General procedure for the synthesis of methyl 5-chloro-4-nitrothiophene-2-carboxylate from ethyl 5-chlorothiophene-2-carboxylate: ethyl 5-chlorothiophene-2-carboxylate (2.7 g, 14.1 mmol) was added in batches to concentrated sulfuric acid (5 mL) and the stirred solution was cooled to below 0 °C under methanol/ice bath conditions. Fuming nitric acid (1.78 g, 1.2 mL, 28.3 mmol) was added slowly and dropwise, ensuring that the reaction temperature was maintained below 0 °C throughout the addition. After addition, the stirred mixture was removed from the cold bath and stirred for 2 h at room temperature. Upon completion of the reaction, the reaction was quenched by the addition of an ice-water mixture (100 mL), at which point the formation of a viscous solid was observed. The product was extracted with dichloromethane (2 x 100 mL) and the organic phases were combined and dried over anhydrous magnesium sulfate. After filtration, the solvent was removed under reduced pressure to give an orange oily crude product. The crude product was purified by silica gel column chromatography using a solvent mixture of hexane and ethyl acetate (95:5, v/v) as eluent, resulting in the title compound methyl 5-chloro-4-nitrothiophene-2-carboxylate as a solid (2.23 g, 67% yield).1H NMR (CDCl3) δ: 8.14 (1H, s), 4.37 (2H, q, J = 7.11 Hz ), 1.37 (3H, t, J = 7.11 Hz).
