General procedure: 5-bromo-2-(methylamino)pyridine (2.0 g, 10.7 mmol) and di-tert-butyl dicarbonate (2.8 g, 12.8 mmol, 3.0 mL) were dissolved in tetrahydrofuran (20 mL) under nitrogen protection and cooled to 0 °C. NaHMDS (1 M, 12 mL) was added slowly, keeping the reaction temperature at 0 °C. After addition, the reaction mixture was gradually warmed to 20 °C and stirred continuously for 13 hours. The progress of the reaction was monitored by thin layer chromatography (TLC, unfolding agent: petroleum ether/ethyl acetate = 3/1) and a small amount of raw material residue and a new point of lower polarity (product point) were observed. Upon completion of the reaction, the reaction was quenched with saturated sodium bicarbonate solution (30 mL) and extracted with ethyl acetate (50 mL × 2). The organic phases were combined, washed with saturated saline (30 mL), dried over anhydrous sodium sulfate, filtered and concentrated under reduced pressure to give the crude product. The crude product was purified by silica gel column chromatography (eluent: petroleum ether/ethyl acetate = 10/1~3/1) to afford 5-bromo-2-(N-methyl-N-(tert-butoxycarbonyl)amino)pyridine (2.8 g, 91% yield) as a light yellow liquid. The structure of the product was confirmed by 1H NMR (400 MHz, CDCl3) and ESI-MS: 1H NMR δ 8.40-8.39 (m, 1H), 7.71-7.65 (m, 2H), 3.37 (s, 3H), 1.49 (s, 9H); ESI-MS (m/z): 287.0 ([M+H]+).
