First total syntheses of dihydroflustramine C and flustramine E, alkaloids from the marine bryozoan Flustra foliacea

We have developed a simple and practical method for providing the common tricyclic skeleton of physostigmine type alkaloids, and demonstrated its utility for indole alkaloid synthesis. Thus, we achieved the first total syntheses of (±)-dihydroflustramine C and (±)-flustramine E, as well as the total syntheses of their debromoanalogues from the corresponding 2-hydroxyindolenines in five steps with 31, 27, 39 and 23% overall yields, respectively. The structures of some intermediates were confirmed by single crystal X-ray diffraction analyses.