P-54 ヒドロキシアレンの環化反応とポリ環状エーテル合成への応用(ポスター発表の部)
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Stereoselective construction of cyclic ethers is one of the major problems in the synthesis of fused-polycyclic ether marine toxins. We have explored a new method for the synthesis of vinyl-substituted tetrahydropyrans by metal-mediated cyclization of hydroxy allenes. Hydroxy allenes 6, 7, 13, and 14 were synthesized by reactions of aldehyde 1 with chromium reagents prepared from 3-trimethylsilylpropargyl bromide or 1-bromo-2-butyne and chromium (II) chloride. Treatment of δ-hydroxy allenes having an oxygenated substituent at the α-position with silver and mercuric salts produced vinyl-substituted tetrahydropyran derivatives in fairly good yields. Yield and stereochemistry of the products were highly dependent on both metal salt employed and the stereogenic center at the α-position. Silver perchlorate induced the cyclization of 6 to give 8 as a single isomer, whereas cyclization of its diastereomer 7 provided a mixture of products 9 and 10. This silver-mediated cyclizaion was further applied to the synthesis of methyl and vinyl-substituted tetrahydropyrans. Cyclizations of hydroxy methyl-substituted allenes 13 and 14 by silver salts were less effective. The best results were obtained by using mercuric triflate to give 15 from 13 as a single isomer and a mixture of 17 and 18 from 14. Iterative application of this silver-mediated cylization to 8 enabled the synthesis of trans-fused tricyclic ring system 23. Moreover, the 6-membered E ring of gambierol, a neurotoxin isolated from culture cells of Gambierdiscus toxicus, was constructed by silver-mediated cyclization of hydroxy allene 27.
- 天然有機化合物討論会の論文
- 2008-09-01
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