The carbocation continuum in terpene biosynthesis--where are the secondary cations?

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.