Transition-Metal-Free Sequential Two-Step Synthesis of Pyrido[3,2-c]pyridazin-4(1H)-ones via Intramolecular Cyclization of α-Keto Hydrazones

Sequential two-step protocol for the construction of pyrido[3,2-c]pyridazin-4(1H)-one fused system under metal-free conditions has been developed. The condensation cascade between β-keto-ester/nitrile (pyridine derivative) and diazonium salts derived from aniline analogues to obtain diaza compounds, which undergo intramolecular cyclization with high regioselectivity to furnish pyrido[3,2-c]pyridazin-4(1H)-one heterocycles with moderate to good yields.