Dimethylformamide → 4-Chloro-5-fluoropyridine-3-carbaldehyde

4-Chloro-3-fluoropyridine (1 g) was dissolved in tetrahydrofuran (15 ml), and the solution was then cooled to −78° C. Lithium diisopropylamide (1.1 M solution in tetrahydrofuran and n-hexane, 7.6 ml) was added dropwise thereto, and the mixture was stirred at the same temperature as above for 5 hours. Then, N,N-dimethylformamide (1 ml) was added thereto, and the mixture was further stirred for 30 minutes while being heated to room temperature. Saturated saline was added to the reaction solution, followed by extraction with ethyl acetate. The extract was washed with water and saturated saline in this order and then dried over anhydrous sodium sulfate. The solvent was distilled off under reduced pressure. The obtained residue was purified by silica gel chromatography (n-hexane-ethyl acetate) to obtain the title compound (521 mg).