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Step 3: Synthesis of 5-bromo-4-methoxy-2-aminopyridine To a solution of 4-methoxy-2-aminopyridine (12.4 g, 100 mmol) in acetonitrile (500 mL) was added N-bromosuccinimide (17.8 g, 100 mmol) in batches, and the reaction was carried out at 0 °C with continuous stirring. After the addition was completed, the reaction mixture was continued to be stirred at room temperature for 1 hour. After completion of the reaction, the solvent was removed by rotary evaporation and the residue was dissolved with dichloromethane. The resulting solution was washed with saturated sodium bicarbonate solution, and the organic phase was dried over anhydrous sodium sulfate and concentrated to afford 5-bromo-4-methoxy-2-aminopyridine (20.3 g, 100% yield), which could be used for subsequent reactions without further purification. Mass spectrum (ESI) m/z: 203.0 [M + H]+.

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