The Witkop Cyclization: A Photoinduced CH Activation of the Indole System

Investigations of excited‐state redox processes have an exceptional impact on the field of organic photochemistry and its application to the synthesis of complex target molecules. In such a photochemical process a single‐electron transfer takes place to produce ion‐radical intermediates, if the reduction and oxidation potentials, as well as excited‐state energies of electron donors and acceptors are chosen appropriately. The Witkop cyclization constitutes an intramolecular variant of such a process, typically with an indole heterocycle as an electron donor. The specific synthetic value of this reaction lies in a CC bond formation without requiring any prefunctionalization of the indole system. Although this photoreaction has been applied to the total synthesis of natural products, it has still not been used to its full capacity. The following review details synthetic efforts using the Witkop cyclization, and aims to incite further applications of reaction in the synthesis of complex molecular architectures. To wit: The title reaction resembles a photoinduced electron‐transfer process, and allows the direct formation of medium‐sized lactams by CH activation of the indole nucleus. Therefore it is a versatile tool for the construction of polycyclic indole alkaloid scaffolds.