Microwaves in organic synthesis: Synthesis of pyridazinones, phthalazinones and pyridopyridazinones from 2-oxo-arylhydrazones under microwave irradiation

The phenylhydrazones 1a-d condensed with ethyl cyanoacetate to yield pyridazinones 2a-d that reacted with sulphur in presence of piperidine to yield the aminothienopyridazineones 3a,b that reacted with electron poor olefins and acetylenes to yield phthalazines 10-12. The condensed aminothiophenes 3a,b reacted with dimethylformamide dimethylacetal to yield amidines 13a,b. Compounds 2a,b condensed with dimethyl form amide dimethylacetal to yield the trans enamines 16a,b that cyclized readily into the pyridopyridazinones 17a,b on treatment with ammonium acetate in presence of acetic acid. Compounds 2a-d reacted also with benzylidenemalononitrile to yield the phthalazinones 21a-d. The reactions were conducted both by microwave heating and conventional heating. Better yields in much shorter reaction times were achieved by microwave heating.