Stylissamides E and F, Cyclic Heptapeptides from the Caribbean Sponge Stylissa caribica

In addition to the four known stylissamides, A (1), B (2), C, and D, two new cyclic heptapeptides, stylissamides E (3) and F (4), were isolated from the Caribbean sponge Stylissa caribica. The structures of 3 and 4 were elucidated from a combination of mass spectrometric and NMR spectroscopic data as cyclo-(cis-Pro¹-Tyr²-trans-Pro³-Ala⁴-Ile⁵-Gln⁶-Ile⁷) for stylissamide E (3) and cyclo-(trans-Pro¹-cis-Pro²-Phe³-Asp⁴-trans-Pro⁵-Arg⁶-Phe⁷) for stylissamide F (4).