Regiospecific Ficini [2 + 2] Cycloaddition of Ynamides with Cyclic Isoimidium Salts under Catalyst-Free Conditions

The regiospecific [2 + 2] cycloaddition of cyclic isoimidium salts with ynamides is described. This effort led to the development of the first successful example of a catalyst-free, thermally driven Ficini [2 + 2] cycloaddition process of ynamides with α,β-unsaturated carbonyl compounds, giving the stable cyclobutenamides in excellent yields (up to 99%).