Modern Annulation Strategies for the Synthesis of Cyclo[ b ]fused Indoles

2,3-Annulated indoles exhibit a broad spectrum of biological activities. Various annulation strategies are applied to generate these scaffolds from prefunctionalized aniline or indole derivatives. Only a few methodologies allow the direct annulation of indole itself, often associated with regioselectivity issues or restrictions on available substitution patterns. More recently, ruthenium-catalyzed cascade transformations of readily available propargyl alcohols have been applied to the selective synthesis of various cyclo[ b ]fused indoles directly from indole. These efficient processes provide rapid access to intricate molecular structures from simple starting materials and facilitate the preparation of drug-like molecules.