Oxalyl chloride → methyl 1,5-dimethyl-3-oxo-2-phenyl-2,3-dihydro-1H-pyrazole-4-carboxylate

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Reaction Steps

2,3-dimethyl-5-oxo-1-phenyl-2,5-dihydro-1H-pyrazole-4-carboxylic acid (7.55 g, 33 mmol) was dissolved in dichlormethane (145 ml) and oxalyl chloride (4.1 ml, 46 mmol) was added via syringe over about 10 minutes, resulting in vigorous bubbling. After stirring at room temperature for about 30 minutes, the reaction was cautiously quenched with MeOH (100 ml). The methanol was added slowly at first as vigorous gas evolution was observed. The reaction was stirred at room temperature for 1 hour, concentrated, and then partitioned between dichlormethane (125 ml) and saturated sodium bicarbonate (125 ml). More dichlormethane and saturated sodium bicarbonate were added, and the layers of the biphasic, homogeneous solution were separated. The aqueous phase was extracted with dichlormethane (3×100 ml), and the organic phases were collected, dried over sodium sulfate, filtered, and concentrated to give desired methyl 1,5-dimethyl-3-oxo-2-phenyl-2,3-dihydro-1H-pyrazole-4-carboxylate (7.82 g, 79% purity by HPLC, 25 mmol, 77% yield). MS (ESI pos. ion) m/z: 247 (MH+). Calc'd exact mass for C13H14N2O3: 246.

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