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.