ent-Pimarane Diterpenoids from Siegesbeckia orientalis and Structure Revision of a Related Compound

Six new ent-pimarane diterpenoids, namely, ent-14β,16-epoxy-8-pimarene-3β,15α-diol (1), 7β-hydroxydarutigenol (2), 9β-hydroxydarutigenol (3), 16-O-acetyldarutigenol (4), 15,16-di-O-acetyldarutoside (5), and 16-O-acetyldarutoside (6), were isolated from the ethanol extract of Siegesbeckia orientalis. Their structures were elucidated on the basis of spectroscopic studies, and the absolute configuration of 1 was established by the modified Mosher's method. Furthermore, the structure of ent-12β,16-epoxy-8-pimarene-2α,15β,19-triol, recently isolated from the same plant, should be revised as ent-14β,16-epoxy-8-pimarene-2α,15α,19-triol (10).