121195-03-7

36983-36-5

121195-03-7

GENERAL METHOD: To a stirred solution of ethyl 2,4-dioxo-4-(thiophen-2-yl)butyrate (0.1 mol) in anhydrous toluene (200 mL) was added a suspension of NaH in mineral oil (60%; 0.2 mol) in batches and the mixture was heated to 50°C. The mixture was then heated to 50°C. The solution of ethyl oxalate (0.15 mol) in anhydrous toluene (60 mL) was added slowly. At this temperature, a solution of diethyl oxalate (0.15 mol) in anhydrous toluene (60 mL) was added slowly and dropwise. The reaction mixture was refluxed for 1.5 hours. After cooling to room temperature, acetic acid (0.25 mol) was added slowly and dropwise. The reaction mixture was washed with water (200 mL) and the organic phase was dried over MgSO4 and concentrated to dryness. The crude product obtained was dissolved in anhydrous ethanol (250 mL), hydrazine dihydrochloride (0.11 mol) was added and the mixture was refluxed for 3 hours. After evaporation of the solvent, the residue was treated with water (200 mL) and cooled in an ice bath for 1 hour. The crystals were collected by filtration and dried in air to give ethyl 3-(thiophen-2-yl)-1H-pyrazole-5-carboxylate. If necessary, the product can be further purified by recrystallization from aqueous ethanol.