Asymmetric aldol additions:: Use of titanium tetrachloride and (-)-sparteine for the soft enolization of N-acyl oxazolidinones, oxazolidinethiones, and thiazolidinethiones
Asymmetric aldol additions:: Use of titanium tetrachloride and (-)-sparteine for the soft enolization of N-acyl oxazolidinones, oxazolidinethiones, and thiazolidinethiones
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DOI：
10.1021/jo001387r
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发表时间：
2001-02-09
影响因子：
3.6
通讯作者：
Chaudhary, K
中科院分区：
文献类型：
--
作者：
Crimmins, MT;King, BW;Chaudhary, K
使用氯钛烯醇化n-酰基氧杂唑烷酮、恶唑烷硫酮和噻唑烷硫酮丙酸酯进行不对称醛醇加成，对Evans或非Evans合成产物具有高的非对映选择性，这取决于所使用碱的性质和量。以1等量的四氯化钛和2等量的(-)-sparteine为碱或1等量的(-)-sparteine和1等量的n -甲基-2-吡咯烷酮为碱，对n -丙酰基恶唑烷酮、恶唑烷硫酮和噻唑烷硫酮合成的Evans合成醛的选择性为97:3 ~ 99:1。通过改变刘易斯酸/胺碱比，可与恶唑烷硫酮和噻唑烷硫酮合成非埃文斯合成醛加合物。醛醇加入物表面选择性的变化被认为是在螯合和非螯合过渡态之间的机制途径切换的结果，助剂可以被还原去除或被亲核酰基取代切割。还可以实现具有高非对映选择性的迭代aldol序列。
Asymmetric aldol additions using chlorotitanium enolates of N-acyloxazolidinone, oxazolidinethione, and thiazolidinethione propionates proceed with high diastereoselectivity for the Evans or non-Evans syn product depending on the nature and amount of the base used. With 1 equiv of titanium tetrachloride and 2 equiv of (-)-sparteine as the base or 1 equiv of(-)-sparteine and 1 equiv of N-methyl-2-pyrrolidinone, selectivities of 97:3 to >99:1 were obtained for the Evans syn aldol products using N-propionyl oxazolidinones, oxazolidinethiones, and thiazolidinethiones. The non-Evans syn aldol adducts are available with the oxazolidinethione and thiazolidinethiones by altering the Lewis acid/amine base ratios. The change in facial selectivity in the aldol additions is proposed to be a result of switching of mechanistic pathways between chelated and nonchelated transition states, The auxiliaries can be reductively removed or cleaved by nucleophilic acyl substitution. Iterative aldol sequences with high diastereoselectivity can also be accomplished.