Amaurones A–K: Polyketides from the Fish Gut-Derived Fungus <i>Amauroascus</i> sp. CMB-F713

Abstract

Using a molecular networking guided strategy, chemical analysis of the Australian mullet fish gastrointestinal tract-derived fungus <i>Amauroascus</i> sp. CMB-F713 yielded a family of polyketide pyrones, amaurones A-I (<b>1</b>-<b>9</b>), featuring an unprecedented carbon skeleton. Structures were assigned to <b>1</b>-<b>9</b> by detailed spectroscopic analysis (including X-ray analysis of <b>1</b>), biosynthetic considerations, and chemical interconversions. For example, the orthoacetate <b>5</b> was unstable when stored dry at room temperature, transforming to the monoacetates <b>2</b> and <b>3</b>, while mild heating (40 °C) prompted quantitative conversion of <b>3</b> to <b>2</b>, via an intramolecular <i>trans</i>-acetylation. Likewise, during handling, the monoacetate <b>1</b> was prone to intramolecular <i>trans</i>-acetylation, leading to an equilibrium mixture with the isomeric monoacetate amaurone J (<b>10</b>), confirmed when partial hydrolysis of the diacetate <b>2</b> yielded the monoacetates <b>1</b> and <b>10</b> and the triol amaurone K (<b>11</b>).