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, (2 R ,3(8) E ,4 S ,6 R )‐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.