Domino reactions of thiopyrano[4,3-b]indole-3(5H)-thiones and dimethyl acetylenedicarboxylate: Quantum chemical investigation and experiment

Reaction of thiopyrano[4,3-b]indole-3(5H)-thiones and dimethyl acetylenedicarboxylate (DMAD) proceeds via two competing cascade pathways. Initially, both the pathways begin from thiocarbonyl sulfur and acetylene carbon atoms interaction. Then two parallel processes take place: an alkyne–thiocarbonyl metathesis and a (3 + 2) cycloaddition. In the next stages, in both cases, thiophene ring formation and thiopyran ring opening proceed. Finally, (4 + 2) cycloaddition reactions of intermediate thioketones and a second equivalent of DMAD leads to the resulting thiopyrano[4,3-b]indole derivatives bearing thienyl substituent. The kinetic and thermodynamic characteristics of both pathways were compared on the basis of DFT and ab initio 6-311++G(d,p) quantum chemical calculations.