| Izvleček: | Focused on our interest to develop novel antiparasistic agents, the present study was aimed to evaluate the biological activity of an extract of <i>Laurencia johnstonii</i> collected in Baja California Sur, Mexico, against an <i>Acantamoeba castellanii</i> Neff strain. Bioassay-guided fractionation allowed us to identify the amoebicidal diastereoisomers α-bromocuparane (<b>4</b>) and α-isobromocuparane (<b>5</b>). Furthermore, bromination of the inactive laurinterol (<b>1</b>) and isolaurinterol (<b>2</b>) yielded four halogenated derivatives, (<b>6</b>)⁻(<b>9</b>), which improved the activity of the natural sesquiterpenes. Among them, the most active compound was 3α-bromojohnstane (<b>7</b>), a sesquiterpene derivative which possesses a novel carbon skeleton johnstane.
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