Synthesis of New 3-Substituted Thieno[2,3-b]quinoxalines by Sonogashira Coupling and Intramolecular Heck Reactions

Quinoxalines are of interest sincesome natural compounds bearing the quinoxaline unit exhibit high biological activitiessuch as the anti-microbial, anti-inflammatory and anti-cancer ones [1-3].Palladium-catalyzed coupling reactions have become a powerful tool in organic synthesis.In this work, we present a procedure for the synthesis of some new thieno[2,3- b]quinoxalines 3viathe Sonogashira coupling followed by an interamolecular Heck reaction.Treatment of 2-chloro-3-(prop-2-yn-1-ylthio)quinoxaline1with various aryl iodides in the presence of Pd-Cu catalyst afforded 2-(3-arylprop-2-ynylthio)-3-chloroquinoxalines 2 in good-to-high yields. The resulting compoundsunderwent cyclization to 3-substituted thieno[2,3-b]quinoxalines3under the Heck reaction conditions(Figure 1).The products obtained were characterized by theIR, 1H NMR, 13C NMR, and mass spectroscopic techniques. Alsothese quinoxaline derivatives were evaluated for anti-microbial activity against the microorganisms such as Escherichia coli(Gram negative bacteria) and Bacillus subtilis (Gram positive bacteria).