Ring transformations forming pyridazines

Several important examples of ring transformation are referred to elsewhere in this chapter. These include the extrusion of nitrogen from tetraazaphenanthridines and S02 from dipyridothiophene dioxides to yield diazabiphenylenes (Section 7.23.8.1.1), the benzylic-type ring contraction of phen-anthridine diones to give diazafluorenones (Section 7.23.8.2.1), and the extrusion of nitrogen from the Diels-Alder adducts formed from enamines and pyridazo[4,5-d]pyridazines (Section 7.23.8.3.2). 

Acyclic dialkylamines are not very reactive as nucleophiles in the oxidative alkylamination, however they are very prone to oxidation and, therefore, they are capable for unexpected behavior. Presumably, transformation 33 41 starts from oxidation of dialkylamme into imine 42, that is in equilibrium with enamine 43 (Scheme 28). The latter, as bifunctional C,W-nucleophile, attacks C-4 atom of the pyridazine ring to form cr -adduct 44, which then undergoes oxidative aromatiza-tion. Subsequent intramolecular oxidative amination of the intermediate 45 yields pyrrole derivative 41. The participation of imines in this process has been confirmed experimentally. In the presence of AgPy2Mn04, pyrimidopyridazine 33 reacts with authentic aldimines and ketimines 42 to give pyrroles 41. Transformation 33 41 represents not only a rare example of the tandem processes but... 

Several transformations of pyrazoles into pyridazines have been reported. In a carbene ring expansion, 3,4,5-trimethylpyrazole was treated with dichlorocarbene under neutral conditions. A very small amount of a pyridazine (51) together with a pyrimidine and two pyrazole derivatives were obtained. Under basic conditions no pyridazine was formed and the major product was the pyrazole 52, which could be rearranged with sodium ethoxide at 125° into the pyridazine 53. Pyridazines are formed also from certain pyrazoles upon heating, for example 54, - or from some pyrazolines. ... 

In the last decade new reactions have been elaborated in which pyridazines can be transformed into a variety of other heterocycles. Esters of pyridazine-carboxylic acids and 1-diethylaminopropyne undergo cycloaddition reactions with inverse electron demand. The orientation of the two reactants depends on the position of the carboxylate group in the pyridazine ring. For example, from methyl pyridazine-4-carboxylate the pyridine derivative 245 was formed via the adduct 244 and subsequent elimination... 

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