Phakefustatins A–C: Kynurenine-Bearing Cycloheptapeptides as RXRα Modulators from the Marine Sponge Phakellia fusca

A neutral-loss scanning mass method was used to explore new kynurenine-containing cycloheptapeptides, phakefustatins A–C (1–3), from the marine sponge Phakellia fusca. Their structures were elucidated by spectroscopic analysis and the advanced Marfey's method. 1 was total synthesized via a final-stage ozonolysis strategy by the combination of solid/solution-phase synthesis. Phakefustatin A (1) was identified as a RXRα modulator to inhibit cancer cell growth, and its pharmacophores could be Kyn and guanidine groups.