General procedure for the synthesis of 2-(4-aminophenyl)-5-aminobenzo[d]oxazole from 2-(4-nitrophenyl)-5-nitrobenzo[d]oxazole: 2-(4-nitrophenyl)-5-nitrobenzo[d]oxazole (3.22 g, 0.01 mol) was added to a 100 mL single-necked flask followed by the addition of 50 mL of water at room temperature. The pH of the reaction system was adjusted with phosphoric acid to 1. Then, sodium thiosulfate solid (8.5 g, 0.06 mol) was added and the reaction system was warmed up to 100 °C for 16 h. The reaction was completed by adding a solid sodium thiosulfate (8.5 g, 0.06 mol). After completion of the reaction, it was cooled to room temperature and filtered. To the filtrate, 25% by weight of aqueous sodium carbonate was added and neutralized to pH 7. At this point, the product 2-(4-aminophenyl)-5-aminobenzo[d]oxazole was precipitated from the reaction system, and after filtration, several washes, and drying, 2.14 g of the target product was obtained (95% yield, HPLC purity >99.9%).
