General procedure for the synthesis of 2,6-diamino-5-nitropyrimidin-4-ol from 2,6-diaminopyrimidin-4(1H)-one: 126 g of 2,6-diaminopyrimidin-4(1H)-one was dissolved in 730 g of the filtrate from the above batch with stirring, and the temperature of the reaction was controlled to be between 30 and 35°C. Subsequently, 20 g of concentrated sulphuric acid with a mass concentration of 98% was slowly added. Under the condition of maintaining the same temperature, 68.5 g (1.01 mol) of 93% fuming nitric acid was added dropwise. After the dropwise addition, the reaction was continued for 2 hours. Upon completion of the reaction, the reaction mixture was cooled to -5 to 0°C to promote crystallization, followed by filtration to obtain about 730 g of brown-red filtrate, which can be used as a solvent for the next batch of the reaction. The solid obtained by filtration was first washed with 100 mL of dichloromethane and then with 200 mL of potable water, and drained to obtain 225 g (165 g on a dry weight basis) of brownish-yellow to brownish-red solid, i.e., the target product 2,6-diamino-5-nitropyrimidin-4-ol (molecular weight 171.11). The purity of the product was 98.7% by HPLC analysis, and the yield was 96.4% as 2,6-diaminopyrimidin-4(1H)-one.
