Diterpenes with Unprecedented Skeletons from the Red Alga Sphaerococcus coronopifolius

Three previously unknown diterpenes – spirosphaerol (1), anthrasphaerol (2) and corfusphaeroxide (3) – have been isolated from the red alga Sphaerococcus coronopifolius. Their structures were elucidated by spectral analysis, including NMR and MS. The isolated metabolites were evaluated for their in vitro antitumour activities against one murine and five human cancer cell lines. Chemical investigation of the red alga Sphaerococcus coronopifolius led to the isolation and structure elucidation of the three new minor metabolites 1–3, featuring rearranged tricyclic diterpene skeletons. The in vitro antitumour activities of 1–3 were evaluated against one murine cancer cell line and five human cancer cell lines.