Catalindoles A–C: brominated indole alkaloids from the starfish Thromidia catalai, which feeds on Theonella sponges

Marine sponges harbor diverse secondary metabolites with a variety of biological activity, many of which are considered as chemical defense substances for the sponges themselves. Recently, we observed that Thromidia catalai, one of the largest species of starfish, feeds on the sponge Theonella swinhoei (chemotype Y), known to contain diverse biologically active metabolites. Here we report the isolation of three novel brominated indole alkaloids, catalindoles A–C, from the arms of T. catalai as well as the elucidation of their structures. Their structures were determined by a combination of spectroscopic data analyses and chemical methods, and they were further confirmed by chemical syntheses of the molecules. Moreover, we revealed the predator–prey relationship between T. catalai and Theonella sponges in terms of chemical components, noting that T. catalai feeds not only on T. swinhoei (chemotype Y) but also on T. swinhoei (chemotype W).