A new isochromane derivative from a marine-associated <i>Penicillium sp.</i> and its potential antiviral and antimicrobial activity

Marine-derived endophytic fungi are a rich source of structurally unique and bioactive natural products. In this study, we isolated a new isochromane derivative, (6<i>S</i>,7<i>R</i>)-7-methoxy-10,11,12,13-tetramethylisochromane-1,3-diol (compound <b>1</b>), along with seven known compounds (compounds <b>2-8</b>), from <i>Penicillium sp</i>. Fx1.1.3, an endophyte obtained from the stem of <i>Tetragonia tetragonioides</i> collected in Beigong Island, China. Structural elucidation of compound <b>1</b> was performed by 1D and 2D NMR, HR-ESI-MS, and ECD spectral analyses, confirming its absolute configuration as 6<i>S,</i>7<i>R</i>. Antimicrobial screening showed that compound <b>1</b> exhibited moderate inhibitory activity 125 μg/mL (494 µM) against <i>Staphylococcus aureus</i>, <i>Escherichia coli,</i> and <i>Candida albicans</i>, while compounds <b>2</b> and <b>4</b> showed more potent inhibitory activity 31.25 μg/mL or 125 µM and 82 µM, respectively against <i>S. aureus</i>. In addition, compounds <b>1</b> and <b>6</b> displayed significant antiviral activity against A/Puerto Rico/8/34 (H1N1) virus with IC₅₀ values of 8.99 ± 3.32 and 4.04 ± 1.10 μg/mL (35.52 ± 13.12 and 11.40 ± 3.10 μM), respectively.