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

To a solution of 2-bromo-4-chloropyridine (1.6 g, 8.0 mmol) in anhydrous tetrahydrofuran (40 mL) cooled at −70° C. was added the solution of lithium diisopropyl-amide (5.0 mL, 10.0 mmol, 2.0 M) over a period of 5 minutes and stirred at −70° C. for another 1 h. Anhydrous DMF (1.3 g) was introduced over a period of 3 minutes and the mixture was stirred for another 30 minutes. It was then quenched with saturated NH4Cl (30 mL) and extracted with ethyl acetate (20 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 103a as a yellow solid (900 mg, 48%). 1H NMR (500 MHz, DMSO) δ 10.21 (s, 1H), 8.52 (d, J=5.5 Hz, 1H), 7.79 (d, J=5.0 Hz, 1H).