New and unusual sesquiterpenes: Kelsoene, prespatane, epi-gamma-gurjunene, and T-cadinthiol, from the tropical marine sponge Cymbastela hooperi

From the dichloromethane solubles of the tropical marine sponge Cymbastela hooperi were isolated four novel terpenoid metabolites. One of these, (1R*,2S*,5R*,6R*,7S*,8R*)-1,5-dimethyl-7-(1'-methylethenyl)- tricyclo[6.2.0.3(2,6)]decane (kelsoene, 1), possesses a most unusual carbocyclic skeleton and is the first member of a new class of sesquiterpenes, the kelsoanes. The remaining three compounds, (1R*,2S*,5R*,6R*,7R*,8S*)-1,5-dimethyl (prespatane, 2),(1R*,4R*,7S*,10S*)-4,l0-dimethyl-7-(l'-methylethenyl)bicyclo[5.3.0]dec-5-ene (epi-gamma-gurjunene, 3), and (1R*,6R*,7S*,10S*)-4,l0-dimethyl-7-(1'-methylethyl)-l0- mercaptobicyclo[4.4.0]- dec-4-ene (T-cadinthiol, 5), are unusual natural products, particularly as sponge metabolites. All structures and their relative configurations were established by spectroscopic methods, particularly H-1-H-1 and H-1-C-13 shift correlated 2D NMR spectroscopy and accurate mass measurement (HREIMS). Compound 5 has weak in vitro antimalarial activity.