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

Abstract

Arnebidin, a heptacyclic naphthoquinone dimer with a tricyclo[3.3.0.0^1,3]octane core, which was first isolated from Arnebia hispidissima and later found from A. euchroma. Its structure was identified by the analysis of NMR and EI-MS data. In the present work, a heptacyclic naphthoquinone dimer ((±)-1) was obtained from the petroleum ether portion of the ethanol extract of Arnebia euchroma. By comparing the NMR data of 1 with the literatures, compound 1 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 1 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 1.