General procedure: 800 ml of concentrated sulfuric acid (80% concentration) was slowly added to 200 ml of water to form a reaction system. To this system was added 50.0 g of 2-propylbenzimidazole, followed by the slow dropwise addition of 30 mL of hydrogen peroxide to 500 mL of the reaction mixture. The reaction is exothermic and causes reflux, and the rate of titration needs to be controlled to maintain the reaction solution at a boil. After completion of the reaction, stirring was continued for 3 hours. Subsequently, 1000 mL of water was added to the reaction mixture and cooled to room temperature. The mixture was stirred for 3 hours at 0 to 5°C to promote precipitation of solids. The solid was collected by filtration and washed sequentially with water and 5% aqueous sodium sulfite solution (100 mL). The resulting solid was purified by column chromatography using water: methanol = 2:1 as eluent to give the final imidazole-4,5-dicarboxylic acid 3-lg with a melting point of 267.3 °C (simultaneous decomposition).
