Pyridopyridazines, synthesis

The only example of a simple pyridopyridazine synthesis of this kind from a pyridine intermediate involves a variant of a well known cinnoline synthesis in which the Japp-Klingemann intermediate (363) gives the pyrido[3,4-c]pyridazine (364) with PPA (69JHC977). 

No examples of this type of synthesis have been noted in the pyridopyridazine series. 

Microwave-assisted solvent-free synthesis of pharmacologically important pyrido fused-ring systems has recently been accomplished and is an improved method for assembly of a variety of pyridopyridazine and quinoline derivatives. Benz-1,3-oxazine formation has also been investigated in dry media using AI2O3-KF as a solid base (Scheme 8.75) [187]. 

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