5-Methoxypyridazin-3(2H)-one (250 mg, 1.98 mmol) was used as starting material and suspended in phosphoryl chloride (1.5 mL, 16 mmol). The reaction mixture was stirred at 100 °C for 10 min. After completion of the reaction, the mixture was slowly poured into ice water and the pH was adjusted to alkaline by adding sodium carbonate. Subsequently, the mixture was extracted with ether and the organic phases were combined and dried with anhydrous magnesium sulfate. After filtration, the solvent was removed by distillation under reduced pressure. The resulting residue was recrystallized with 1,2-dichloroethane to give 130 mg of 3-chloro-5-methoxypyridazine in 45% yield. The product was analyzed by LC-MS (Method 5): r = 0.85 mm; MS (ESIpos): m/z = 145 [M + H]+. 1H-NMR (400 MHz, DMSO-d6) δ (ppm): 2.495 (0.78), 2.499 (1.07), 2.503 (0.84), 3.317 (11.94). 3.944 (16.00), 7.528 (2.28), 7.534 (2.29), 8.988 (2.34), 8.994 (2.34).
