Cytotoxic Polyketides with an Oxygen-Bridged Cyclooctadiene Core Skeleton from the Mangrove Endophytic Fungus <i>Phomosis</i> sp. A818

Plant endophytic microorganisms represent a largely untapped resource for new bioactive natural products. Eight polyketide natural products were isolated from a mangrove endophytic fungus Phomosis sp. A818. The structural elucidation of these compounds revealed that they share a distinct feature in their chemical structures, an oxygen-bridged cyclooctadiene core skeleton. The study on their structure–activity relationship showed that the α,β-unsaturated δ-lactone moiety, as exemplified in compounds 1 and 2, was critical to the cytotoxic activity of these compounds. In addition, compound 4 might be a potential agonist of AMPK (5′-adenosine monophosphate-activated protein kinase).