Using Calcium Carbide as an Acetylene Source for Cascade Synthesis of Pyrrolo[2,3-b]quinoxalines via Copper-Free Sonogashira Coupling Reaction

A palladium-catalyzed cascade protocol has been established for the synthesis of 4-methyl-1-(1H-pyrrolo[2,3-b]-quinoxalin-2-yl)cyclohexanols and 2-phenyl-1-(1H-pyrrolo[2,3-b]quinoxalin-2-yl)propan-1-ols through the reaction of N-alkyl(aryl)-3-chloroquinoxalin-2-amines with calcium carbide and cyclohexanones or 2-phenylpropanal. This one-pot process, carried out without any copper salt in the key step of the Sonogashira coupling reaction, provides an efficient method for the synthesis of 2,3-disubstituted pyrrolo[2,3-b]quinoxalines in the presence of catalytic amounts of Pd(PPh3)(2)Cl-2 in DMSO/H2O with high yields. The benefit of this strategy is the use of a commercially available, inexpensive, and less hazardous primary chemical feedstock, calcium carbide, as an acetylene source in a wet solvent.