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An unreported isocoumarin, (<i>3S</i>,4<i>R</i>)-4-hydroxy-6-methoxymellein (<b>2</b>), an undescribed propylpyridinium anthraquinone (<b>4</b>), and an unreported C-glucosyl resorcinol derivative, acetyl carnemycin E (<b>5c</b>), were isolated, together with eight previously reported metabolites including <i>p</i>-hydroxybenzaldehyde (<b>1</b>), 1,3-dimethoxy-8-hydroxy-6-methylanthraquinone (<b>3a</b>), 1,3-dimethoxy-2,8-dihydroxy-6-methylanthraquinone (<b>3b</b>), emodin (<b>3c</b>), 5[(3<i>E</i>,5<i>E</i>)-nona-3,5-dien-1-yl]benzene (<b>5a</b>), carnemycin E (<b>5b</b>), tajixanthone hydrate (<b>6a</b>) and 15-acetyl tajixanthone hydrate (<b>6b</b>), from the ethyl acetate extract of the culture of a marine sponge-derived fungus, <i>Aspergillus stellatus</i> KUFA 2017. The structures of the undescribed compounds were elucidated by 1D and 2D NMR and high resolution mass spectral analyses. In the case of <b>2</b>, the absolute configurations of the stereogenic carbons were determined by comparison of their calculated and experimental electronic circular dichroism (ECD) spectra. The absolute configurations of the stereogenic carbons in <b>6a</b> and <b>6b</b> were also determined, for the first time, by X-ray crystallographic analysis. Compounds <b>2</b>, <b>3a</b>, <b>3b</b>, <b>4</b>, <b>5a</b>, <b>5b</b>, <b>5c</b>, <b>6a</b>, and <b>6b</b> were assayed for antibacterial activity against four reference strains, viz. two Gram-positive (<i>Staphylococcus aureus</i> ATCC 29213, <i>Enterococcus faecalis</i> ATCC 29212) and two Gram-negative (<i>Escherichia coli</i> ATCC 25922, <i>Pseudomonas aeruginosa</i> ATCC 27853), as well as three multidrug-resistant strains. However, only <b>5a</b> exhibited significant antibacterial activity against both reference and multidrug-resistant strains. Compound <b>5a</b> also showed antibiofilm activity against both reference strains of Gram-positive bacteria.