| Summary: | Cyclic imine toxins are neurotoxic, macrocyclic compounds produced by marine dinoflagellates. Mass spectrometric screenings of extracts from natural plankton assemblages revealed a high chemical diversity among this toxin class, yet only few toxins are structurally known. Here we report the structural characterization of four novel cyclic-imine toxins (two gymnodimines (GYMs) and two spirolides (SPXs)) from cultures of <i>Alexandrium ostenfeldii</i>. A GYM with <i>m</i>/<i>z</i> 510 (<b>1</b>) was identified as 16-desmethylGYM D. A GYM with <i>m</i>/<i>z</i> 526 was identified as the hydroxylated degradation product of (<b>1</b>) with an exocyclic methylene at C-17 and an allylic hydroxyl group at C-18. This compound was named GYM E (<b>2</b>). We further identified a SPX with <i>m</i>/<i>z</i> 694 as 20-hydroxy-13,19-didesmethylSPX C (<b>10</b>) and a SPX with <i>m</i>/<i>z</i> 696 as 20-hydroxy-13,19-didesmethylSPX D (<b>11</b>). This is the first report of GYMs without a methyl group at ring D and SPXs with hydroxyl groups at position C-20. These compounds can be conceived as derivatives of the same nascent polyketide chain, supporting the hypothesis that GYMs and SPXs are produced through common biosynthetic genes. Both novel GYMs <b>1</b> and <b>2</b> were detected in significant amounts in extracts from natural plankton assemblages (<b>1</b>: 447 pg; <b>2</b>: 1250 pg; <b>11</b>: 40 pg per mL filtered seawater respectively).
|
|---|
