Quinolone analogues 2.: A facile synthesis of novel 3-substituted 1-alkyl-4-oxo-1,4-dihydropyridazino[3,4-b]quinoxalines

The reaction of the 2-(1-alkylhydrazino)-6-chloroquinoxaline 4-oxides 1a,b with diethyl acetone-dicarboxylate or 1,3-cyclohexanedione gave ethyl 1-alkyl-7-chloro-3-ethoxycarbonylmethylene-1,5- dihydropyridazino[3,4-b]quinoxaline-3-carboxylates 5a,b or 6-alkyl-10-chloro-1-oxo-1,2,3,4,6,12-hexahydroquinoxalino[2,3-c]cinnolines 7a,b, respectively. Oxidation of compounds 5a,b with nitrous acid afforded the ethyl 1-alkyl-7-chloro-3-ethoxycarbonylmethylene-4-hydroxy-1,4-dihydropyridazino[3,4-b]quinoxaline-4-carboxylates 9a,b, whose reaction with base provided the ethyl 2-(1-alkyl-7-chloro-4-oxo-1,4-dihydropyridazino[3,4-b]quinoxalin-3-yl)acetates 6a,b, respectively. On the other hand, oxidation of compounds 7a,b with N-bromosuccinimide/water furnished the 4-(1-alkyl-7-chloro-4-oxo-1,4-dihydropyridazino[3,4-b]quinoxalin-3-yl)butyric acids 8a,b, respectively. The reaction of compound 8a with hydroxylamine gave 4-(7-chloro-4-hydroxyimino-1-methyl-1,4-dihydropyridazino[3,4]quinoxalin-3-yl)-butyric acid 12.