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Two novel oxazole-thiazole containing cyclic hexapeptides, bistratamides M (1) and N (2) have been isolated from the marine ascidian Lissoclinum bistratum (L. bistratum) collected in Raja Ampat (Papua Bar, Indonesia). The planar structure of 1 and 2 was assigned on the basis of extensive 1D and 2D NMR spectroscopy and mass spectrometry. The absolute configuration of the amino acid residues in 1 and 2 was determined by the application of the Marfey’s and advanced Marfey’s methods after ozonolysis followed by acid-catalyzed hydrolysis. The interaction between zinc (II) and the naturally known bistratamide K (3), a cyclic hexapeptide isolated from a different specimen of Lissoclinum bistratum, was monitored by 1H and 13C NMR. The results obtained are consistent with the proposal that these peptides are biosynthesized for binding to metal ions. Compounds 1 and 2 display moderate cytotoxicity against four human tumor cell lines with GI50 values in the micromolar range.

Three novel cyclic 3-alkylpyridinium salts, named njaoaminiums A, B, and C (1-3), were isolated from the marine sponge Reniera sp., collected off the coasts of Pemba Island, Tanzania. The structural determination of the compounds was based on 1D and 2D NMR studies and mass spectral determinations. Njaoaminiums B (2) and C (3) are the first examples of cyclic 3-alkylpyridinium salts bearing a methyl substituent on the alkyl chains. These compounds are assumed to be biosynthetic precursors of the njaoamines, previously isolated from the same sponge. The absolute configurations of the methyls of 2 and 3 were assigned by comparison between experimental and TDDFT calculated circular dichroism spectra on the most stable conformer. Compound 2 showed weak cytotoxicity against the three human tumor cell lines MDA-MB-231, A549, and HT29.

Two new xenicanes, named protoxenicins A (1) and B (2), were isolated from an organic extract of the soft coral Protodendron repens, collected off the coast of Okuza (Tanzania), being the first chemical study of an organism belonging to this genus. Their planar structures were determined by 1D and 2D NMR and HRESIMS techniques, while the relative configurations were elucidated by comparison of their chemical shifts and coupling constants with the literature values of their congeners, as well as by ROESY experiments, chemical derivatization, and molecular mechanics calculations. This is the first report of a xenicin acylated with a long saturated fatty acid. Furthermore, the absolute configuration of the stereogenic centers of the cyclononane ring and at C-1 in 1 was determined by Mosher's method. Protoxenicin B (2) is present in solution as a mixture of two conformers in a 2:1 ratio deduced by H NMR. Both xenicanes display significant cytotoxic activity against a panel of different tumor cell lines.