Synthesis of Cyclobutane-Fused Angular Tetracyclic Spiroindolines via Visible-Light-Promoted Intramolecular Dearomatization of Indole Derivatives

An
intramolecular dearomatization of indole derivatives based on
visible-light-promoted [2+2] cycloaddition was achieved via energy
transfer mechanism. The highly strained cyclobutane-fused angular
tetracyclic spiroindolines, which were typically unattainable under
thermal conditions, could be directly accessed in high yields (up
to 99%) with excellent diastereoselectivity (>20:1 dr) under mild
conditions. The method was also compatible with diverse functional
groups and amenable to flexible transformations. In addition, DFT
calculations provided guidance on the rational design of substrates
and deep understanding of the reaction pathways. This process constituted
a rare example of indole functionalization by exploiting visible-light-induced
reactivity at the excited states.