Metal-Free Intramolecular Carbonyl-Olefin Metathesis of ortho-Prenylaryl Ketones

On treatment with boron trifluoride etherate as a Lewis acid, acetophenone and benzophenone derivatives bearing a prenyl (or a related) side chain in ortho-position were shown to undergo intramolecular carbonyl-olefin metathesis, in the absence of any transition-metal catalyst. The cationic cyclization process is supposed to proceed via an oxetane intermediate, which fragments to give the cyclization product (indene or 1,2-dihydronaphthalene) and a ketone (acetone) as a stoichiometric by-product. Several substrates were shown to afford the metathesis products with up to 93% yield.