ethyl 5-oxo-1-phenyl-4H-pyrazole-4-carboxylate

Conditions: temp:0.00; pressure:0.00; stir:; See reaction.notes.procedure_details.

Procedure: To a mixture of 2,4-dihydro-2-phenyl-4-ethoxycarbonyl-3H-pyrazol-3-one (5.8 g; 25 mmol) in 16 ml of ethanol/water (1:1) 4 g of KOH was added and the resulting mixture was heated at reflux for 4 hours. The mixture was cooled in an ice/bath, conc. HCl solution was added (until pH=1-2), and the resulting mixture was refluxed overnight. The reaction mixture was cooled, concentrated in vacuo, and the residue was dissolved in water. The aqueous layer was extracted with ethyl acetate (2×), the combined organic layer was dried over magnesium sulfate and concentrated in vacuo. The residual yellow oil was purified by silica gel column chromatography (25-30% ethyl acetate/hexane) to afford 1.5 g (38%) of 2,4-dihydro-2-phenyl-3H-pyrazol-3-one (Formula III: R1 =Ph; R2 =H; R3 =H) as a solid, m.p. 113°-114° C. was added the resulting mixture was heated at reflux for 4 hours The mixture was cooled in an ice/bath the resulting mixture was refluxed overnight The reaction mixture was cooled concentrated in vacuo the residue was dissolved in water The aqueous layer was extracted with ethyl acetate (2×) the combined organic layer was dried over magnesium sulfate concentrated in vacuo The residual yellow oil was purified by silica gel column chromatography (25-30% ethyl acetate/hexane)