General procedure for the synthesis of 2,5-dimethyl-3-thiophenecarboxaldehyde from 2,5-dimethylthiophene and 1,1-dichlorodimethyl ether: a solution of 2,5-dimethylthiophene (7.80 g, 69.5 mmol) in dichloromethane (DCM, 15 mL) was added simultaneously and slowly to a solution of titanium tetrachloride (TiCl4, 19.1 mL) in dichloromethane (DCM, 20 mL). mL) solution were added simultaneously and slowly dropwise to a dichloromethane (DCM, 20 mL) solution of titanium tetrachloride (TiCl4, 19.1 mL), keeping the reaction temperature below 5 °C. The reaction mixture was stirred at 0 °C for 2 h, followed by slow warming to room temperature for 30 min. The reaction mixture was poured into ice water acidified with hydrochloric acid (HCl, 20 mL). The mixture was partitioned with dichloromethane and water, the organic layer was washed with water, dried over anhydrous sodium sulfate and concentrated under reduced pressure. Purification by Kugelrohr distillation (membrane pump, set temperature ca. 175 °C) gave 2,5-dimethyl-3-thiophenecarboxaldehyde as a colorless liquid (5.81 g, 60% yield).1H NMR (CDCl3) δ 9.93 (s, 1H), 7.00 (d, J = 1.1 Hz, 1H), 2.70 (s, 3H), 2.40 (d, J = 0.4 Hz, 3H). Mass spectrum measured value: [M + H]+ = 141.1.
