| Summary: | Global natural products social (GNPS) molecular networking is a useful tool to categorize chemical space within samples and streamline the discovery of new natural products. Here, we demonstrate its use in chemically profiling the extract of the marine tunicate <i>Synoicum kuranui,</i> comprised of many previously reported rubrolides, for new chemical entities. Within the rubrolide cluster, two masses that did not correspond to previously reported congeners were detected, and, following MS-guided fractionation, led to the isolation of new methylated rubrolides T (<b>3</b>) and (<i>Z</i>/<i>E</i>)–U (<b>4</b>). Both compounds showed strong growth inhibitory activity against the Gram-positive bacteria <i>Bacillus subtilis</i>, with minimum inhibitory concentration (MIC) values of 0.41 and 0.91 μM, respectively.
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