4-Amino-3-nitrobenzoic acid (50 g, 270 mmol, 1 eq.) was suspended in methanol (600 mL) at room temperature. Subsequently, sulfoxide chloride (20 mL, 270 mmol, 1 eq.) was slowly added dropwise to the above suspension. The reaction mixture was heated to reflux for 16 h. After completion of the reaction, it was cooled to room temperature. The resulting yellow solid, methyl 4-amino-3-nitrobenzoate (D1) (53 g, 100% yield), was collected by filtration, and the product could be used in subsequent reactions without further purification. Mass spectrometry analysis showed [M + H]+ = 197.3, retention time (RT) = 2.42 min.
