Dihydro Derivatives Containing a Carbonyl Group in the Ring

2 Dihydro Derivatives Containing a Carbonyl Group in the Ring 

3 Dihydro Derivatives without a Carbonyi Group in the Ring 

The role of carbonyl compounds is dominant in the chemistry of dihydro derivatives of pyridazines. Dihydro-3 (2/ -pyridazinone derivatives are more stable than the corresponding simple dihydropyridazine derivatives and constitute the majority of dihydro compounds reported. The analogous derivatives are of course not possible in the phthalazine and cinnoline series, though 1,4-dihydro-3 (2/ -cinnolinones are known. 

In an unusual reaction, treatment of 2-benzyl-6-phenyl-4,5-dihydro-3(2//)-pyridazinone with a mixture of phosphorus oxychloride and pentachloride resulted in debenzylation, aromatization, and chlorination (Equation (16)). The presence of the benzyl group in the substrate is important, as the dichloropyridazine product is only formed in very low yield from the corresponding reaction of 6-phenyl-4,5-dihydro-3(2 0 Pyt dazinone 83JHC1473 . 

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