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 in vitro test, compound 2 showed potent neuroprotective activity against serum-deprivation induced PC12 cell damage at a concentration of 10 µmol/L.