Lepadins D-F: antiplasmodial and antitrypanosomal decahydroquinoline derivatives from the tropical marine tunicate Didemnum sp.

From a new tunicate species, belonging to the genus Didemnum, three alkaloids possessing an unusual and extremely rare decahydroquinoline skeleton and showing significant and selective antiplasmodial and antitrypanosomal activity were obtained as follows: (2R*,3S*,4aR*,5R*,8aS*)-decahydro-3-hydroxy-5-(5'-hydroxyoctyl)-2-methylquinoline (lepadin D,1), its quaternary nitrogen derivative (2), (2R*,2"E,3S*,4aR*,5R*,8aS*)-decahydro-3-hydroxy-5-(5'-hydroxyoctyl)-2-methyl-3-quinolinyl ester 2"-octenoic acid (lepadin E, 3), and (2S*,2"E,3S*,4aR*,5R*,8aS*)-decahydro-3-hydroxy-5-(5'-hydroxyoctyl)-2-methyl-3-quinolinyl ester 2"-octenoic acid (lepadin F, 4). These isolates may well serve as lead structures for the development of new antimalarial drugs.