Cytotoxic oplopane sesquiterpenoids from <i>Arnoglossum atriplicifolium</i>

<p>Pale Indian plantain (<i>Arnoglossum atriplicifolium</i> (L.) H. Rob.) is a plant with traditional medicinal usage among the Cherokee Native American tribe for treating cancer. Two oplopane sesquiterpenoids were isolated from an extract of <i>A. atriplicifolium</i> from Western North Carolina. The compounds were isolated by bioassay-guided fractionation using an MCF-7 breast tumour cell line assay. The known compound (1<i>S</i>,6<i>R</i>,7<i>R</i>,8<i>R</i>)-1-acetoxy-6,7-diangeloxy-8,10-epoxy-2-oxo-oplopa-3,14Z,11,12-dien-13-al (<b>1</b>) had an EC₅₀ value of 9.0 μM against MCF-7 cells, while the new compound (1<i>S</i>,3<i>R</i>,6<i>R</i>,7<i>R</i>,8<i>R</i>,11<i>S</i>)-1-acetoxy-6,7-diangeloxy-8,10,11,13-bisepoxyoplopan-2-one (<b>2</b>) had an EC₅₀ value of 96 μM. The compounds were characterised by 1D and 2D NMR spectroscopy and by comparison with literature values in the case for <b>1</b>. Based on NOESY analysis, a correction of the relative configuration for <b>1</b> is presented. The presence of these compounds may help to explain the folk remedy usage of this plant as an anticancer agent.</p