Nardochinins A–D (1–4), four novel sesquiterpenoids, along with four known ones were isolated from the underground parts of Nardostachys chinensis Batal in ethanol. Their structures were determined by extensive spectroscopic methods and single-crystal X-ray diffraction. Nardochinin A (1) possessed a norsesquiterpene skeleton with an unusual 3/6/5/5 tetracyclic ring system, which had not appeared in natural products. Nardochinins B (2) and C (3) were the first time found naturally occurring sesquiterpenoids with a 4,5-seco-nardosinane skeleton. Besides, compound 3 represented an unprecedented 4,5-seco-nardosinane type norsesquiterpenoid with losing an isopropenyl at C-6 compared with 2 in the structural framework. Nardochinin D (4) was a novel, highly oxygenated valerenane-type sesquiterpenoid possessing a rare 3,12-epoxy group and an unusual 9,11-epoxy group. The anti-Alzheimer's disease (AD) activities of 1–4 were also evaluated using the Caenorhabditis elegans AD pathological model, and nardochinin B (2) had the highest anti-AD activity.
