Ocellatuspyrones A‒G,new antibacterial polypropionates from the Chinese mollusk Placobranchus ocellatus

Marine invertebrates serve as rich sources of secondary metabolites with intriguing chemical diversities and a wide spectrum of biological activities.Particularly,marine shell-less sacoglossan mollusks have attracted much attentions due to the fact that mollusks apply complex metabolites as chemical defense agents against to their predators.With the purpose of discovering bioactive secondary metabolites to develop marine-derived medicines from the South China Sea,we have conducted a chemical study on the photosynthetic mollusk Placobranchus ocellatus.As a result,seven newγ-pyrone polypropionates,namely(±)-ocellatuspyrone A(1),(±)-ocellatuspyrone B(2),and ocellatuspyrones C−G(5,9−12),along with fve known polypropionates,have been isolated and characterized from the South China Sea photosynthetic mollusk Placobranchus ocellatus.Extensive spectroscopic analysis,single crystal X-ray difraction analysis,modifed Mosher’s method,ECD comparison,CD exciton chirality method,TDDFT-ECD calculation,and chemical conversion were used to determine the structures and absolute confgurations of the new compounds and the stereochemistry of undefned known compounds 4,6 and 7.All these isolated polypropionates were evaluated in bioassays for their biological activities,including antibacterial,neuroprotective efect,anti-infammatory,PTP1B inhibitory,and antiviral activities.Compounds 7,8 and 11 were found for the frst time to show antibacterial activity against fsh pathogenic bacteria Streptococcus parauberis(the main pathogen causing fsh streptococcal infections and acute death)with MIC values of 35.8,34.2,and 37.4μg/mL,respectively,which might be potential novel antibacterial agents for the treatment of fsh infectious diseases.