A Novel Approach to the Pyrido[4,3-c]pyridazine Ring. Synthesis of Pyrido[4,3-c]pyridazin-5(6H)-one from 3,4-Disubstituted Pyridazines.
Jean-Pierre Vors, J. Heterocyclic Chem. 28, 1043, (1991)
The synthesis of new [4,3-c]pyridazin-5(6H)-one from ethyl 3-methyl-4-pyridazinecarboxylate
using an enamination/ring closure sequence is described. The starting material is easily available from a hetero Diels-Alder reaction between a 1,2-diaza-1,3-diene and ethyl vinyl ether and oxidation of the resulting 1,4,5,6-tetrahydropyridazines.
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Further Reading: pyridines synthesis and properties at Chemsynthesis.