Step 1: 4-Amino-2-nitrobenzoic acid (9.895 g, 54.326 mmol) was suspended in water (17 mL) under stirring conditions and concentrated sulfuric acid (12 mL) was slowly added to form a homogeneous solution. The reaction system was cooled in an ice bath and additional water (10 mL) was added to improve stirring. Crushed ice (50 mL) was then added and a solution of sodium nitrite (NaNO2, 4.124 g, 59.759 mmol) in water (9 mL) was added dropwise, keeping the reaction temperature below 5 °C. After the dropwise addition, stirring was continued for 1.25 h at 0 °C. The reaction mixture was quenched by addition of urea (6 g), stirred at 0 °C for 10 min and filtered. The filtrate was added dropwise to a mixture of concentrated sulfuric acid (39 mL) and water (27 mL) preheated to boiling. After completion of dropwise addition, the reaction mixture was refluxed for 3 hours and subsequently cooled to room temperature. The reaction mixture was extracted with ethyl acetate (6.50 mL), the organic phases were combined, washed with saturated saline, filtered and concentrated to a small volume. Purification was carried out by rapid chromatography on silica gel using ethyl acetate, 20% methanol/ethyl acetate, 10% methanol/chloroform and 60% ethyl acetate/hexane sequentially as eluents. The final 4-hydroxy-2-nitrobenzoic acid (5.331 g, 29.112 mmol, 54% yield) was obtained as an orange solid and decomposed.1H NMR (DMSO-d6) δ 7.05 (dd, 1H), 7.15 (s, 1H), 7.78 (d, 1H), 11.10 (s, 1H), 13.38 (s, 1H); mass spectrum [ES (-) )], m/z 182 (M-H)-.
