General procedure for the synthesis of 3-chloro-4-methylpyridazine from 4-methyl-3(2H)-pyridazinone: The product of step c) (12.2 g, 0.112 mol) was dissolved in phosphorochloridic acid chloride (125 mL) and heated to reflux for 3 hours at 90 °C. Upon completion of the reaction, the reaction mixture was cooled to room temperature and concentrated to near dryness under reduced pressure. The concentrated residue was carefully poured into an ice/water mixture and the pH was adjusted to basic by slowly adding 4N NaOH solution. Subsequently, extraction was carried out with ether and the aqueous phase was then subjected to a secondary extraction with dichloromethane (DCM). All organic phases were combined, washed with water, dried over anhydrous sodium sulfate, filtered and concentrated to dryness under reduced pressure. Finally, the residue was purified by rapid chromatography on silica gel with 2:1 ethyl acetate-isohexane as eluent to give an orange solid product (11.2 g, 78% yield). The product was characterized by 1H NMR (400 MHz, CDCl3): δ 8.97 (1H, d, J = 4.9 Hz), 7.35 (1H, d, J = 4.9 Hz), 2.44 (3H, s) ppm.
