Step 2: Synthesis of tert-butyl 3-formylpyridin-2-ylcarbamate (9b)
To a solution of compound 9a (14.0 g, 72 mmol) in anhydrous ether (500 mL) at -78 °C and under nitrogen protection, tert-butyllithium (1.7 M, 106 mL, 180 mmol) was slowly added. The reaction mixture was slowly warmed to 0 °C and stirred at this temperature for 1 hour. Subsequently, anhydrous N,N-dimethylformamide (8.0 mL, 103 mmol) was added rapidly under vigorous stirring. The reaction mixture was continued to be stirred at 0 °C for 10 min and then quenched with a half-saturated aqueous ammonium chloride solution. The aqueous layer was separated and extracted twice with ethyl acetate (EA). The organic layers were combined, dried with anhydrous sodium sulfate and concentrated under reduced pressure. The residue was purified by column chromatography (eluent: petroleum ether/ethyl acetate = 3/1) to afford the target product 9b (8.0 g, 52% yield) as a white solid.
