| Summary: | Extracts of the mantle and viscera of the Indo-Pacific nudibranchs <i>Goniobranchus aureopurpureus</i> and <i>Goniobranchus</i> sp. 1 afforded 11 new diterpenoids (<b>1</b>–<b>11</b>), all of which possess a tetracyclic spongian-16-one scaffold with extensive oxidation at C-6, C-7, C-11, C-12, C-13, and/or C-20. The structures and relative configuration were investigated by NMR experiments, while X-ray crystallography provided the absolute configuration of <b>1</b>, including a 2′<i>S</i> configuration for the 2-methylbutanoate substituent located at C-7. Dissection of animal tissue revealed that the mantle and viscera tissues differed in their metabolite composition with diterpenes <b>1</b>–<b>11</b> present in the mantle tissue of the two nudibranch species.
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