The general procedure for the synthesis of ethyl 3-bromopyrazole-4-carboxylate from 3-amino-4-ethoxycarbonylpyrazole was as follows: to a solution of ethyl 3-amino-1H-pyrazole-4-carboxylate (5.0 g, 32 mmol) and copper(II) bromide (7.2 g, 32 mmol) in acetonitrile (65 mL) was slowly added isoamyl nitrite (12 mL, 86 mmol). The reaction mixture was heated to 50 °C and stirred overnight. Upon completion of the reaction, the mixture was cooled to room temperature and the reaction was quenched with 1N aqueous hydrochloric acid (150 mL). The mixture was extracted with ethyl acetate (3 x 100 mL). The organic phases were combined, washed with water, dried over anhydrous sodium sulfate, filtered and concentrated to afford ethyl 3-bromopyrazole-4-carboxylate as a brown oil, which was partially cured overnight under vacuum (7.1 g, 100%). The product was characterized by 1H NMR (400 MHz, CDCl3): δ 9.78 (br.s, 1H), 8.10 (br.s, 1H), 4.33 (q, J = 7.22 Hz, 2H), 1.36 (m, 3H).
