A mixture of 6-(4-amino-2,6-dichloro-phenylsulfanyl)-4-isopropyl-2H-pyridazin-3-one (85) (3.40 g, 10.4 mmol) in water (135 mL) and concentrated hydrochloric acid (68 mL) cooled to 0° C. was treated with a solution of sodium nitrite (853 mg, 12.36 mmol) in water (7.0 mL) via Pasteur pipette under the surface of the reaction mixture. This was followed by a water rinse (1.0 mL) of the Pasteur pipette. The resulting yellow mixture was stirred at 0° C. for 30 min. More concentrated hydrochloric acid (7.0 mL) was added. The reaction was stirred at 0° C. for an additional 1.3 h. The solids were removed by filtration through filter paper and were rinsed with water. The clear, yellow diazonium salt solution of the filtrate was quickly poured into a solution of cyanoacetylurethane (1.77 g, 11.33 mmol), pyridine (75 mL) and water (204 mL) cooled to 0° C. Upon mixing, orange-red solids immediately formed. This mixture was stirred at 0° C. for 1 h. At this time, the solids were collected by filtration through filter paper. The solids were washed with water, air-dried under house vacuum for 2 h, and then dried under vacuum to afford (2-cyano-2-{[3,5-dichloro-4-(5-isopropyl-6-oxo-1,6-dihydro-pyridazin-3-ylsulfanyl)-phenyl]-hydrazono}-acetyl)-carbamic acid ethyl ester (86) (2.98 g, 58%) as an orange solid; ES(+)-HRMS m/e calcd for C19H18Cl2N6O4S (M+H)+ 497.0560, found 497.0559. Exact Mass=496.0487; Molecular weight=497.36
