Labdane Diterpenoids from Leonurus sibiricus

Six new labdane diterpenoicis, preleosibirone A (1), 13-epi-preleosibirone A (2), isopreleosibirone A (3), leosibirone A (4), leosibirone B (5), and 15-epi-leosibirone B (6), Were isolated from the leaves of Leonurus sibiricus. The absolute,configurations of,1,. 2, 5, and 6 were established by X.-ray crystallographic analyses, and leosibirone A (4) was shown to be an artifact of the isolation process.