Highly Stereoselective Assembly of α-Carbolinone Skeletons via N-Heterocyclic Carbene-Catalyzed [4 + 2] Annulations

A series of iminoindoline-derived alkenes was found to be a new class of excellent aza-diene electrophiles in NHC-catalyzed asymmetric [4 + 2] cyclizations. This transformation is mainly characterized by excellent compatibility, which allows aza-diene substrates to incorporate various substituents and functionalities, including (hetero)­aryl, (linear or branched)­alkyl, alkenyl, alkynyl, and ester groups. Forty examples of the desired tetrahydro-α-carbolinones were facilely synthesized using this method, with up to 99% yield and >99% ee.