Dolabellanes with Antibacterial Activity from the Brown Alga Dilophus spiralis

Seventeen diterpenes featuring the dolabellane skeleton (1−17) were isolated from the organic extracts of the brown alga Dilophus spiralis. Seven compounds are new natural products (1, 3, 5, 6, 11, 14, 15) and eight are structurally revised (2, 4, 7−10, 12, 13), among which three are reported for the first time from a natural source (4, 9, 10). The structure elucidation and the assignment of the relative configurations of the isolated natural products were based on detailed analyses of their spectroscopic data. The structure of metabolite 10 was confirmed by single-crystal X-ray diffraction analysis, whereas the absolute configurations of compounds 2, 4−10, 12, and 13 were determined using the modified Mosher’s method on the semisynthetic product 18 and chemical interconversions. The antibacterial activities of compounds 1−18 were evaluated against six strains of Staphylococcus aureus, including multidrug- and methicillin-resistant variants.