By Cleavage of a Second fused Ring

The synthesis of pyridazines by cleavage of a second fused ring was not discussed in CHEC-I, but general examples of this approach include oxidative degradation of benzene rings of benzohet- 

A hydrogenolysis-hydrolysis sequence applied to isoxazolo[4,5-r/]pyridazin-4(5//)-ones and to 7-methoxy-3-methylisoxazolo[4,5- /]pyridazine gives moderate to good yields of novel 4-acyl- 

Reduction of diaryloxadiazolo[3,4-d]pyridazines with borohydride has been used as a route to symmetrically substituted 3,6-diaryl-4,5-diaminopyridazines (Equation (42)) 92JHC87 . 

3-Amino-4-carboxamidopyridazine can be obtained in high yield by aqueous base hydrolysis of 4(3//)-pyrimido[4,5-c]pyridazinone (Equation (43)). The pyrimido[4,5-c]pyridazinone, which is readily prepared from 6-hydrazino-4(3/ -pyrimidinone by condensation with glyoxal sodium bisulfite followed by treatment with trifluoroacetic acid, is even hydrolyzed in boiling water. This lability is shared by some derivatives 85JOC346 . 

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