Bipinnatins K-Q, minor cembrane-type diterpenes from the West Indian gorgonian Pseudopterogorgia kallos:: Isolation, structure assignment, and evaluation of biological activities

An extensive chemical study of the secondary metabolites found in the crude organic extract of the gorgonian octocoral Pseudopterogorgia kallos has led to the isolation of seven new cembranolides, bipinnatins K-Q (2-8), and one known compound, bipinnatin E (9). The molecular structures of compounds 2-8, many of which contain unusual structural features, were assigned mainly by 2D NMR spectroscopic methods and X-ray crystallographic analysis. The discovery of compounds 2-8 may lend support to previously proposed mechanisms for the biosynthesis of recently isolated bioactive natural products from the same gorgonian specimen. The in vitro cytotoxicity of bipinnatins 4, 6, and 7 against the NCI tumor cells MCF breast cancer, NCI-H460 non-small cell lung cancer, and SF-268 CNS cancer was evaluated. However, only bipinnatin Q (6) displayed significant cytotoxic activity. Some of the compounds isolated proved to be inhibitors of acetylcholine receptors.