Cascade Assembly of 4,5,6,7-Tetrahydroindole from Cyclohexanone Oxime and Acetylene in the KOH/DMSO Superbase Medium: A Quantum Chemical Study.

Pyrrole synthesis from ketoximes and acetylene in the KOH/dimethyl sulfoxide (DMSO) superbase medium (here abbreviated as the KOA reaction) provided access to a wide variety of 2-substituted and 2,3-disubstituted pyrroles from the available starting materials (enolizable ketones and acetylene). All steps of the KOA reaction mechanism are studied, for the first time, in detail at a uniform theoretical level for the cascade assembly of 4,5,6,7-tetrahydroindole from cyclohexanone oxime and acetylene. Our results explain the reasons why some earlier postulated intermediates have not been detected during the reaction. Alternative channels for the formation of intermediates of 3H-pyrroles are considered. The qualitative agreement of the obtained results with kinetic studies is demonstrated.