| Summary: | Mycousfurans (<b>1</b> and <b>2</b>), two new usnic acid congeners, along with (−)-mycousnine (<b>3</b>), (−)-placodiolic acid (<b>4</b>), and (+)-usnic acid (<b>5</b>), were isolated using high-performance liquid chromatography-ultraviolet (HPLC-UV)-guided fractionation of extracts of <i>Mycosphaerella</i> sp. isolated from a marine sediment. The planar structures of <b>1</b> and <b>2</b> were elucidated using 1D and 2D NMR spectra. The relative configurations of the stereogenic carbons of <b>1</b> and <b>2</b> were established via analysis of their nuclear Overhauser spectroscopy (NOESY) spectra, and their absolute configurations were determined using a comparison of experimental and calculated electronic circular dichroism (ECD) spectra. Compounds <b>1</b> and <b>2</b> were found to have antibacterial activity, showing moderate activity against <i>Kocuria rhizophila</i> and <i>Staphylococcus aureus</i>.
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