Highly Stereoselective Formal [3 + 3] Cycloaddition Reactions of Chiral Vinylogous Amides with α,β-Unsaturated Iminiums

Highly stereoselective formal [3 + 3] cycloaddition reactions of chiral vinylogous amides with α,β-unsaturated iminiums are described. A mechanistic model is proposed to rationalize the observed stereoselectivity. The 6π-electron electrocyclic ring closure appears to be reversible, and a preferred rotation of the alkenyl group, one of the three 2π-components, during the ring closure step provides the thermodynamically favored diastereomer as the major product.