18 生合成推定経路を基盤とした縮環型ポリ環状エーテルの構築(口頭発表の部)

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trans-Fused polycyclic ethers are the common frameworks of various marine polyether compounds, such as brevetoxins, ciguatoxins, and maitotoxin that have attracted many chemists from their characteristic structures, biosyntheses, and biological activities. The total syntheses of hemibrevetoxin B, brevetoxin B, and brevetoxin A had already been achieved. Nakanishi and Shimizu proposed the biogeneses in which these polyethers might be produced from the requisite polyepoxy precursors (Scheme 1). Their hypothetical routes seem to be plausible, though there has been not any chemical evidence that suggests the reaction mechanism. Many of the chemical syntheses of assembled polycyclic ethers from polyepoxy compounds has been performed through successive exo-cyclizations, while few successful endo-selective cyclizations in the polyepoxide system have been reported. We describe the first chemical realization of the biomimetic construction of the six-membered trans-fused tricyclic ether. In order to control the cyclization in an endo-mode, we have planned to activate an epoxide regioselectively by the coordination of Lewis acid between the oxygen atoms of a methoxymethyl group and an epoxide (Scheme 2). After many attempts, it was found that the use of La(OTf)_3 afforded 6- and 7-endo cyclization predominantly in the presence of La_2O_3 and H_2O. 8-endo-Selective cyclization was also achieved in the absence of H_2O (Scheme 3). We have extended the above results to the biomimetic reaction. anti-Diepoxy (11) and anti, anti-triepoxy alcohols (13) were prepared by Shi's asymmetric epoxidation of the respective (Z,Z)-diene 6 and (Z,Z,Z)-triene 12, which have been constructed by the procedure involving the syn-seiective aidol coupling followed by the syn-dehydration. On treatment with a 4:1 mixture of La(OTf)_3 and La_2O_3, in the presence of H_2O, racemic 9 corresponding to 11 provided a trans-fused bicyclic ether 14 in 52% yield, along with a trans-fused perhydropyranofuran 15 as well as monocyclic compounds 16 and 17 (Scheme 7). Under the same conditions, 13 afforded a trans-fused tricyclic ether 19 in 9.3% yield (Scheme 9). Thus we have succeeded in the first biomimetic construction of trans-fused bi- and tricyclic ethers from the di- and triepoxy alcohols in one pots, respectively.
天然有機化合物討論会の論文
2000-10-01

18 生合成推定経路を基盤とした縮環型ポリ環状エーテルの構築(口頭発表の部)

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