Coupled Electrocyclization/Prototropic Shift in the Biosynthesis of Crotinsulidane Diterpenoids.

The energetic viability of proposed mechanisms for the formation of the five-membered ring in crotinsulidane diterpenes is assessed using density functional theory calculations. A protonation-assisted coupled four-electron electrocyclization/prototropic shift mechanism is predicted to have a low (biologically relevant) barrier. [ABSTRACT FROM AUTHOR]