Labdane Diterpenes of Leonurus sibiricus

Seven new labdane diterpenes, sibiricinones A−E (1−4, 6) and 15-epi-sibiricinones D and E (5 and 7), and the flavone genkwanin were isolated from the aerial parts of Leonurus sibiricus. Sibiricinone D (4) and 15-epi-sibiricinone D (5), and sibiricinone E (6) and 15-epi-sibiricinone E (7), respectively, were isolated as C-15 epimeric pairs. These secondary metabolites were identified on the basis of 1D and 2D NMR including 1H−1H COSY, HSQC, and HMBC spectroscopic techniques. The stereochemical configurations of compounds 4−7 were assigned through 2D T-ROESY and selective NOE experiments.