Step 3 Synthesis of methyl 4,6-dichloropyridazine-3-carboxylate: 4,6-dihydroxypyridazine-3-carboxylic acid methyl ester (10.5 g, 61.7 mmol) was mixed with phosphorous trichloride (70 mL), heated to 95 °C, and the reaction was carried out for 5 hours. After completion of the reaction, the excess phosphorous trichloride was evaporated under reduced pressure. The crude product was slowly poured into ice water (250 mL) and extracted with ethyl acetate (3 x 100 mL). The organic phases were combined, dried with anhydrous sodium sulfate and concentrated under reduced pressure to give the crude product. The crude product was purified by silica gel column chromatography (100-200 mesh, 30% ethyl acetate in hexane solution as eluent) to afford methyl 4,6-dichloropyridazine-3-carboxylate (9.2 g, 72% yield) as an off-white solid.LC-MS: 207.0 [M + H]+.
