Synthesis of Cyclic Alkenyl Triflates by a Cationic Cyclization Reaction and its Application in Biomimetic Polycyclizations and Synthesis of Terpenes.

Cyclic alkenyl triflates are useful intermediates in organic synthesis usually synthesized from ketones through a reaction involving enolization and trapping with a triflating agent. This sequence suffers from some stereochemical drawbacks owing to the basic conditions required. Herein, we describe a new acid-mediated cationic cyclization reaction of enyne derivatives (or alkynols) to access cyclic alkenyl triflates. This new atom-economical process is high yielding, scalable, technically very simple, proceeds without the need of any metallic reagent or catalyst, and more importantly, it complements and challenges conventional methodologies. We have also developed new biomimetic cationic cyclization reactions to yield interesting polycyclic compounds. As a demonstration of the potential of this method in the context of total synthesis, we have synthesized two terpenes: austrodoral and pallescensin A. Using the cationic cyclization in the key step of the synthetic routes allowed the synthesis of these natural products in a very simple, concise, scalable, and efficient way.