Theoretical study on the formation of a pentacyclo-undecane cage lactam

► The multistep mechanism of the pentacyclo-undecane (PCU) cage lactam was investigated with DFT. ► B3LYP/6-31 + G(d) calculations in gas phase and with the SCRF solvent model is reported. ► Kinetic and thermodynamic properties of all species involved was investigated. ► The final product obtained is both thermodynamically and kinetically preferred. ► The observed rate determining step compared favourably to the experimentally observed intermediate.