Silvaticusins A–D: ent-kaurane diterpenoids and a cyclobutane-containing ent-kaurane dimer from Isodon silvaticus

Three new ent -kaurane diterpenoids, silvaticusins A–C ( 1 – 3 ), along with a new ent -kaurane dimer silvaticusin D ( 4 ) were isolated from the aerial parts of Isodon silvaticus . The structures of these new compounds were established mainly by comprehensive analysis of their NMR and MS data. The absolute configuration of compounds 1 and 4 were determined using a single-crystal X-ray diffraction and computational methods, respectively. Compounds 2 and 3 were found to exhibit remarkable cytotoxic effects against five human tumor cell lines (HL-60, A-549, SMMC-7721, MDA-MB-231, and SW-480), with IC 50 values spanning from 1.27 ± 0.08 to 7.52 ± 0.33 μM.