Direct annelation of six-membered heterocycles with a quinoxaline ring
In previous communications we have shown that N-alkylpyrazinium and quinoxalinium cations as such, without introduction of any substituents, are also applicable for the synthesis of condensed pyrazines and quinoxalines with annelated furan, pyrrole, imidazole, thiazole, and other five-membered rings [4]. These cyclizations are based on the diaddition of 1,3-bisnucleophiles and lead to hydrogenated analogs of those pyrazines and quinoxalines that can be obtained in cyclizations with replacement of chlorine atoms in the 2 and 3 positions of the pyrazine ring.