5-Fluoroquinazoline-2,4(1H,3H)-dione (B-1, 2.6 g, 14.43 mmol) was used as starting material and suspended in 29 mL of toluene and heated to 50 °C. Subsequently, phosphoryl chloride (9.88 mL, 108.25 mmol) was slowly added dropwise, followed by DBU (4.31 mL, 28.87 mmol). The reaction mixture was stirred vigorously at 120 °C overnight. Upon completion of the reaction, the mixture was cooled to room temperature and quenched by slow dropwise addition to ice water. The aqueous layer was extracted with ethyl acetate, the organic phase was washed with brine, dried over anhydrous sodium sulfate and concentrated under reduced pressure to give the crude product. The crude product was purified by silica gel column chromatography with toluene as eluent to give 2,4-dichloro-5-fluoroquinazoline (B-2, 3.41 g, 80.8% yield) as white powder. The product could be used directly in the subsequent reaction without further purification.
