Di- and Triterpenoids from the Rhizomes of Isodon amethystoides and Their Anti-inflammatory Activities.

Amethystoidesic acid ( 1 ), a triterpenoid with an unprecedented 5/6/6/6 tetracyclic skeleton, and six undescribed diterpenoids, amethystoidins A-F ( 2 - 7 ), were isolated from the rhizomes of Isodon amethystoides along with 31 known di- and triterpenoids ( 8 - 38 ). Their structures were fully elucidated via extensive spectroscopic analysis including 1D and 2D NMR, high-resolution electrospray ionization mass spectrometry (HRESIMS), and electronic circular dichroism (ECD) calculations. Compound 1 is the first example of a triterpenoid possessing a rare ring system (5/6/6/6) derived from a contracted A-ring and the 18,19- seco -E-ring of ursolic acid. Compounds 6 , 16 , 21 , 22 , 24 , and 27 significantly inhibited nitric oxide (NO) production in lipopolysaccharide (LPS)-stimulated RAW264.7 cells, which could be partly mediated by the downregulation of LPS-induced inducible nitric oxide synthase (iNOS) protein expression.