Direct Pyridazine Ring Synthesis from β-Cyano Esters. A Facile Synthesis of the Derivatives of Tetrahydro-3,6-pyridazinedione 3-Hydrazone

4-Substituted tetrahydro-3,6-pyridazinedione 3-hydrazones (2) (or their 3-hydrazino tautomers 3), useful intermediates in the synthesis of bicyclic pyridazine derivatives, were prepared in satisfactory yields in the reaction of the corresponding alkyl 3-substituted 3-cyanopropionates with hydrazine hydrate.