Targeted Isolation of Rubrolides from the New Zealand Marine Tunicate Synoicum kuranui

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 Synoicum kuranui, 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 (3) and (Z/E)–U (4). Both compounds showed strong growth inhibitory activity against the Gram-positive bacteria Bacillus subtilis, with minimum inhibitory concentration (MIC) values of 0.41 and 0.91 μM, respectively.