N‑Heterocyclic Carbene-Catalyzed Diastereoselective and Enantioselective Reaction of 2‑Aroylvinylcinnamaldehydes with α,β-Unsaturated Imines: Complete Control and Switch of Diastereoselectivity by <i>N</i>‑Substituents of Catalysts

Highly diastereoselective and enantioselective reactions between 2-aroylvinylcinnamaldehydes and α,β-unsaturated imines were achieved under asymmetric catalysis of chiral triazole carbene catalysts. In the presence of N-anisylindeno­[2,1-b]­triazolo­[4,3-d]­[1,4]­oxazinium salt, the reaction of 2-aroylvinylcinnamaldehydes with α,β-unsaturated imines afforded indeno­[2,1-c]­pyran-1-one derivatives 4 with 90%–99% ee, while enantiopure indenocyclopentan-1-ones 5 (>99% ee) were obtained under the catalysis of N-mesitylindeno­[2,1-b]­triazolo­[4,3-d]­[1,4]­oxazinium salt. A slight variation of an N-substituent on triazole carbenes was found to switch completely the diastereoselectivity of the reaction for the formation of indeno­[2,1-c]­pyran-1-ones.