A Novel Sc(OTf)3‐Catalyzed (2+2+1)‐Cycloannulation/Aza‐Friedel–Crafts Alkylation Sequence toward Multicyclic 2‐Pyrrolines

The rapid assembly of molecular complexity continues to be at the forefront of novel reaction development. In the pursuit of that goal, we herein report a novel Sc(OTf)3‐catalyzed, one‐pot multicomponent reaction that furnishes complex multicyclic 2‐pyrrolines with excellent overall yields and perfect diastereocontrol. This process is based on our previously established (2+2+1)‐cycloannulation of in situ generated 1‐azaallyl cations, 1,3‐dicarbonyls and primary amines. The newly formed and highly reactive aminal moiety is readily substituted with indoles and pyrroles both as external and internal π‐nucleophiles to provide densely functionalized N‐heterocycles with four new σ‐bonds and two vicinal quaternary stereogenic centers. In addition, DFT calculations have been conducted to further characterize the intermediate 1‐azaallyl cations.