Light-Driven [2 + 2] Cycloaddition Strategies for the Synthesis of Cyclobuta[b]indoles and Their Derivatives

The dearomatization of indoles is a powerful strategy for transforming simple substrates into architecturally complex molecular frameworks. This review provides a comprehensive overview of photoinduced methodologies developed for the construction of three-dimensional polycyclic systems bearing indolyl-cyclobutane motifs. We summarize classical ultraviolet(-driven [2 + 2] cycloaddition reactions alongside recent advances in photocatalysis, including sensitizer- or photocatalyst-mediated energy-transfer and single-electron transfer processes. Attention is also given to photoinduced hetero-[2 + 2] cycloadditions, specifically the Büchi and aza-Büchi reactions, as well as photodimerization pathways of indole derivatives. Together, these approaches provide valuable and versatile tools for the rapid assembly of complex molecular architectures and drug-like scaffolds.