New diterpenes from the gorgonian Solenopodium excavatum.

Seven new briariane diterpene lactones were isolated from the gorgonian Solenopodium excavatum collected in New Guinea. One structure (4) was determined by X-ray diffraction and the remainder were elucidated by spectroscopic analysis. Six of the new briareolides, 4-9, exist in a conformation that is quite uncommon for representatives of this fairly large family of compounds, namely one in which the bonds joining the 10-membered ring to the 6-membered ring are nearly diaxially oriented. Conformational energy calculations are discussed as are the structural features that are responsible for the uncommon conformation. Mild cytotoxic activity was confirmed for 4.