Bioassay-guided fractionation of the extract of a species of Lyngbya from Palau has yielded Palau'amide (1), which had an IC(50) value of 13 nM against KB cells. The structure of 1 was elucidated by NMR analysis in a variety of solvents. In particular, a gradient-enhanced band-selective HMBC experiment allowed unambiguous assignment of HMBC correlations to carbon signals that were separated by 0.1 ppm. The relative stereochemistry and the absolute configurations of all but one (C-37) of the nine chiral centers were determined by NMR analysis of the alpha-methoxyphenylacetic acid (MPA) derivatives and chiral HPLC of the degradation products of 1. The assignment of S rather than R stereochemistry at C-37 is discussed in detail.

Ulongapeptin (1), a cyclic depsipeptide, was isolated from a Palauan marine cyanobacterium Lyngbya sp. The gross structure was elucidated through one-dimensional TOCSY experiments and other spectroscopic techniques. The absolute and relative stereochemistry of the /?-amino acid, 3-amino-2-methyl-7-octynoic acid (AMO), in 1 was determined by synthesis of the saturated a-alkyl-/3-amino acid and Marfey's analysis of the acid hydrolysate of tetrahydro-1. Ulongapeptin (1) was cytotoxic against KB cells at an IC 50 value of 0.63 fM..Cyanobacteria are photosynthetic microorganisms that inhabit diverse habitats ranging from marine to terrestrial. These ancient prokaryotes are believed to have played a key role in the evolution of the modern ecosystem since they were presumably the first organisms to produce molecular oxygen. 1 Filamentous cyanobacteria of the genus Lyngbya are the most frequently encountered cyanobacteria in tropical areas and have the ability to fix nitrogen through heterocysts. With few exceptions most secondary metabolites from Lyngbya arise by the incorporation of nitrogen into a combination of polyketide and peptide biosynthetic pathways. 2 Here we describe the isolation of a new nitrogen-containing compound, ulongapeptin (1), from a Palauan collection of Lyngbya sp.Results and Discussion.Lyophilized VP755 was extracted with 1:1 methanol/ ethyl acetate and the concentrated extract fractionated by normal and reversed-phase chromatography. After repeated RP-HPLC 1.1 mg of ulongapeptin (1), which had an IC50 of 0.63 ftM against KB cells, 3 was isolated in a yield of 0.10%.Ulongapeptin (1) was found to have a molecular weight of 808 Da based on FABMS pseudo-molecular ion peaks