Structure Revision of Heptacyclic Naphthoquinone Dimer Arnebidin Based on NMR Spectroscopic Analysis and X-ray Crystallography

Arnebidin, a heptacyclic naphthoquinone dimer with a tricyclo[3.3.0.0^1,3]octane core, which was first isolated from <i>Arnebia hispidissima</i> and later found from <i>A. euchroma</i>. Its structure was identified by the analysis of NMR and EI-MS data. In the present work, a heptacyclic naphthoquinone dimer ((±)-<b>1</b>) was obtained from the petroleum ether portion of the ethanol extract of <i>Arnebia euchroma</i>. By comparing the NMR data of <b>1</b> with the literatures, compound <b>1</b> should be identified as arnebidin. However, the single-crystal X-ray diffraction analysis followed by reinterpretation of the NMR data revealed that the structure of <b>1</b> was a heptacyclic naphthoquinone dimer with a tricyclo[3.2.1.0^2,7]octane core. Thus, we proposed that arnebidin should be revised to <b>1</b>.