GENERAL STEPS: Oxalyl chloride (0.3 mL) was slowly added dropwise to cooled (ice bath) N,N-dimethylformamide (3 mL) under stirring conditions. The reaction mixture was stirred continuously at 0 °C for 1 hour. Subsequently, 6-fluoroindole (4 mmol) was dissolved in N,N-dimethylformamide (1.5 mL) and the resulting solution was added slowly dropwise to the above reaction mixture. The reaction system was stirred at room temperature for 5 hours. Upon completion of the reaction, 2 N sodium hydroxide solution (2 mL) was added and the mixture was heated to 100 °C and held for 10 minutes. The reaction solution was cooled and extracted with ethyl acetate (3 x 50 mL). The organic phases were combined and washed sequentially with water and saturated brine. The organic layer was dried over anhydrous sodium sulfate and concentrated under reduced pressure to give the crude product. The crude product was purified by fast column chromatography with the eluent of ethyl acetate/petroleum ether (3:1, v/v) to obtain pure 6-fluoroindole-3-carbaldehyde.
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