Rearrangements of 5-isoxazolylhydrazines: 1-amino- and 4-aminopyrazolin-5-ones

5-Isoxazolylhydrazines rearranged to 1-aminopyrazolin-5-ones on heating. Under the same conditions, 5-(3-methyl-4-phenylisoxazolyl)hydrazine also gave 3-methyl-4-amino-4-phenylpyra-zolin-5-one. The structures of aminopyrazolones were assigned on the basis of spectroscopic properties and chemical behavior.