72(P2-2) 豪州産軟体サンゴ、Pachyclavularia speciesの新ジテルペノイド(ポスター発表の部)  [in Japanese] 72(P2-2) NEW DITERPENES FROM AN AUSTRALIAN PACHYCLAVULARIA SPECIES (COELENTERATA, ANTHOZOA, OCTOCORALLIA)  [in Japanese]

Soft corals belonging to the genera Pachyclavularia and Briareum on the Great Barrier Reef, Australia are now known to be extremely close taxonomically. Based on classical taxonomic characters, it is difficult to distinguish between colonies of the two genera, and yet Pachyclavularia belongs to the order Stolonifera, while Briareum is a Gorgonacean coral. Chemical studies of the two groups suggest that Pachyclavularia species produce cembranoid diterpenes, while Briareum species generally yield briarane diterpenes. Chemical investigation of a soft coral tentatively ascribed to the genus Pachyclavularia afforded the known diterpene, acetoxycladiellin (3), and three new briarane diterpenes (4), (5) and (6). The major briarane (4) was found to be ichthyotoxic to the mosquitofish Gambusia affinis at 2-5 ppm. The basic skeleton and the common substitution pattern of the cyclohexane ring in the briaranes (4), (5) and (6) was deduced from ^1H and ^<13>C-n.m.r. spectral comparisons with the known metabolite (2) (Tables 1 and 2.). High field n.m.r. experiments, including COSY-45 and short and long range ^<13>C-^1H shift correlated 2-D n.m.r. spectroscopy, permitted the broad structural features to be deduced for (4) and (5). N.O.e. difference spectroscopy permitted stereochemical detail to be added to these structures. Compound (6) was isolated in only trace amounts, and was not stable. Only ^1H-n.m.r. data was available for this compound, and the structure should be regarded as tentative. The ^1H-n.m.r. spectrum of (4) at room temparature in CDCl_3 was extremely broad due to slow exchange between several conformations. Molecular mechanics calculations on compound (4) revealed three major conformations for (4), and their likely proportions in an equilibrium mixture as 4a, 52%; 4b, 39%; 4c, 9%. At 345K in C_6D_6, the ^1H n.m.r. spectrum sharpened, but the coupling constants measured under these conditions were averaged conformational values. They did not relate to the J values calculated for the various minimised conformations. The isolation of briarane diterpenes from a soft coral attributed to the genus Pachyclavularia castes some doubt on its taxonomy.