General method: 2,4-dibromothiophene (0.5 g, 2.07 mmol) was dissolved in THF (20 ml) under argon protection and cooled to -78 °C. A n-butyllithium solution (2.5 M hexane solution, 0.91 ml, 2.28 mmol, 1.1 eq.) was slowly added dropwise, keeping the temperature at -78 °C. Subsequently, the reaction mixture was slowly warmed to 0 °C over 30 min. Anhydrous N,N-dimethylformamide (182 mg, 2.48 mmol, 1.2 eq.) was added and the reaction mixture was stirred at room temperature for 2 hours. Cooled again to -78 °C, tert-butyllithium solution (1.7 M pentane solution, 1.58 ml, 2.69 mmol, 1.3 eq.) was added and stirred for 5 min. An excess of anhydrous N,N-dimethylformamide (1 ml) was then added and the reaction mixture was stirred overnight at room temperature. After completion of the reaction, the solution was poured into 1N HCl (10 ml) and stirred for 20 min. After neutralization with aqueous NaHCO3, the aqueous phase was extracted with dichloromethane (3 x 30 ml). The organic phases were combined, dried with anhydrous MgSO4 and concentrated under reduced pressure to remove the solvent. The crude product was purified by silica gel column chromatography (eluent: hexane/ethyl acetate=6:1) to afford thiophene-2,4-dicarboxaldehyde (123 mg, 42%) as a yellow amorphous solid.
