Excavatolides N-T, new briaran diterpenes from the Western Australian gorgonian Briareum excavatum

A Western Australian sample of the gorgonian Briareum excavatum, collected at Rowley Shoals, afforded one known (10) and seven new briaran diterpenes (1), (3) and (5)-(9). Gross structures and relative stereochemistries were determined by spectroscopic methods and by comparison of spectral data with those of previously reported briarans. Support for a proposed conformation of the 10-membered ring (stabilized by hydrogen-bonding) to explain atypical H-1 n.m.r. chemical shifts and coupling constants in (9) was obtained from energy minimization of the proposed structure by an ab initio density functional method.