94(P-22) フランの特性に基づくスピロ環化反応を用いたマンザミンAの合成研究(ポスター発表の部)
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概要
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We have recently completed the total synthesis of nakadomarin A utilizing furan-iminium cation cyclization, that is, intramolecular vinylogous Mannich reaction of furan. In this time, further application of this reaction to the synthesis of manzamine A was investigated. We expected that 6/6/5 tricyclic core of manzamine A would be constructed by furan-iminium cation cyclization between spiro iminium cations 1 and 2-furyl group connected to A ring with 1 or 2 carbon unit. Firstly, two cyclization precursors, 12 and 19, were prepared. From spirolactam 5, C1 unit and 2-furyl group were introduced to give furyl ketone 7 by carbonylative Stille coupling. Then deprotection and 1,2-reduction of enone gave secondary alcohol 10. Following protection of alcohol and reduction of lactam gave hemiaminal 12. On the other hand, C2 unit and 2-furyl group were introduced to 5 stepwisely. Wittig reaction of 5 gave exo olefin 13 which was diastereoselectively hydrogenated after deprotection of PMB to give 15. Then the ester 15 was converted to 2-furyl ketone 16 via Weinreb amide. After the protection of carbonyl group of ketone, hemiaminal 19 was obtained in similar protocol as above. Then, furan-iminium cation cyclization was examined. Both 12 and 19 cyclized regioselectively at 2-position of furan under mild acidic conditions. Furthermore, the stereochemistry of spiro center was completely controlled to give the ABC ring system of manzamine A with correct stereocenters. In conclusion, we have developed the new strategy of furan-iminium cation cyclization. This methodology would be effective in construction of carbon flameworks. Total synthesis of manzamine A using this strategy is now under investigating.
- 天然有機化合物討論会の論文
- 2004-10-01
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