The propensity of terpenes to invoke concerted reactions in their biosynthesis

Recent advances in the understanding of mechanistic aspects of terpene biosynthesis is primarily due to computational chemistry ranging from DFT to QMMM MD. In such a way, it has been determined that terpene biosynthetic pathways make extensive use of multi-step concerted reactions that involve carbocation intermediates, including cyclizations, alkyl and [1,2]hydride shifts. The rearrangements that occur in the mechanism of the bisabolyl carbocation to form a number of terpene products involve a [1,2]hydride shift followed by two cyclizations each of which produces a new cyclopentane rings. It was found earlier, using the B3LYP functional, that while two rings were formed via a typical, concerted reaction, the hydride shift did not, but instead an intermediate was produced along the pathway. Being aware of the deficiencies in treating the formation of carbocycles with the B3LYP functional, the two pathways of conformers of the bisabolyl carbocation were revisited here, applying the M062X functional which includes dispersion. In both analyzed cases the [1,2]hydride shift occurred in concert with the two subsequent cyclizations.