Compelling Computational Evidence for the Concerted Cyclization of the ABC Rings of Hopene from Protonated Squalene

The long-standing question of what is the nature of the cyclization of squalene to form tetracyclic and pentacyclic triterpenes has been addressed computationally. Using the DFT method with an intrinsic reaction coordinate calculation, we find that the first three rings of protonated squalene were formed without the intermediacy of mono- or bicyclic carbocations. The cyclization, calculated in the gas phase, proceeds in a highly asynchronous, concerted reaction to yield the tricyclic, tertiary carbocation with a 5-membered C ring. The fourth double bond of squalene is not properly oriented for the ring expansion of the C ring in concert with the formation of the 5-membered ring.