The general procedure for the synthesis of 1,5-dimethyl-3-oxo-2-phenyl-2,3-dihydro-1H-pyrazole-4-carboxylic acid from 4-antipyrinecarboxaldehyde is as follows:
1. 1-benzyl-5-methyl-3-oxo-2-phenyl-2,3-dihydro-1H-pyrazole-4-carboxaldehyde (1 g, 4.624 mmol) was dissolved in butanol at 0 °C to prepare a mixture.
2. An aqueous solution of NaClO2 (1.254 g, 13.873 mmol) and an aqueous solution of potassium dihydrogen phosphate monohydrate (3.146 g, 23.12 mmol) were slowly added to the above mixture.
3. the reaction mixture was slowly heated to room temperature with continuous stirring for 10 hours.
4. During the reaction, NaClO3 (1 g) was further added and the progress of the reaction was monitored.
5. After addition of sodium chlorite, stirring of the reaction mixture was continued followed by extraction with ethyl acetate.
6. The organic layer was washed, dried with Na2SO4 and filtered.
7. The filtrate was concentrated under reduced pressure to give a residue.
8. The residue was washed with hexane solution of 20% ethyl acetate to afford the target compound 1,5-dimethyl-3-oxo-2-phenyl-2,3-dihydro-1H-pyrazole-4-carboxylic acid as a white solid (808 mg, 3.48 mmol, 75% yield).
Product characterization data: 1H NMR (400 MHz, DMSO): δ 12.22 (br s, 1H), 7.61-7.42 (m, 5H), 3.36 (s, 3H), 2.59 (s, 3H).
