Okadaic Acid, a Cytotoxic Polyether from Two Marine Sponges of the

J. Am. Chem. SOC.1981, 103, 2469-2471 it has two spirofused six-membered ketal ring systems in place of the more commonly encountered spiroketal involving six- and fixe-membered rings. The presence of two fused tetrahydropyran rings (D, E) is also unique. Acanthifolicin exhibits ED5