General procedure: n-Butyl lithium (n-BuLi, 320 mL, 0.8 mol) was slowly added dropwise to a stirred solution of anhydrous THF (1000 mL) of 2-chlorothiazole (80 g) at -78 °C for 1 hour. Maintaining the reaction temperature at -78 °C, ethyl formate (74 g) was slowly added dropwise to the above solution, and the reaction continued to be stirred for 1 hour after the dropwise addition was completed. Saturated ammonium chloride (NH4Cl) solution was added to the reaction mixture and stirred for 30 minutes. Subsequently, the reaction mixture was diluted with ethyl acetate. The aqueous phase was separated and extracted with ethyl acetate. The organic phases were combined, washed sequentially with water and saturated brine and dried over anhydrous sodium sulfate. The organic phase was concentrated under reduced pressure to give the crude product. The crude product was purified by recrystallization from hexane/ethyl acetate mixed solvent to give 2-chloro-1,3-thiazole-5-carbaldehyde (72 g, yield: 73%). The product was characterized by 1H NMR (400 MHz, CDCl3): δ 9.96 (s, 1H), 8.21 (s, 1H).
