Ent-Kaurane Diterpenoids from Rabdosia rubescens and Their Cytotoxic Effects on Human Cancer Cell Lines

Two new ent-kaurane diterpenoids, 16,17-exo-epoxide-oridonin (1) and 11,15-O,O-diacetyl-rabdoternins D (2), together with thirteen known ones, were isolated from the aerial parts of Rabdosia rubescens. Their structures were established on the basis of high-field 1D and 2D NMR methods supported by HRMS. All diterpenoids were tested for cytotoxicity against human Hep G2, CO-LO 205, MCF-7, and HL-60 cancer cells. The compounds oridonin (3), 14-O-acetyl-oridonin (4), 1,14-O,O-diacetyl-oridonin (5), rosthorin (6), effusanin E (7), and ponicidin (8), as well as six alpha-methylene gamma-ketone bearing diterpenoids, were modestly active in these assays.