2.3- Disubstituted quinoxalines, formation

The reaction between aromatic 1,2-diamines and 1,2-diketones affords 2,3-disubstituted quinoxaline derivatives (see table below) " however, a number of hindered a-ketones fail to react. " Unsymmetrical diketones give mixtures of isomers whose ratio varies with the acidity of the medium in some examples. A mechanistic study of quinoxaline formation from benzene-1,2-diamine and benzil has been described." ... 

Photochemical decomposition of 2-azido-l-methoxyphenazine forms singlet and triplet nitrenes, which are trapped with the azide leading in acetonitrile at —20°C to the dimeric product 3-[3-[[(l-methoxy-2-phenazinyl)imino]methoxymethyl]-2-quinoxalinyl]-2-propenenitrile <9lJOC6993>. Higher reaction temperatures result in the additional formation of aminophenazines and other 2,3-disubstituted quinoxalines <87J0C1245>. 

A study of the scope of this reaction has been performed which shows that excess phenylhydrazine will convert the disubstituted quinoxalines 19 into the pyrazolo[3,4-h]quinoxaline 20. Analogously hydroxylamine gave the isoxazolo compound 21 from these starting materials. The compounds 19 (R = Me or Ph) did not undergo cyclization with phenylhydrazine. A reaction mechanism has been suggested (see Scheme 1) which is analogous to osazone formation in sugar chemistry. 

One-pot S5mthesis of both 1,2-disubstituted benzimidazoles and quinoxalines has been carried out in water at room temperature with excellent yields [69]. In the presence of trimethylsilyl chloride, o-phenylenediamine gives benzimidazole with aldehyde and quinoxaline with a-bromoketone. Isotope-labeling experiment carried out in the reaction with benzaldehyde and o-phenylenediamine demonstrated the 1,3-hydride migration process during the formation of 1,2-disubstituted benzimidazoles (Scheme 47). 

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