Diisopropylamine (33.6 mL, 0.24 mol) was dissolved in tetrahydrofuran (400 mL) under argon protection and cooled to -65 °C. The solution of 1.6 M n-butyllithium in hexane (156 mL) was added slowly. A hexane solution (156 mL) of 1.6 M n-butyllithium was slowly added. after 20 min, a solution of 3,5-dichloropyridine (29.6 g, 0.20 mol) in tetrahydrofuran (150 mL) was added dropwise, and stirring was continued for 30 min. Subsequently, a tetrahydrofuran (50 mL) solution of dimethylformamide (23.2 mL, 0.30 mol) was added, kept at a constant temperature and stirred for 1 hour. The reaction mixture was poured into 5% aqueous ammonium chloride solution (1,000 mL) and extracted with ethyl acetate. The organic layer was washed sequentially with water and saturated brine, dried over anhydrous sodium sulfate and concentrated under reduced pressure. The residue was purified by silica gel column chromatography to afford 3,5-dichloropyridine-4-carbaldehyde (27.2 g, 77% yield).1H-NMR (CDCl3) δ: 8.63 (2H, s), 10.44 (1H, s).
