Keywords: Oximes / Tautomerism / X-ray diffraction / NMR spectroscopy / Density functional calculations 1 H and 13 C NMR spectroscopic investigations of 4-benzoyl-5-methyl-2-phenyl-1,2-dihydro-3H-pyrazol-3-one oxime (4) and different methylation products thereof (5−11) indicate that 4 exists predominantly as 4-enaminopyrazolone in [D 6 ]DMSO solution. Single-crystal X-ray analysis revealed that in the solid state the same tautomeric structure of 4 was present, and closely resembles that of the corresponding N-

Treatment of 1,(3)‐(di)substituted 4‐benzoyl‐5‐hydroxypyrazoles with phosphorus oxychloride affords the corresponding 4‐benzoyl‐5‐chloropyrazoles. Reaction of the latter with hydroxylamine leads to oximes, which can be cyclized to novel 3‐phenyl‐6H‐pyrazolo[4,3‐d]isoxazoles by treatment with sodium hydride in dimethyl formamide. Detailed nmr spectroscopic studies (1H, 13C) with all obtained compounds are presented.

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