39 (-)-イルシニアニンの全合成と(+)-ウィスタリンへの変換(口頭発表の部)

抄録

(-)-Ircinianin (1) and (+)-wistarin (2) are a rare cyclic furanosesterterpenetetronic acid as a secondary metabolite in marine sponge of Ircinia sp. Hofheinz reported the isolation of 1 from genus ircinia and determined the relative structure in 1977. The structurally related 2 was also found in sponge, ircinia wistarii at by Gregson in 1982. However the absolute structures have not been revealed. Herein, we like to report the first asymmetric total synthesis of 1 and 2, and the reported structure of (+)-wistarin is revised. Our synthesis started methyl R-(-)-3-hydroxy-2-methylpropionate to reach tetronic acid intermediate 5 in 14 steps. On the other hand, iodo triene intermediate 6 was derived from 3-furfural in 12 steps involving the key stereospecific trisubstituted iodo olefin formation. A key coupling of 5 with 6 was performed by NiCl2-CrCl2 mediated reaction. The coupling products were readily cyclized by heating in xylene to give the tricyclic carbon skeleton. Deoxygenation and deprotection completed the total synthesis of 1 in good yields. All the physical and spectroscopic data including specific rotation of the synthetic 1 were identical with the reported data in literature. Iodo ether formation of 1 with iodine in the presence of K_2CO_3 followed by radical reduction with tributyltin hydride afforded 2 which was identified by a comparison with the reported data of natural wistarin. However, the reported structure conflicts with our result concerning the quaternary center at C-8. The structure of the iodo ether suggests the furanopropyl group exists at an equatorial position, which means the quaternary carbon should possesses R-configuration.