Pyridazino pyridazine, reactivity

Another pathway which is often used in the preparation of this ring system involves the Diels-Alder reaction. 3,6-Pyridazinedione (139) is known to be exceptionally reactive toward dienes. This dienophile readily condenses with butadiene, 2,3-dimethylbutadiene and coumalic acid to give the respective Diels-Alder adducts (140). 3,6-Pyridazinedione is also reported to decompose with evolution of nitrogen gas at temperatures below 0 °C, giving the pyridazino[l,2-a]pyridazine-l,4,6,9-tetrone (44) as the major product (62JA966). 

The reactivity of pyridazines in Pd-catalyzed reactions was of interest. For example, the Heck alkenylation at C5 of 6-phenyl-3(2//)-pyridazinones was investigated, with the aim of suppressing production of 4-phenyl-6-substituted-2-phthalazinone byproducts <04TL3459>. In another study, the reactivity of 5-iodopyridazin-3(2f/)-ones in Pd-catalyzed reactions was investigated to develop an efficient route to 2,5-disubstituted pyridazin-3(2/f)-ones <04T12177>. Other pyridazine syntheses relied on condensation approaches. Benzo[g]pyridazino[l,2-i>]-phthalazine-6,13-diones 16 and 17 related to certain anthracyclinones were obtained by cycloaddition of 1,3-dienes to benzo[g]phthalazine-l,4-dione <04H(63)1299>, and pyridazine C-nucleosides synthesized by 14-i-2 cyclocondensation of alkynyl C-nucleosides with substituted tetrazines afforded, upon extrusion of a nitrogen atom, pyrrole C-nucleosides in good yields... 

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