Pyridazine 1,2-dioxides

Phototransformation of pyridazine 1,2-dioxides sharply contrasts with that of pyridazine 1-oxides. Pyridazine 1,2-dioxide derivatives give 3a,6a-dihydroisoxazolo[5,4- f]isoxazoles (53) through postulated bisiminoxyl radicals. 3,6-Diphenylpyridazine 1,2-dioxide gives, besides the corresponding bicyclic derivative (53), 3-phenylisoxazole (54) and 4,5-diphenyl-furoxan (55). The last two products can be explained by generation of the nitrile oxide from the intermediate (53) with subsequent dimerization to the furoxan (55 Scheme 18) (79T1267). 

Syntheses of this type have been reported only recently (78H(9)1367, 79JOC3524). Unsaturated 1,4-dioximes are transformed by oxidative cyclization into pyridazine 1,2-dioxides... 

Scheme 59). If phenyliodoso bistrifluoroacetate is used as the oxidizing agent the major product is an isoxazoloisoxazole, but pyridazine 1,2-dioxides are formed in minor amounts together with other products. With lead tetraacetate, in general, pyridazine 1,2-dioxides are the major products. 

Other heterocycles which rearrange to isoxazoles are pyridazine 1,2-dioxides (77CC856) and pyridinium salts (80CPB2083), although these transformations are of little synthetic importance. 

Pyridazine 1,2-dioxides undergo photoisomerization to 3a,6a-dihydroisoxazolo[5,4-d]isoxazoles. The proposed mechanism involves cleavage of the N—N bond to give bis(im-inoxyl) radicals which rearrange and recyclize (equation 65) (79T1267). 

Oxidation of 1,4-disubstituted 2-butene-1,4-dione dioximes with lead tetraacetate or phenyliodoso bis(trifluoroacetate) leads to mixtures containing 3,6-disubstituted dihy-droisoxazolo[5,4-c ]isoxazoles and 3,6-disubstituted pyridazine 1,2-dioxides (equation 78)... 

Pyridazine 1,2-dioxides can be prepared in poor yield under forcing condition (68TL1855 70CPB1211). 

Pyridazines were also formed from dioximes by oxidative cyclization [78H(9)1367 79JOC3524 82M118]. As oxidants, lead tetraacetate or phe-nyliodoso bis(trifluoroacetate) were used, and substituted pyridazine 1,2-dioxides 55 are obtained accompanied by isoxazoles, isoxazoloisoxazoles, and open chain compounds. It was also established that a compound to which the structure of 3,6-diphenylpyridazine 1,2-dioxide was previously assigned is, in fact, a dihydroisoxazoloisoxazole derivative (79JOC3524). 

This section covers the deoxygenation of compounds containing the structural units (24) and (25). Compounds of the first type include not only aliphatic and aromatic azoxy compounds, but also heterocyclic compounds, such as pyridazine /V-oxides those of the second type include dimers of nitroso compounds and heterocycles, such as pyridazine 1,2-dioxides. 

Pyridazine 1,2-dioxides were first obtained in 1968 in poor yield by direct N-oxidation. (for other references see Section III,A). Recently, they have... 

Pyridazine 1,2-dioxides can be reduced with hydrogen in the presence of palladized charcoal to monoxides. For example, the dioxide 223 gives a mixture of the isomeric monoxides 224 and 225, but with three moles of hydrogen compound 226 is formed. Pyridazine-3(2H)-one was reduced over platinum to l,4,5,6-tetrahydropyridazine-3(2if)-one. ... 

Polarographic reduction of pyridazines and pyridazine 1,2-dioxide has been studied. " The polarographic behavior of pyridazine is typical of an overall two-electron reduction process. In alkaline solution it is reduced polarographically to 1,4-dihydropyridazine, but in acid solutions this compound gives an open-chain aminoimine. ... 

Few reports on the light-induced reactions of pyridazine 1,2-dioxides have been published. Irradiation of pyridazine 1,2-dioxides, first thought to give... 

The first syntheses of pyridazine 1,2-dioxides have been described,225 in low yield by direct N-oxidation with 50-90% hydrogen peroxide in acetic acid.294 The dioxides are fairly resistant to deoxygenation with phosphorus trichloride. 

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