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Herein, we develop a new approach to directly access architecturally complex polycyclic indolines from readily available indoles and bicyclo1.1.0butanes (BCBs) through formal cycloaddition promoted by commercially available Lewis acids. The reaction proceeded through a stepwise pathway involving a nucleophilic addition of indoles to BCBs followed by an intramolecular Mannich reaction to form rigid indoline-fused polycyclic structures, which resemble polycyclic indole alkaloids. This new reaction tolerated a wide range of indoles and BCBs, thereby allowing the one-step construction of various rigid indoline polycycles containing up to four contiguous quaternary carbon centers.
Ni et al. (Wed,) studied this question.