Quinolone analogues 5.: Synthesis of 1-methylpyridazino[3,4-b]quinoxalin-4(1H)-ones

The reaction of the quinoxaline N-oxide (8a) with diethyl ethoxymethylenemalonate gave the 1,4-dihydropyridazino[3,4-b]quinoxaline-4,4dicarboxylate (10c), whose reaction with a base afforded the 1,5-dihydropyridazino[3,4-b]quinoxaline-4-carboxylate (6a). The oxidation of compound (6a) with nitrous acid provided the 1,4-dihydro-4-hydroxypyridazino[3,4b]quinoxaline-4-carboxylate (7), whose reaction with potassium hydroxide gave 7-chloro-1-methylpyridazino[3,4-b]quinoxalin-4(1H)-one (5a). On the other hand, the reaction of the quinoxaline N-oxide (8 b) with acetylacetaldehyde dimethyl acetal afforded 4-acetyl-1,5-dihydro-1-methylpyridazino[3,4-b]quinoxaline (6b), whose oxidation with selenium dioxide provided 1-methylpyridazino[3,4-b]quinoxalin4(1H)-one (5 b).