Leonuketal, a Spiroketal Diterpenoid from Leonurus japonicus

An architecturally complex spiroketal diterpenoid, leonuketal (1), was isolated from the aerial parts of the plant Leonurus japonicus. This compound possessed an unprecedented tetracyclic skeleton that comprised a bridged spiroketal moiety fused with a ketal-gamma-lactone unit. The structure and absolute configuration were determined by spectroscopic analyses, a modified Moshers method, and ECD (electronic circular dichroism) calculations. Leonuketal (1) showed significant vasorelaxant activity against KCl-induced contraction of rat aorta, with the EC50 value of 2.32 mu M.