Raocyclamides A and B, novel cyclic hexapeptides isolated from the cyanobacterium Oscillatoria raoi

Raocyclamides A and B, two novel oxazole- and thiazole-containing cyclic hexapeptides, were isolated from the cyanobacterium Oscillatoria raoi (TAU strain IL-76-1-2). The gross structures of raocyclamides A (1) and B (2) were elucidated by homonuclear and inverse-heteronuclear 2D-NMR techniques and HREIMS. The absolute stereochemistry of compounds 1 and 2 was determined by Marfey's HPLC Method. Raocyclamide A (1) was established as cyclo-thiazole-D-Ile-oxazole-L-Ala-D-oxazoline-D-Phe and raocyclamide B (2) as cyclo-thiazole-D-Ile-oxazole-L-Ala-D-Ser-D-Phe. Raocyclamide A (1) exhibits a moderate cytotoxicity against sea urchin embryos.