Indium(III) triflate-catalysed [4 + 2] benzannulation reactions of o-alkynylbenzaldehydes with enolisable carbonyl compounds: selective synthesis of naphthyl ketones.

Indium(III) triflate is found to be an effective catalyst for the benzannulation reactions of o-alkynylbenzaldehydes with enolisable aldehydes, ketones, 1,3-diketones and β-keto esters. The reactions produce naphthyl ketones through ring opening of adducts arising from the inverse electron demand Diels-Alder reactions between in situ generated isochromenylium cations and enols. The main feature of the method is selective formation of naphthyl ketones in a highly regioselective manner without any decarbonylation.