New and Unusual Sesquiterpenes:  Kelsoene, Prespatane, Epi-γ-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-8-(1‘-methylethenyl)tricyclo[5.3.0.2^2,6]decane (prespatane, 2), (1R*,4R*,7S*,10S*)-4,10-dimethyl-7-(1‘-methylethenyl)bicyclo[5.3.0]dec-5-ene (epi-γ-gurjunene, 3), and (1R*,6R*,7S*,10S*)-4,10-dimethyl-7-(1‘-methylethyl)-10-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 ¹H−¹H and ¹H−¹³C shift correlated 2D NMR spectroscopy and accurate mass measurement (HREIMS). Compound 5 has weak in vitro antimalarial activity.