Phosphorus oxychloride → methyl 3-chloro-1-methyl-4,5-dihydro-1H-pyrazole-5-carboxylate

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To a solution of POCl3 (31.8 ml, 341 mmol) and Net3 (47.6 ml, 341 mmol) in CHCl3 (100 ml) at −5° C. was added a solution of methyl 3-hydroxy-1-methyl-4,5-dihydro-1H-pyrazole-5-carboxylate (36 g, 228 mmol) in CHCl3 (100 ml). After 1 h at this temperature, the mixture was stirred for another 20 h under warming to rt. Then, the reaction mixture was poured on ice water and extracted with CHCl3. The combined organics were washed with water, dried over Na2SO4 and evaporated under reduced pressure. This yielded the title compound (21 g, 52%) which was used in the next step without further purification. [LCMS RtJ=0.63 min; [M+H]+=177.0, 179.0]

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