Pyridazin-3-ones, 2-aryl-5-hydroxy

Maleic anhydride and hydrazine give the hydroxy-pyridazinone ( maleic hydrazide ) directly, " the additional unsaturation in the 1,4-dicarbonyl component meaning that an oxidative step is not reqnired conversion of 3-hydroxypyridazin-6-one into 3,6-dichloropyridazine makes this useful intermediate very easily available. Mucohalo acids (18.1.1.4), synthons for 4-carboxy-aldehydes, are an oxidation level down and produce l-aryl-pyridazin-3-ones on reaction with arylhydrazines. ... 

The synthesis of pyridazine derivatives from hydrazones includes the thermal cyclization of 2-arylhydrazono-3-oxo-5-phenyl-4-pentenenitriles (readily obtained from ethyl cinnamate by condensation with acetonitrile followed by Japp-Klingemann type reactions) to l-aryl-3-cyano-6-phenyl-5,6-dihydro-4(l//)-pyridazinones (Scheme 85) <86JHC93>, and base-induced cyclization of a hydrazone of a 4-chloro-l-arylbutan-l-one to prepare a 2,3,4,5-tetrahydropyridazine (Scheme 85) <88JHC1543>. An earlier route to 6-carboxy-5-hydroxy-2-phenyl-3(2//)-pyridazinone via condensation of benzenediazonium chloride and dimethyl acetonedicarboxylate has been adapted to give a series of aryl derivatives either as esters (by thermal cyclization) or as acids (by cyclization with hydroxide). Both cyclizations proceed in high overall yield (Scheme 86) and decarboxylation of the acids also proceeds in high yield <89JHC169>. 

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