Halogenated Cyclic Monoterpenoids with Anti‐Biofouling Activity from the Okinawan Red Marine Algae Portieria Hornemannii

The red algal genus Portieria is a prolific producer of halogenated monoterpenoids. In this study, we isolated and characterised monoterpenoids from the Okinawan red algae Portieria hornemannii. A new polyhalogenated cyclic monoterpenoid, 2(R)‐chloro‐1,6(S)‐dibromo‐3(8)(Z)‐ochtoden‐4(R)‐ol (1), along with three known monoterpenoids, (2R,3(8)E,4S,6R)‐6‐bromo‐2‐chloro‐1,4‐oxido‐3(8)‐ochtodene (2), 1‐bromo‐2‐chloroochtoda‐3(8),5‐dien‐4‐one (3), and 2‐chloro‐1‐hydroxyochtoda‐3(8),5‐dien‐4‐one (4) were isolated from the methanol extract of three populations of P. hornemannii. These compounds were characterised using a combination of spectroscopic methods and chemical synthesis, and the absolute stereochemistry of compounds 1 and 2 was determined. In addition, all isolated compounds were screened for their anti‐biofouling activity against the mussel Mytilus galloprovincialis, and 1 exhibited strong activity. Therefore, halogenated monoterpenoids have the potential to be used as natural anti‐biofouling drugs.