Ulongamides A-F, new beta-amino acid-containing cyclodepsipeptides from Palauan collections of the marine cyanobacterium Lyngbya sp.

Six new beta-amino acid-containing cyclic depsipeptides, termed ulongamides A-F (1-6), have been isolated from collections of apratoxin-producing cyanobacteria from Palau. Their planar structures have been determined by NMR spectroscopic techniques. The absolute stereochemistry of the hydroxy acid and all the alpha-amino acid-derived units was ascertained to be S by chiral HPLC analysis of degradation products. The stereochemistry of the beta-amino acid moiety, 3-amino-2-methylhexanoic acid, was established by advanced Marfey analysis of the acid hydrolyzates and found to be 2R,3R in compounds 1-3 but 2S,3R in compounds 4-6. All compounds except 6, which lacks an aromatic amino acid moiety, were weakly cytotoxic against KB cells.