The Structures of Cytotoxic Diterpenes Containing Bromine from the Marine Red Alga Laurencia obtusa (Hudson) Lamouroux

Abstract
Four new brominated diterpenes possessing parguerane or isoparguerane skeleton have been isolated from the title alga collected in the Japanese waters along with several known parguerol, isoparguerol, and deoxyparguerol congeners. The structures of three new compounds were determined by the chemical correlation with the corresponding known compounds. The structural elucidation of the remaining one was carried out on the basis of a combination of chemical and spectroscopic means including 2D NMR technique. One of the new metabolites has proved to be cytotoxic. The cytotoxic properties of the several chemically derived compounds in addition to the known diterpenes are also described.