General procedure for the synthesis of 1-(dimethylaminosulfonyl)-3-bromopyrazole from 1-(N,N-dimethylsulfonyl)pyrazole: A hexane solution of n-butyllithium (1.3 M, 31.1 mL) was added slowly and dropwise over 15 min to a tetrahydrofuran (75 mL) solution of N,N-dimethyl-1H-pyrazole-1-sulfonamide (6.74 g) at -78 °C. The reaction mixture was kept stirred at -78 °C for 15 min, followed by the dropwise addition of a tetrahydrofuran (25 mL) solution of 1,2-dibromo-1,1,2,2-tetrachloroethane (13.8 g) over 10 min. The reaction mixture was continued to be stirred at -78 °C for 15 minutes, then warmed to room temperature and stirred for 1 hour. Upon completion of the reaction, the mixture was poured into water and extracted with ethyl acetate. The combined organic phases were washed with saturated brine, dried over anhydrous sodium sulfate and concentrated under reduced pressure to remove the solvent. The residue was purified by silica gel column chromatography (eluent: ethyl acetate/hexane) to afford the target product 1-(dimethylaminosulfonyl)-3-bromopyrazole (8.03 g). The product was characterized by 1H NMR (400 MHz, CDCl3): δ 3.06 (6H, s), 6.42 (1H, d, J = 1.6 Hz), 7.60 (1H, d, J = 1.6 Hz).
