Placidenes G–J, Bioactive <i>γ</i> ‐Pyrone Derivatives From the Marine Mollusk <i>Placida dendritica</i>

Three pairs of undescribed enantiomeric γ-pyrone derivatives placidenes G-I (1-3), together with two new congeners placidene J (4)/isoplacidene J (5) and two known compounds 6 and 7, were isolated from the marine mollusk Placida dendritica. Compounds 1a/1b-3a/3b were further separated by chiral-phase high-performance liquid chromatography. The structures were elucidated by comprehensive spectroscopic analysis, high-resolution electrospray ionization mass spectrometry, time-dependent density functional theory/electronic circular dichroism calculation, and X-ray diffraction analysis. In bioassays, compounds 1-5 exhibited moderate nitric oxide inhibition from 28.2% to 51.9% on RAW264.7 inflammatory cells at 20 µM without notable cytotoxicity.