Synthesis of 3,4-Fused Tricyclic Indoles Using 3-Alkylidene Indolines as Versatile Precursors

In this personal account, our recent studies of novel synthetic methods of 3,4-fused tricyclic indole derivatives using 3-alkylidene indoline derivatives as versatile precursors are discussed. Two types of cascade reactions producing 3,4-fused tricyclic 3-alkylidene indolines were developed based on a palladium-catalyzed intramolecular Heck insertion to an allene-allylic amination cascade and a platinum-catalyzed intramolecular Friedel-Crafts type C-H coupling-allylic amination cascade. Furthermore, three types of 3,4-fused tricyclic indoles were accessible from a single 3-alkylidene indoline precursor via acid-promoted olefin isomerization or oxidative treatments. The application of the developed methods to the synthesis of natural products bearing a 3,4-fused tricyclic indole skeleton, (-)-aurantioclavine, fargesine, and synthetic studies of dragmacidin E are also highlighted.