| Summary: | Mangrove-associated fungi are important sources for the discovery of new bioactive natural products. Three new isocoumarins (<b>1</b>–<b>3</b>) and one new pyrone derivative (<b>4</b>) were isolated from the ethyl acetate extract of the fermentation broth of the mangrove endophytic fungus <i>Phomopsis</i> sp. DHS-11. Nuclear magnetic resonance (NMR) spectroscopy (one-dimensional and two-dimensional) and mass spectrometry were used to determine the structures of these new compounds. The absolute configurations for the new isocoumarins <b>1</b>–<b>3</b> were determined by comparing their experimental and calculated electronic circular dichroism (ECD) spectra, while the configuration for the new pyrone-derivative <b>4</b> was tentatively solved by comparison of its <sup>13</sup>C NMR data with reported data. In the biological activity test, compounds <b>1</b> and <b>3</b> showed cytotoxic activity against HeLa cells with IC<sub>50</sub> values of 11.49 ± 1.64 µM and 8.70 ± 0.94 µM, respectively. The initial structure and activity relationship (SAR) analysis revealed that the length of the side chain at C-3 for isocoumarin-type compounds <b>1</b>–<b>3</b> could affect the cytotoxicity against HeLa cells. Compound <b>4</b> exhibited cytotoxic activities against human hepatoma cells HepG2 with an IC<sub>50</sub> value of 34.10 ± 2.92 µM. All compounds have no immunosuppressive activity.
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