Hydrochloric Acid → 5-chloro-1-[2-(2-methoxyethoxy)-ethyl]-1H-pyrazole-4-carboxylic acid ethyl ester

To a mixture of t-butyl nitrite (8.5 mL, 64.5 mmol), anhydrous cuprous chloride (6.54 g, 66.1 mmol) and anhydrous acetonitrile (80 mL) was added 5-amino-1-[2-(2-methoxyethoxy)-ethyl]-1H-pyrazole-4-carboxylic acid ethyl ester (10.38 g, 40.3 mmol) over 10 minutes at 0° C. The reaction mixture was stirred at room temperature for 1 h, then at 70° C. for 2 h. The mixture was then cooled to room temperature and poured carefully into 6.0 N aqueous HCl (45 mL). The aqueous phase was extracted with methylene chloride (3×150 mL). The combined organic extracts were combined, dried over anhydrous Na2SO4, filtered and concentrated in vacuo to provide the crude material as an oil. The product was purified by column chromatography on silica gel (eluting with ethyl acetate/hexanes, using a gradient of 1:3 to 1:0) to provide 5-chloro-1-[2-(2-methoxyethoxy)-ethyl]-1H-pyrazole-4-carboxylic acid ethyl ester as a light yellow oil (3.43 g, 30%). Mass spectrum: m/z: 277.0 (M+1).