The Structure of 4‐Benzoyl‐5‐methyl‐2‐phenylpyrazol‐3‐one Oxime and Its Methyl Derivatives

1 H and 13 C NMR spectroscopic investigations of 4‐benzoyl‐5‐methyl‐2‐phenyl‐1,2‐dihydro‐3 H ‐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 ‐methylamino product ( 6 ). Both compounds are stabilised by an intramolecular hydrogen bond between the pyrazolone C=O group and the N‐OH proton. A 1,2‐dihydro‐3 H ‐pyrazol‐3‐one structure was found in a clathrate of O ‐methyloxime 5 and dichloromethane. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003)