Maoecrystal V(1,Scheme1),which was isolated in 2004 by Sun and co-workers from the leaves of a Chinese medicinal herb called Isodon eriocalyx,is a novel C19diterpenoid1 and displays potent and selective inhibitory activity against HeLa cells(IC50=60nM).Maoecrystal V possesses an unprecedented and highly congested pentacyclic framework with six stereocenters,among which three are vicinal quaternary stereocenters.This structure has been confirmed by X-ray crystallography.