Raocyclamides A and B, Novel Cyclic Hexapeptides Isolated from the Cyanobacterium <i>Oscillatoria raoi</i>

Abstract

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.