Condensation of cyclohexanediones with pyrrole under acidic conditions: unusual products and interesting structural features

Abstract

Reaction of the three isomers of cyclohexanediones (1,2-, 1,3- and 1,4-diones) with pyrrole under acidic conditions are explored. The rigid cyclohexane scaffold led to the formation of the bisdipyrromethanes in case of both 1,4- and 1,3-cyclohexanedione, not observed in case of their acyclic analogues (2,5-hexanedione and 2,4-pentanedione respectively). While the former exclusively forms the desired bisdipyrromethane, the latter produced in addition the mono-condensed product along with a 2-alkenylpyrrole derivative and a highly unexpected 5,5-dipropyldipyrromethane, albeit in low yield. Surprisingly, 1,2-cyclohexanedione produced a tripyrrole derivative and a pyrrole substituted bicyclic dihydroindole (a ring annulated product) along with the mono-condensed product. The structure of these products was comprehensively established by various spectroscopic means including single crystal X-ray diffraction analysis.