Hydroxycinnolines oxidation

Hydroxycinnoline (LXXVI) [101] in acid solution is reducible in two, two-electron steps. The product from the first two-electron reduction is 4-keto-l,2,3,4-tetrahydrocinno-line (LXXVII). This compound is a derivative of phenylhydrazine and can thus be oxidized anodically in alkaline solution to an azo compound (LXXVIIT), which then tautomerizes to LXXVI. The carbonyl group of LXXVII is activated by the phenyl ring and—in contrast to 3-keto-l,2,3,4-tetrahydrocinnoline—is thus further reducible the reduction yields 4-hydroxy-1,2,3,4-tetrahydrocinnoline (LXXIX). On standing, this compound loses water with the formation of 1,4-dihydrocinnoline. If, however, the compound is oxidized in alkaline solution, the product (LXXX) loses water and forms cinnoline. 

Other 1,2-diazines synthesized included 4-amino-3-cinnolinecarboxylic acids <97PHA91 >, I -aryl-6-chloro-l,4-dihydro-4-oxothieno[2,3-c]pyridazine-3-carboxylic acids <97JPR284>, 2H-benzo[2,3-g]pyridazino[4,5-<7,e]quinolin-3-ones <97M681>, 3-chloro-4-carbamoyl-5-aryl-6-methylpyridazine N-oxides <97F67>, 2-aroyl-6-(hetero substituted)-3(2//)-pyridazinones <97HCM267>, and 5-(4-hydroxycinnolin-3-yl)tetrazoles 2-methyl-5-(4-acetoxycinnolin-3-yl)-... 

Note. These reactions of acylcinnolines have been covered already reduction to alkylcinnolines (Section 2.2.1), some cyclocondensations (Section 4.1.2.2), and reduction to extranuclear hydroxycinnolines (Section 4.2). Their oxidation to cinnolinecarboxylic acids appears to be unrepresented in the more recent literature. Some other reactions are illustrated in the examples that follow. 

Lithium tetrahydridoaluminate/mercuric oxide Cinnolines from 4-hydroxycinnolines s. 16, 137 Li[AlHJ/HgO... 

Lithium tetrahydridoaluminatelmercuric oxide Cinnolines from 4-hydroxycinnolines... 

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