Phosphorus oxybromide → pentyl 3-bromo-1-(3-chloro-2-pyridinyl)-4,5-dihydro-1H-pyrazole-5-carboxylate

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A mixed solution comprising 1.2 g of pentyl 2-(3-chloropyridin-2-yl)-5-oxopyrazolidine-3-carboxylate, 0.59 g of phosphorus oxybromide and 18 ml of acetonitrile was gradually heated, and 25 minutes later, the mixed solution was refluxed with heating for one hour. After the reaction liquid was stood to cool, it was slowly added to a saturated sodium hydrogen carbonate aqueous solution, followed by stirring for 5 minutes. The mixed liquid was subjected to extraction with ethyl acetate, and the organic layer was washed with a sodium chloride aqueous solution and dried over anhydrous sodium sulfate. The solvent was distilled off under reduced pressure, and the residue was purified by silica gel column chromatography (eluent: n-hexane/ethyl acetate=9/1 to 8/2) to obtain 1.06 g of the desired product (melting point: 39 to 42° C.) in the form of pale yellow crystals.

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