Structural Characterization of New 8,17-Epoxybriarane Diterpenoids from the Octocoral Briareum stechei

An encrusting, algae-associated octocoral, Briareum stechei, afforded three 8,17-epoxybriarane diterpenoids, including the known compound briaexcavatolide B (1) and two previously undescribed analogues, briastecholides Q (2) and R (3). The absolute configuration of 1 was established for the first time by single-crystal X-ray diffraction analysis. The planar structures and relative configurations of 2 and 3 were determined through extensive spectroscopic analyses. In bioactivity evaluation, compound 2 exhibited cytotoxic effects toward the human acute lymphoblastic leukemia cell lines Molt-4 and CCRF-CEM.