Clavulones were isolated from the Okinawan soft coral, Clavularia viridis, and have been recognized as marine prostanoids having unique structures and significant biological activities. Our continuous studies to find new prostanoids from C. viridis have resulted in the isolation of about 50 congeners of clavulones. The present study deals mainly with the true producer for clavulones, the host soft coral or its symbiotic alga, and the biosynthetic pathway for clavulones. The cultured symbiotic alga, isolated from C. viridis and identified as a dinoflagellate of the genus Symbiodinium, was extracted with MeOH. The AcOEt soluble portion of the MeOH extract was examined to find clavulones, however clavulones as well as their congeners were not detected. Arachidonic acid, the biosynthetic precursor for clavulones, was also not detected in the AcOEt soluble portion by gas chromatographic analysis. The quantitative analysis of clavulones in the fractions obtained by the centrifugal separations of C. viridis indicated the majority of clavulones in the coral fraction rather than the symbiotic alga fraction. These findings strongly suggested that clavulones are produced by the host soft coral itself. In order to obtain biosynthetic intermediates in the biosynthesis for clavulones, the hexane extract of freeze-dried C. viridis was searched, to give preclavulone-A methyl ester (1) and its stereoisomer 2 as the minor prostanoids. Preclavulone-A is an important intermediate in the biosynthetic pathway for clavulones mediated by lipoxygenase, but has not been found as a natural product so far. The biosynthetic experiment using a crude enzyme system (acetone powder) prepared from C. viridis was also carried out. Arachidonic acid in a Tris-HCl buffer solution was treated with the acetone powder at room temperature to give clavulones, whose spectral data were identical with those for the natural products. These findings provided the evidences for the pathway shown in Scheme 1 for the biosynthesis of clavulones proposed by Corey et al. The molecular phylogenetic analysis for the soft corals of the genus Clavularia was also performed.
