| Summary: | A novel 2,3-epoxy naphthoquinol, named (6R,7R,8R)-theissenone A (1), possessing an oxatricyclo[5.4.0.03,5]undeca-trien-2-one skeleton, together with two known compounds, (6S,7R,8R)-theissenone (2) and arthrinone (3), were produced by an endophytic fungus, Arthrinium marii M-211, which was isolated from mangrove plants. The structure of 1, including the absolute stereochemistry, was elucidated by analysis of nuclear magnetic resonance (NMR) and mass spectrometry (MS) data and time-dependent density functional theory (TDDFT) calculations of electronic circular dichroism (ECD) spectra. Additionally, the absolute structure of 2 was deduced as a diastereomer of 1 using ECD spectral data analysis. Compounds 1, 2 and 3 exhibited cytotoxic activity against the H4IIE rat hepatoma cells, with IC50 values of 67.5, 46.6 and 13.4 µM, respectively.
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