Structures and Antibacterial Activities of Minor Dolabellanes from the Brown Alga Dilophus spiralis

Thirteen new minor diterpenes featuring the dolabellane skeleton (1-13) were isolated from the organic extracts of the brown alga Dilophus spiralis, collected in Elafonissos Island. The structures and relative configurations of the isolated natural products were elucidated on the basis of extensive analyses of their spectroscopic data (NMR, MS, IR). The absolute configuration of 1 was determined by using the modified Mosher's method, whereas the structure of 2 was confirmed by single-crystal X-ray diffraction analysis. The antibacterial activities of metabolites 1-9 and 13 were evaluated against six strains of Staphylococcus aureus, including multidrug- and methicillin-resistant variants. Compounds 1 and 4 exhibited noteworthy levels of activity with MIC values in the range of 2-8 μg/mL.