Cyclolobatriene, a novel prenylated germacrene diterpene, from the soft coral Lobophytum pauciflorum

A new 10-membered-ring diterpene, cyclolobatriene (1), along with three other known diterpenes, lobatriene (2), eunicol (3), and fuscol (4), were isolated from the Okinawan soft coral Lobophytum pauciflorum. Their structures were established by extensive NMR spectroscopic analyses. Cyclolobatriene (1) is an additional example of rare prenylated germacrenes. Although 1, due to a 10-membered-ring structure, exists as an equilibrium mixture of three conformers, the NMR measurement in CDCl(3) at 7°C enabled us to assign the NMR signals of the three, which is the first example of the complete NMR assignment of all the existing conformers of germacrene-type compounds. Cyclolobatriene (1) was thermally unstable and converted into 2 through Cope rearrangement upon heating at 70°C. Eunicol (3) also possesses the same prenylated germacrene structure as 1, showing similar physico-chemical properties to 1. All four compounds 1-4 showed cytotoxic effect with IC(50)’s of 0.64, 0.41, 0.35 and 0.52 μM, respectively, against human epidermoid carcinoma A431 cells.