Scandium triflate and secondary amine promoted AA′B 2:1 coupling and formal inverse electron demand Diels–Alder reactions of dienals

Reaction prospecting studies resulted in the discovery of a Sc(OTf)3 promoted novel AA′B 2:1 coupling of imidazolone or benzofuran substituted enals with morpholine. A related Sc(OTf)3/morpholine promoted reaction was demonstrated for the coupling of a benzofuran substituted enal with dihydrofuran. The formation of these adducts is consistent with formal inverse electron demand Diels–Alder cycloadditions, most likely in a stepwise manner via a domino Michael–Mannich annulation process, involving iminium ion activation of the diene. A purely amine promoted formal inverse electron demand Diels–Alder cycloaddition approach was also demonstrated in the 2:1 coupling of 2,4-hexadienal with methanol. These reactions demonstrate the concepts of dual metal/amine catalysis and amine promoted formal inverse electron demand Diels–Alder cycloadditions. They differ from known examples of organocatalyzed Diels–Alder reactions, in which iminium ion activation of the dienophiles or enamine activation of the dienes occur.