Synthesis of 4H-1,3,4-oxadiazino[5,6-b]quinoxalines from 2-substituted quinoxaline 4-oxides

The reaction of 6-chloro-2-(1-methylhydrazino)quinoxaline 4-oxide 8 with acetic anhydride resulted in the intramolecular cyclization to give 8-chloro-2,4-dimethyl-4H-1,3,4-oxadiazino[5,6-b]quinoxaline 7a, while the reaction of compound 8 with acetic anhydride/pyridine or acetic anhydride/acetic acid afforded 3-(2,2-diacetyl-1-methylhydrazino)-7-chloro-2-oxo-1,2-dihydroquinoxaline 9, effecting no intramolecular cyclization. The reaction of 2-(2-acetyl-1-methylhydrazino)-6-chloroquinoxaline 4-oxide 10a or 6-chloro-2(1-methyl-2-trifluoroacetylhydrazino)quinoxaline 4-oxide 10b with phosphoryl chloride provided compound 7a or 8-chloro-4-methyl-2-trifluoromethyl-4H-1,3,4-oxadiazino[5,6-b] quinoxaline 7b, respectively. The reaction of compound 7b with phosphorus pentasulfide gave 7-chloro-3-(1-methyl-2-trifluoroacetylhydrazino)-2-thioxo-1,2-dihydroquinoxaline 11, whose dehydration with sulfuric acid in acetic acid afforded 8-chloro-4-methyl-2-trifluoromethyl-4H-1,3,4-thiadiazino [5,6-b]quinoxaline 12.