THE BRIARELLINS, NEW EUNICELLIN-BASED DITERPENOIDS FROM A CARIBBEAN GORGONIAN, BRIAREUM-ASBESTINUM

The Caribbean gorgonian Briareum asbestinum has been shown to contain representatives from three skeletal classes, asbestinins, briareins, and eunicellins. The structural diversity of the diterpenoids isolated suggest a common biosynthetic pathway. Briarellius -A (1), -B (3), -C (5), and -D (6) are members of a new class of tetracyclic diterpenoids of the eunicellin family distinguished by the presence of a seven-membered lactone ring formed between C-3 and C-16. Seco-briarellin e), which possesses a novel carbon skeleton, is the first representative of a new class of ether-cyclized eunicellin diterpenes known as seco-eunicellins. The briarellins are also unique among the compounds in this series in having the opposite relative stereochemistry at C-11 to that found in eunicellin-based compounds isolated from South Pacific soft corals. The structures of the new eunicellin-type diterpenoids were established after extensive analysis of spectral data and those of 3 and 5 were confirmed by chemical interconversions. Briarellin-A (1) displayed modest in vitro cytotoxicity against HeLa cells.