Ring transformation of 3-halo-4-methoxycoumarins into pyrazoles with hydrazines

3‐Chloro‐ and 3‐bromo‐4‐methoxycoumarins 1a,b were readily transformed into 4‐halo‐5‐(2‐hydroxyphenyl)‐3‐oxo‐2,3‐dihydropyrazoles 2a,b with hydrazines. In the reaction of 1a,b with excess hydrazine and phenylhydrazine in boiling ethanol, unexpected 4‐hydrazono‐3‐(2‐hydroxyphenyl)‐2‐pyrazolin‐5‐ones 3, 5 were obtained. The structure of 3 was determined by X‐ray diffraction analysis.