38 環状ヘミケタールのヒドリド還元制御に基づくテトラヒドロフラン系天然物の立体選択的不斉合成(口頭発表の部)
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We now describe totally stereocontrolled method for the synthesis of both trans- and cis-2,5-substituted tetrahydrofurans 3 based on the stereocontrolled hydride reduction of cyclic hemiketals 1 followed by the stereospecific one-step cyclodehydration of the resulting anti- and syn-1,4-diols 2. Highly stereoselective hydride reduction of the cyclic hemiketal 1 which provides either anti- or syn-1,4-diols (anti-2 or syn-2) depending on hydride reagents employed has been developed. Thus, high degree of and selectivity (5: 1〜>99: 1) is obtained in a series of reactions with LiAlH4 in THE at -78 ℃. In striking contrast, a series of reactions with NaBH_4 in ethanol at 0 ℃ displays good syn selectivity (3: 1〜5: 1). The one-step cyclodehydration of anti-2 and syn-2 with p-TsCl in pyridine proceeds effectively at room temperature to produce trans and cis tetrahydrofurans (trans-3 and cis-3) in complete stereospecificity, respectively. Based on these methods, we have demonstrated the stereoselective asymmetric synthesis of the marine natural product isolated from the brown alga, Notheia anomala, by starting from the optically active epoxide 9. We have also performed the stereocontrolled synthesis of the multifunctional 2,5-substituted tetrahydrofurans 28-31 by starting from the optically active epoxides 18 and 19, which are our key intermediates for the synthesis of a series of polyketide-derived fatty acid derivatives, Annonacious acetogenines, isolated from tropical and subtropical trees.
- 1992-09-10
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