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