-carboxylate, ethyl 5-oxo-2-(pyrimidin-2-yl)-2,5-dihydroisoxazole-4-carboxylate, ethyl 2-(6-chlorpyridazin-3-yl)-5-oxo- 2,5-dihydroisoxazole-4-carboxylate, ethyl 2-(benzothiazol-2-yl)-5-oxo- 2,5-dihydroisoxazole-4-carboxylate, 2- and 6-(4-ethoxycarbonyl-5-oxo-2,5-dihydroisoxazol-2-yl)pyridine-3-carboxylic acid, ethyl 5-oxo- 2-(2-phenylquinazolin-4-yl)-2,5-dihydroisoxazole-4-carboxylate and ethyl 2-(2,4-diaminotriazin-2-yl)-5-oxo-2,5-dihydroisoxazole-4- carboxylate . The compounds generally cause loss of motor control in mice, but are relatively toxic.
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.