Dimethylformamide → 4-Bromo-2-chloropyridine-3-carbaldehyde

To a solution of 4-bromo-2-chloropyridine (12.0 g, 60.0 mmol) in anhydrous tetrahydrofuran (300 mL) cooled at −70° C. was added the solution of lithium diisopropylamide (30.0 mL, 60.0 mmol, 2.0 M) over a period of 30 minutes and stirred for another at −70° C. 2 h. Anhydrous DMF (12.0 g) was introduced over a period of 10 minutes and stirred for another 30 minutes. It was then quenched with saturated NaHCO3 (200 mL), extracted with ethyl acetate (100 mL×3). The combined organic layer was dried over anhydrous Mg2SO4, filtered, and evaporated under reduced pressure. The residue was purified by silica-gel column chromatography eluting with petroleum ether/ethyl acetate (20:1) to afford 104a as a yellow solid (4.0 g, 29%). 1H NMR (500 MHz, DMSO) δ 10.23 (s, 1H), 8.44 (d, J=5.5 Hz, 1H), 7.94 (d, J=5.5 Hz, 1H).