Arnequinol A and arnequinone A, two unique meroterpenoids from Arnebia euchroma

Arnequinol A (1), featuring an unprecedented 6/6/3 tricyclic carbon skeleton fused with a heptatomic oxo-bridge, together with arnequinone A (2) bearing a highly conjugated methyl-shifting benzogeijerene skeleton, were isolated from Arnebia euchroma. Their structures were elucidated by extensive spectroscopic methods and quantum chemical calculations of the 13C nuclear magnetic resonance (NMR) data and electronic circular dichroism (ECD) spectra. The plausible biosynthetic pathways for 1 and 2 were presented. In an in vitro test, compound 2 showed potent neuroprotective activity against serum-deprivation induced PC12 cell damage at a concentration of 10 μmol/L.