Biomimetic Cyclizations of Functionalized Isoprenoid Polyenes: A Cornucopia of Synthetic Opportunities

The synthetic aspects of biomimetic cyclizations of isoprenoid polyenes, simulating one of the key steps in the biosynthesis of terpenoids in living organisms, have been reviewed, with emphasis on transformations carried out on carbocyclic systems. These reactions are made possible by a clever choice of initiating and terminating units of cyclization, and by introducing suitable functional groups on the acyclic precursor which can be manipulated after cyclization. Complex functionalized polycyclic structures are thus assembled with an efficiency paralleling or even superior that of purely biochemical processes. In particular, synthetic strategies towards alkyl and hydroxyl-substituted compounds have been examined in detail. The last part of the review has been dedicated to a thorough illustration of carbenium ion initiated polyene cyclizations, with their synthetic applications and a discussion of the different mechanisms proposed.