Enantioselective Synthesis of Benzofuran-Fused N‑Heterocycles via Chiral Squaramide Catalyzed [4 + 2] Cyclization of Azadienes with Azlactones

An asymmetric cyclization reaction of azadienes and azlactones was investigated by employing a Cinchona squaramide catalyst, which could afford a series of benzofuran-fused six-membered heterocycles containing a α,α-disubstituted amino acid unit in a highly diastereoselective (>20:1 dr) and enantioselective (up to 99% ee) manner with good to excellent yields (up to 92%). A plausible pathway was proposed to explain the reaction process.