Cytotoxic labdane alkaloids from an ascidian Lissoclinum sp.: Isolation, structure elucidation, and structure–activity relationship

Four new cytotoxic labdane alkaloids, haterumaimides N–Q were isolated from the ascidian Lissoclinum sp. Their structures and SAR studies of this series of alkaloids are described. Four labdane alkaloids, haterumaimides N–Q ( 1– 4), were isolated from an ascidian Lissoclinum sp. and their structures were elucidated by chemical and spectral analyses. Investigation of the structure–activity relationships of haterumaimides J–K, N–Q, and 14 related compounds suggested that the presence of hydroxyl groups at C-6, C-7, C-12, and C-18, a chlorine atom at C-2, and an imido NH in ring C should be essential for cytotoxicity against P388 cells.