General procedure for the synthesis of 2-chloro-3-methylpyridine-4-carboxaldehyde from 2-chloro-N,3-dimethyl-N-phenyl-isonicotinamide: 2-chloro-N,3-dimethyl-N-phenyl-isonicotinamide (124.9 g, 479 mmol) was dissolved in CH2Cl2 (1300 mL) at -78 °C, and the slow dropwise addition of DIBAL (1 M THF solution. 719 mL, 719 mmol) over 1 hour. The reaction mixture continued to be stirred at this temperature for 2 hours. Subsequently, DIBAL (1M THF solution, 281 mL, 281 mmol) was added again and the reaction mixture was warmed to -60°C and stirred for 30 minutes. Slow addition of saturated aqueous sodium potassium tartrate (500 mL) was accomplished over 30 min, the cooling bath was removed, and the mixture was stirred at room temperature overnight. The reaction was quenched by the addition of water (100 mL) and the organic phase was separated. The aqueous phase was extracted with CH2Cl2 (2 x 100 mL) and the organic phases were combined. The organic phase was dried with MgSO4, filtered and concentrated under reduced pressure. The target product 2-chloro-3-methylpyridine-4-carboxaldehyde (58.35 g, 78% yield) was obtained by purification as light yellow crystals.
