Synthesis and Some Reactions of 6H-Indolo[2,3-b]quinoxalines

6H-Indolo[2,3-b]quinoxalines were synthesized by condensation of isatin and 5-nitroisatin with o-phenylenediamine. The alkylation of the title compounds with dimethyl sulfate gave the corresponding quaternary salts which were converted to perchlorates and oxidized with potassium hexacyanoferrate(III) to previously unknown 5-methyl-6H-indolo[2,3-b]quinoxalin-3-one and its 9-nitro derivative. Treatment of 6H-indolo[2,3-b]quinoxalin-3-ones with dimethyl sulfate afforded 3-methoxyindoloquinoxalinium salts which were demethylated by the action of hydrobromic acid at 130–140°C. The structure of the isolated compounds was confirmed by IR and NMR spectra and elemental analyses.