Antibacterial Dolastane Diterpenes from the Brown Alga Canistrocarpus cervicornis

Marine macroalgae, particularly the brown alga Canistrocarpus cervicornis, are prolific sources of structurally diverse diterpenes with significant pharmacological potential. In the context of the growing need for new antibacterial scaffolds, the secondary metabolites of this species were investigated. Two new dolastane derivatives, (4R*,5R*,9S*,12R*,14S*)-4α-acetoxy-9β-methoxy-14α-hydroxydolasta-1(15),7-diene (1) and (5R*,9S*,12R*,14S*)-9β-methoxy-14α-hydroxydolasta-1(15),7-diene (2), have been isolated from C. cervicornis, together with two previously isolated secodolastanes, isolinearol (3) and isolinearol acetate (4), and three dolastanes, (4R*,5R*,8S*,9S*,12R*,14S*)-4α-acetoxy-8,9-epoxy-14α-hydroxy-7-oxodolast-1(15)-ene (5), (4R*,5R*,9S*,12R*,14S*)-4α-acetoxy-9β,14α-dihydroxydolasta-1(15),7-diene (6) and (4R*,5R*,7R*,12S*,14S*)-4α,7α-diacetoxy-14α-hydroxy-10-oxodolasta-1(15),8-diene (7). Their planar structures and relative stereochemistry were elucidated through extensive spectroscopic analyses, including HRMS and 1D and 2D NMR experiments (such as COSY, HSQC, HMBC and NOESY/t-ROESY). Compounds 1 and 6 showed antibacterial activities against clinically important bacteria, with MIC values in the range of 8–64 µg/mL.