General procedure for the synthesis of 4-bromo-5-nitro-2-thiophenecarboxaldehyde from 4-bromo-2-thiophenecarboxaldehyde: A nitration reagent was prepared by dissolving potassium nitrate (110 g, 1.09 mol) in concentrated sulfuric acid (550 mL). The above nitration reagent was slowly added to a mixed solution of anhydrous dichloromethane (5 mL) and concentrated sulfuric acid (1.1 L) containing 4-bromo-2-thiophenecarboxaldehyde (207.6 g, 1.08 mol) at 0 °C, and the addition process was controlled to be completed within 45 min. The reaction mixture was continued to be stirred at 0 °C for 2 h, followed by raising to room temperature and stirring overnight. Upon completion of the reaction, the mixture was slowly poured into ice water, the precipitate was collected by filtration and washed sequentially with water and hexane. The resulting yellow solid was dried overnight to afford the target product 4-bromo-5-nitro-2-thiophenecarboxaldehyde (251.4 g, 98.6% yield). The structure of the product was confirmed by 1H-NMR (DMSO-d6): δ 9.90 (s, 1H, aldehydic hydrogen), 8.56 (s, 1H, thiophene cyclohexane).
