Covering: up to the end of 2010 This review describes applications of quantum chemical calculations in the field of terpene biosynthesis, with a focus on insights into the mechanisms of terpene-forming carbocation rearrangements arising from theoretical studies.

The existence of post-transition state bifurcations on potential energy surfaces for organic and biological reaction mechanisms has been known for decades, but recently, new reports of bifurcations have been occurring at a much higher rate. Beyond simply discovering bifurcations, computational chemists are developing techniques to understand what aspects of molecular structure and vibrations control the product selectivity in systems containing bifurcations. For example, the distribution of products seen in simulations has been found to be extremely sensitive to the local environment of the reacting system (i.e. the presence of a catalyst, enzyme, or explicit solvent molecules). The outlook for the future of this field is discussed, with an eye towards the application of the principles discussed here by experimental chemists to design a reaction setup to efficiently generate desired products.

In this tutorial review, structures encountered in carbocation cascade polycyclization reactions leading to terpene natural products are surveyed. The nature of delocalization in these carbocations is discussed in detail. For select cases, the ability of functional groups in enzyme active sites to modulate this delocalization is discussed. In addition to carbocation intermediates, cationic transition state structures are also described.