Trinor-Guaiane-Type Sesquiterpenes with Unusual Skeletons from the Okinawan Soft Coral Clavularia viridis

Four new trinor-guaiane sesquiterpenes, clavuridin C (1), clavuridin D (2), 5-methoxyclavuridin B (3), and 5-epi-clavuridin B (4), together with three known compounds, clavuridin B (6), clavularin A (7), and clavularin B (8), were isolated from the soft coral Clavularia viridis collected from the coast of Cape Kyan, Okinawa Prefecture, Japan. The structures of the newly isolated compounds 1–4, including their relative configurations, were elucidated through detailed analysis of spectroscopic data, such as IR, NMR, MS, and density functional theory (DFT)-based DP4 analysis, as well as by comparison with previously reported related compounds. The absolute configurations of 2 and 3 were additionally investigated through electronic circular dichroism (ECD) calculations. The absolute configuration of 4 was determined through benzoylation, followed by application of the allylic benzoate rule. Furthermore, structural differences between 4 and antheliol (4a) were addressed through detailed comparison of experimental and calculated NMR data, including DP4 and CP3 analyses, leading to a revision of the previously proposed structure. These results highlight the importance of integrated spectroscopic and computational approaches for accurate structure determination of closely related sesquiterpenes. In addition, isolated compounds 1–4 and 6–8 were evaluated for their antibacterial and antifungal activities.