Clavosines A−C from the Marine Sponge Myriastra clavosa: Potent Cytotoxins and Inhibitors of Protein Phosphatases 1 and 2A

Clavosines A ( 1 ) and B ( 2 ), novel metabolites closely related to calyculins and calyculinamides, have been isolated from the marine sponge Myriastra clavosa . Clavosine C ( 3 ) was also isolated and is assumed to be an artifact from clavosine B ( 2 ). Structures of these compounds were determined by spectroscopic analysis and by comparison of their NMR data with those of the calyculins and calyculinamides. Clavosines A and B were found to be very potent cytotoxins in the National Cancer Institute's screening panel of 60 tumor cell lines and were potent inhibitors of the type 1 and 2A serine/threonine protein phosphatases.