Tricyclic marine alkaloids: synthetic approaches to cylindricines, lepadiformine, and fasicularin

Cylindricines, lepadiformine, and fasicularin are marine alkaloids with a common novel pyrrolo- and pyrido[1,2-j]quinoline skeleton. They have recently been isolated from various tunicates and have shown interesting cytotoxic effects which could be attributed to covalent interactions with DNA, as well as cardiovascular effects. The promising biological activities together with the unique structural features make these alkaloids interesting targets for natural product synthesis. This review focuses on the different approaches developed for their synthesis with a particular emphasis on the key step where the quaternary amino substituted carbon center is created.