N(OC(C)(C)C)=O.[CH2:8]([O:10][C:11]([C:13]1[CH:14]=[N:15][N:16]([CH3:19])[C:17]=1N)=[O:12])[CH3:9].[ClH:20]>C(#N)C>[CH2:8]([O:10][C:11]([C:13]1[CH:14]=[N:15][N:16]([CH3:19])[C:17]=1[Cl:20])=[O:12])[CH3:9]To a mixture of t-butyl nitrite (29.5 mL, 248 mmol), cuprous chloride (17.6 g, 177.8 mmol) and anhydrous acetonitrile (490 mL) was added 5-amino-1-methyl-1H-pyrazole-4-carboxylic acid ethyl ester (25 g, 148 mmol) in portions over 30 minutes at 0° C. The reaction mixture was stirred at room temperature for 1 h, then at 65° C. for 1 h. The mixture was then poured into 6N HCl (600 mL) and extracted with dichloromethane. The aqueous phase was extracted three times with dichloromethane. The combined organic phases were concentrated in vacuo, and the crude residue was purified by flash chromatography eluting with a gradient of 0-20% ethyl acetate/hexanes, then 20% ethyl acetate/hexanes to give 5-chloro-1-methyl-1H-pyrazole-4-carboxylic acid ethyl ester (18 g, 64%).
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