General procedure for the synthesis of 3,4,5,6-tetrachloropyridazine from 3-hydroxy-4,5-dichloro-6-nitropyridazine: a mixture of DMF (14.75 mL, 190 mmol) and POCl3 (17.7 mL, 190 mmol) in anhydrous toluene (150 mL) was stirred for 1 hour at room temperature. Subsequently, 3-hydroxy-4,5-dichloro-6-nitropyridazine (10 g, 48.0 mmol) was added and the reaction mixture was refluxed for 18 hours. After completion of the reaction, the mixture was cooled, poured into ice water and stirred for 30 minutes. The organic phase was separated and the aqueous layer was extracted with EtOAc (4 x 200 mL). The organic layers were combined, dried over MgSO4, filtered and concentrated. The residue was purified by column chromatography (cHexane/EtOAc, 15:1 to 10:1) to afford 3,4,5,6-tetrachloropyridazine as a white solid (6.22 g, 60% yield).HPLC-MS (Method 4): retention time Rt = 4.97 min, [M + H]+ m/z 217-219-221-223.13C NMR (300 MHz, CDCl3) δ 137.48, 154.65.
