59(P-33) Structure of Stolonilactone with a Unique Skeleton from a Soft Coral
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The Okinawan soft coral Clavularia virids is recognized as a rich source of structurally unique antitumor prostanoids such as clavulones and chlorovulones, while other species belonging to the genus Clavularia inhabiting in the Okinawan coral reef contain a number of sesquiterpenoids and diterpenoids instead of prostanoids (Figure 1). Our continuous investigation on chemical constituents of the soft coral belonging to the genus Clavularia resulted in the isolation of a new terpenoid-related compound with a unique skeleton, stolonilactone (1), along with four new sesquiterpenoids (2,3,4 and 5) and two new diterpenoids (6 and 7) from Clavularia koellikeri. From the ethyl acetate soluble portion of the methanol extract of the soft coral, compounds 1-7 as well as (-)-trans-cembranolide and neodollabelenol were isolated by normal phase LCC, and normal and reverse phase HPLC. The plane structure of 1 possessing a fourteen-membered carbocyclic ring with a γ-lactone, bicyclo[2.2.1]heptene, and a cycloheptadiene (Figure 2), was elucidated by the analysis of 1D and 2D NMR spectral data (Table 1, Figure 3). The stereochemistry of three double bonds and the relative configuration of six chiral centers in 1 were assigned based on the NOESY spectral analysis (Figure 4). A possible biogenetic pathway for the unique structure of 1 was proposed: the [4+2] cycloaddition reaction between the clavukerin-type sesquiterpenoid diene and the exomethylene of the cembrane-type diterpenoid may proceed to yield the bicyclo[2.2.1]heptene (Figure 5). The absolute stereochemistry of 1 was thus suggested as shown in Figure 2, because absolute stereochemistries of (-)-trans-cembranolide and clavukerin were previously established.
