Accessing Medium-Sized Rings via Vinyl Carbocation Intermediates

Medium-sized rings (8–11-membered cycles) are often more challenging to synthesize than smaller rings (5–7-membered cycles) due to ring strain. Herein, we report a catalytic method for forming 8- and 9-membered rings that proceeds via the intramolecular Friedel–Crafts reactions of vinyl carbocation intermediates. These reactive species are generated catalytically through the ionization of vinyl toluenesulfonates by a Lewis acidic lithium cation–weakly coordinating anion salt.