Absolute structures of new briarane diterpenoids from junceella fragilis

Four new diterpenoids with the briarane skeleton, (-)-4-deacetyljunceellolide D (2), (+)-11alpha, 20alpha-epoxyjunceellolide D (3), (-)-11alpha, 20alpha-epoxy-4-deacetyljunceellolide D (4), and (-)-11alpha, 20alpha-epoxy-4-deacetoxyjunceellolide D (5), (+)-junceellolide A (6) [the antipodal derivative of the known (-)-junceellolide A], along with three known briaranes, (-)-junceellolide D (1), (-)-junceellin (7), and (-)-praelolide (8), were isolated from the Indonesian gorgonian Junceella fragilis. The structures of the new compounds were established on the basis of extensive NMR studies and by comparison with the spectral data from other briarane compounds. The absolute configurations for four of the compounds were determined by the modified Mosher method and by unambiguous chemical interconversions.