General procedure: 2,3-dichloropyridine (10 g, 67.57 mmol) was dissolved in dry tetrahydrofuran (200 ml) under nitrogen protection and cooled to -78 °C. A hexane solution of n-butyllithium (37.165 ml, 74 mmol, 2M) was added slowly and dropwise. The reaction mixture was stirred at -78°C for 20 min and then dry N,N-dimethylformamide (6.28 ml, 81.087 mmol) was added dropwise. Stirring was continued at -78 °C for 15 min, followed by slow warming of the reaction mixture to room temperature. Upon completion of the reaction, the reaction was quenched with water and extracted with dichloromethane. The organic phases were combined, dried over anhydrous sodium sulfate and concentrated under reduced pressure. The crude product was initially purified by fast column chromatography (dichloromethane as eluent). The fraction containing 2,3-dichloropyridine-4-carbaldehyde was collected and concentrated under reduced pressure to give the crude product. It was further purified by column chromatography (silica gel; dichloromethane/heptane as eluent, gradient elution to 50%). The target fraction was collected and concentrated under reduced pressure to afford the intermediate compound D41 (4.15 g, 34.9% yield) as a white solid.GC-MS retention time (min): 7.9.
