Minor Sesquiterpenes with New Carbon Skeletons from the Brown Alga <i>Dictyopteris </i><i>divaricata</i>

Five minor sesquiterpenes (<b>1</b>−<b>5</b>) with two novel carbon skeletons, together with a minor new oplopane sesquiterpene (<b>6</b>), have been isolated from the brown alga <i>Dictyopteris divaricata</i>. By means of spectroscopic data including IR, HRMS, 1D and 2D NMR, and CD, their structures including absolute configurations were assigned as (+)-(1<i>R</i>,5<i>S</i>,6<i>S</i>,9<i>R</i>)-3-acetyl-1-hydroxy-6-isopropyl-9-methylbicyclo[4.3.0]non-3-ene (<b>1</b>), (+)-(1<i>R</i>,3<i>S</i>,4<i>S</i>,5<i>R</i>,6<i>S</i>,9<i>R</i>)-3-acetyl-1,4-dihydroxy-6-isopropyl-9-methylbicyclo[4.3.0]nonane (<b>2</b>), (+)-(1<i>R</i>,3<i>R</i>,4<i>R</i>,5<i>R</i>,6<i>S</i>,9<i>R</i>)-3-acetyl-1,4-dihydroxy-6-isopropyl-9-methylbicyclo[4.3.0]nonane (<b>3</b>), (+)-(1<i>S</i>,2<i>R</i>,6<i>S</i>,9<i>R</i>)-1-hydroxy-2-(1-hydroxyethyl)-6-isopropyl-9-methylbicyclo[4.3.0]non-4-en-3-one (<b>4</b>), (−)-(5<i>S</i>,6<i>R</i>,9<i>S</i>)-2-acetyl-5-hydroxy-6-isopropyl-9-methylbicyclo[4.3.0]non-1-en-3-one (<b>5</b>), and (−)-(1<i>S</i>,6<i>S</i>,9<i>R</i>)-4-acetyl-1-hydroxy-6-isopropyl-9-methylbicyclo[4.3.0]non-4-en-3-one (<b>6</b>). Biogenetically, the carbon skeletons of <b>1</b>−<b>6</b> may be derived from the co-occurring cadinane skeleton by different ring contraction rearrangements. Compounds <b>1</b>−<b>6</b> were inactive (IC<sub>50</sub> > 10 μg/mL) against several human cancer cell lines.