Highly enantioselective syntheses of homopropargylic alcohols and dihydrofurans catalyzed by a bis(oxazolinyl)pyridine-scandium triflate complex
Highly enantioselective syntheses of homopropargylic alcohols and dihydrofurans catalyzed by a bis(oxazolinyl)pyridine-scandium triflate complex
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DOI：
10.1021/ja011983i
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发表时间：
2001-12-05
影响因子：
15
通讯作者：
Tedrow, JS
中科院分区：
文献类型：
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作者：
Evans, DA;Sweeney, ZK;Tedrow, JS
刘易斯酸促进的烯丙基硅烷和丙二烯基硅烷的反应为天然产物合成提供了重要的结构单元。1例如，三甲基甲硅烷基丙二烯在醛加成反应中充当炔丙基阴离子当量（方程式1，路径A）。2如果硅中心在空间上拥挤，则正常的加成途径被抑制，并产生官能化的二氢呋喃（等式1，路径B）。3虽然这两种转化都可以通过化学计量的四氯化钛来促进，但还没有报道过烯丙基硅烷的对映选择性反应。4在本通讯中，我们描述了三氟甲磺酸钪催化的丙二烯基硅烷与乙醛酸乙酯的高对映选择性加成和成环反应，初步研究表明，[Sc（S，S）-Ph-pybox]（OTf）_3配合物（1）5（10摩尔%，CH 2Cl 2，-55 C）促进1-甲基-1-甲基-N-（2-甲基-1-氧代-N-甲基-N-氧代-将（三甲基甲硅烷基）丙二烯（2a）转化为乙醛酸乙酯（当量2），在用K2 CO 3/EtOH脱甲硅烷基后以高对映选择性得到（R）-3a（98%ee，95%产率）。6催化剂似乎不受少量水的影响（约1当量）或醇（10当量），并且当向反应混合物中加入六氟-2-丙醇（HFIP）时，产率略有提高。该添加剂抑制低聚副产物的形成，但不影响催化剂活性或反应对映选择性。该反应的范围总结在表1中。三氟甲磺酸钪络合物1提供良好的对映选择性和产率，其中丙二烯基硅烷包含线性或非线性的二氟甲磺酸钪络合物。
Lewis acid promoted reactions of allylsilanes and allenylsilanes provide access to important building blocks for natural product synthesis. 1 For example, trimethylsilylallenes function as propargylic anion equivalents in aldehyde addition reactions (eq 1, Path A). 2 If the silicon center is sterically congested, the normal addition pathway is suppressed and functionalized dihydrofurans are produced (eq 1, Path B). 3 Although both transformations may be promoted by stoichiometric amounts of titanium tetrachloride, enantioselective reactions of allenylsilanes have not been reported. 4 In this Communication, we describe highly enantioselective scandium triflate catalyzed addition and annulation reactions of allenylsilanes with ethyl glyoxylate.Initial investigations revealed that [Sc (S, S)-Ph-pybox](OTf) 3 complex (1) 5 (10 mol%, CH2Cl2,-55 C) promotes the addition of 1-methyl-1-(trimethylsilyl) allene (2a) to ethyl glyoxylate (eq 2) to afford (R)-3a in high enantioselectivity following desilylation with K2CO3/EtOH (98% ee, 95% yield). 6 The catalyst does not appear to be affected by small amounts of water (ca. 1 equiv) or alcohol (10 equiv), and yields were slightly improved when hexafluoro-2-propanol (HFIP) was added to the reaction mixture. This additive suppresses the formation of oligomeric byproducts, but does not influence either the catalyst activity or the reaction enantioselectivity. The scope of this reaction is summarized in Table 1. The scandium triflate complex 1 affords good enantioselectivities and yields with allenylsilanes containing either linear