Step 2 (Ib): to a solution of anhydrous methanol (MeOH, 1.0 L) of 4,5-difluoro-2-nitrobenzoic acid (Ia, 40.0 g, 196.947 mmol) was slowly added concentrated sulfuric acid (H2SO4, 60.0 mL), and the reaction mixture immediately became clear. The reaction system was heated to reflux with continuous stirring for 2 days. After completion of the reaction, methanol was removed by rotary evaporator under reduced pressure. The residue was liquefied with ethyl acetate (EtOAc) and water and the organic phase was collected. The organic phase was sequentially washed with saturated brine and dried over anhydrous magnesium sulfate (MgSO4). After filtration, the filtrate was treated with anhydrous sodium sulfate (Na2SO4), filtered again, and the filtrate was concentrated to give methyl 4,5-difluoro-2-nitrobenzoate as a light yellow solid with a yield of 39.71 g and 93% yield.
