The title sulfone derivative is prepared by two alternative methods. Addition of sulfur to the lithio derivative of 5-chloro-3-methylbenzofuran (I) affords thiol (II). Further condensation of (II) with 3-chloro-6-methoxypyridazine (III) yields sulfide (IV). Acidic hydrolysis of methoxypyridazine (IV) leads to pyridazinone (V). The sulfide group of (V) is finally oxidized to the required sulfone employing an AcOH solution of peracetic acid. (1,2)
Alternatively, 3-chloro-6-methoxypyridazine (VI) is converted into the pyridazine thiol (VII) via reaction with thiourea in butanone. Oxidation of thiol (VII) with chlorine in the presence of potassium bifluoride leads to 4-methoxypyridazine-3-sulfonyl fluoride (VIII) (3). The lithio derivative of 5-chloro-3-methylbenzofuran (I) is then condensed with sulfonyl fluoride (VIII) to produce sulfone (IX). Finally, acidic hydrolysis of the methoxypyridazine group of (IX) gives rise to the corresponding pyridazinone compound (1-3).