3,6-Dichloro-4-methylpyridazine (30.6 g, 190 mmol) was suspended in glacial acetic acid (800 mL) at room temperature and subsequently heated in an oil bath at 110-115°C for 3 hours (the mixture became homogeneous after 1 hour of reaction). Upon completion of the reaction, the mixture was cooled to room temperature and the acetic acid was removed by distillation under reduced pressure. The solid residue obtained was slowly added to saturated sodium bicarbonate solution (200 mL) to obtain a light yellow non-homogeneous mixture, which was then adjusted to pH 6 by slowly adding solid sodium bicarbonate with vigorous stirring.The mixture was extracted with dichloromethane (2 x 250 mL), and the combined organic phases were washed with saturated aqueous sodium bicarbonate, then washed with sodium chloride (60 mL) and finally dried and concentrated with anhydrous magnesium sulfate. Finally, it was dried and concentrated to obtain the crude product (20.9 g). The crude product (20.9 g) was purified by column chromatography (elution gradient: 1% methanol/dichloromethane to 5% methanol/dichloromethane), and 6-chloro-4-methyl-2H-pyridazin-3-one (13.4 g, 92.7 mmol, 49% yield) [Rf 0.21 (2.5% methanol/dichloromethane)] and 6-chloro-5-methyl-2H-pyridazin-3-one (5.00 g, 34.6 mmol, 18% yield) [Rf 0.11 (2.5% methanol/dichloromethane)], both colorless solids.NMR data for 6-chloro-4-methyl-2H-pyridazin-3-one: 1H NMR (200 MHz, CDCl3-CD3OD): δ 2.12 (3H, s), 3.55 (1H, br s), 7.05 (1H, s); 13C NMR (50 MHz, CDCl3): δ 16.1 (CH3), 131.7 (CH-5), 139.0 (C-6), 143.0 (C-4), 161.9 (C-3).Physical and spectroscopic data of 6-chloro-5-methyl-2H-pyridazin-3-one: melting point 172-173°C (recrystallized from dichloromethane/light petroleum ether) ; 1H NMR (200 MHz, CDCl3): δ 2.20 (3H, d, J = 1.3 Hz), 7.13 (1H, q, J = 1.3 Hz); 13C NMR (50 MHz, CDCl3): δ 16.3 (CH3), 131.9 (CH-5), 139.2 (C-4), 143.2 (C-6), 161.9 (C-3). Low resolution mass spectrum (EI): m/z 144 ([M+]). Calculated elemental analysis (C5H5ClN2O): C, 41.54; H, 3.49; N, 19.38. Measured values: C, 41.55; H, 3.51; N, 19.22.
