Quinolone analogues 6 [1‐5]. Synthesis of 3‐halogeno‐1‐methylpyridazino[3,4‐b]quinoxalin‐4(1H)‐ones

The reaction of the quinoxaline N ‐oxides 7a,b with diethyl ethoxymethylenemalonate gave the 1‐methylpyridazino[3,4‐ b ]quinoxaline‐4,4‐dicarboxylates 8a,b , whose reaction with N ‐bromosuccinimide or N ‐chlorosuccinimide afforded the 3‐halogeno‐1‐methylpyridazino[3,4‐ b ]quinoxaline‐4,4‐dicarboxylates 9a‐d. The reaction of compounds 9a‐d with hydrazine hydrate resulted in hydrolysis and decarboxylation to provide the 3‐halogeno‐1‐methylpyridazino[3,4‐ b ]quinoxaline‐4‐carboxylates 10a‐d , whose reaction with nitrous acid effected oxidation to furnish the 3‐halogeno‐4‐hydroxy‐1‐methylpyridazino[3,4‐ b ]quinoxaline‐4‐carboxylates 11a‐d , respectively. The reaction of compounds 11a‐d with hydrazine hydrate afforded the 3‐halogeno‐1‐methylpyridazino[3,4‐ b ]quinoxalin‐4‐ols 12a‐d , whose oxidation provided the 3‐halogeno‐1‐methylpyridazino[3,4‐ b ]quinoxalin‐4(1 H )‐ones 6a‐d , respectively. Compounds 6a‐d had antifungal activities in vitro.