5-Isoxazolylhydrazines rearranged to 1-aminopyrazolin-5-ones on heating. Under the same conditions, 5-(3-methyI-4-phenylisoxazolyl)hydrazine also gave 3-methyl-4-amino-4-phenylpyrazolin-5-one. The structures of aminopyrazolones were assigned on the basis of spectroscopic properties and chemical behavior.In a previous paper (l), we reported that 5-lialoisoxazoles (Ia-c) heated with anhydrous hydrazine for a short period gave 5-isoxazolylhydrazines (IIa-c) in 70-90% yield. In a short communication (2), we have described that prolonged heating led to rearrangement of compounds IIa-c to l-aminopyrazolin-5-ones(IIIa-c). In addition under these circumstances, small amounts of the corresponding pyrazolones (IVa-c) were obtained. Furthermore when R = CHJ and R' = C6HS, we isolated 3-methyl-4-amino-4phenylpyrazolin-5-one (V). Increased yields V of this compound were obtained when the reaction was carried out with hydrazine hydrate.The hydrazines (Ha-c) were the intermediates in the reaction since they gave the same products, in the same ratios, under the same conditions.The structures of compounds IIIa-c and V were established by their spectroscopic properties and chemical behavior. The ir spectra show bands in the region 3450-3100 cm-' attributable to imino and/or amino groups. In chloroform solution, strong bands near 1700 cm-' (for compounds IIIa-c) and 1740 cm-' (for compound V) can be attributed to the stretching vibrations of CO groups which are present. In the spectra of IIIa and V, this band is still present in the solid state whereas in the spectra of Y + Ia-c IIa-c NH* D o -c + NH Scheme 1 1Ya-c I220

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