Abstract

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 ¹H 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.