Cascade Energy-Transfer-Mediated [2 + 2] Cycloaddition/Ring-Expansion Rearrangement of Indole Derivatives via Vinylcyclobutane Intermediates

Vinylcyclobutane-based rearrangements hold great potential for molecular reorganization, yet their application in cascade reactions remains largely underexplored. Herein, we report a cascade energy-transfer-catalyzed rearrangement involving vinylcyclobutanes derived from indole frameworks, enabling the synthesis of pharmaceutically relevant carbazole derivatives. Mechanistic investigations confirm that the reaction proceeds through a cascade energy-transfer pathway involving sequential excitation of both the starting material and the in situ-generated cyclobutane intermediate. This work offers valuable insights into the design of vinylcyclobutane-based cascade transformations and the construction of architecturally complex polycyclic scaffolds.