Enantioselective aza-Diels-Alder reaction of aldimines with Danishefsky-Type Diene catalyzed by chiral scandium(III)-N,N′-dioxide complexes

A new kind of complex prepared from scandium(III) triflate and L-proline-derived N,M-dioxides has been developed to catalyze the enantioselective aza-Diels-Alder reaction between 1,3-butadiene (diene 1) and aldimines 2, affording the corresponding 2,5-disubstituted dihydropyridinones in moderate to high yields (up to 92%) with good enantioselectivities (up to 90% ee) at room temperature. A variety of aldimines including aromatic, heteroaromatic, conjugated, and aliphatic imines were found to be suitable substrates. Enantiopure samples (up to 99% ee) were obtained for some products by a single recrystallization. The absolute configuration of the products was determined by X-ray diffraction and CD analysis. On the basis of the investigation of H-1 NMR spectra and the positive nonlinear effect, the catalyst structure was carefully discussed.