A Facile Approach\nto Tetracyclic Indolines: Highly\nDiastereoselective [4+2] Annulation of Indoles with Bicyclic N‑Substituted\nCyclobutanes

A new stereoselective [4+2] annulation method for constructing\ntetracyclic indolines by reacting indoles with bicyclic N-substituted\ncyclobutanes has been developed. Using Sc(OTf)<sub>3</sub> as a catalyst,\na series of tetracyclic indolines with four continued stereogenic\ncarbon centers have been obtained in ≤86% yields as single\ndiastereomers. This reaction offers an accessible way for the rapid\nconstruction of the core structures of biologically active natural\nproducts like paucidirinine, deethylibophyllidine, and ibophyllidine.