Synthetic Approaches to Tetracyclic Indolines as Versatile Building Blocks of Diverse Indole Alkaloids.

Indole alkaloids bearing the tetracyclic indoline scaffolds of 1H-pyrrolo[2,3-d]carbazole have shown fascinating chemical diversity and significant biological activities. The development of efficient synthetic methodologies for such a tetracyclic scaffold remains highly desirable in both synthetic chemistry and medicinal chemistry. This review outlines key strategies for the construction of the tetracyclic indoline scaffolds in total syntheses of many indole alkaloids. The key strategies include nucleophilic additions, Diels-Alder reactions, radical cyclizations, and palladium-catalyzed coupling reactions. The representative examples and their applications in the total syntheses are described here and discussed in depth.