Complex-induced proximity effects: The effect of varying directing-group orientation on carbamate-directed lithiation reactions
Complex-induced proximity effects: The effect of varying directing-group orientation on carbamate-directed lithiation reactions
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DOI：
10.1021/ja002662u
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发表时间：
2001-01-17
影响因子：
15
通讯作者：
Beak, P
中科院分区：
文献类型：
--
作者：
Gross, KMB;Beak, P
研究了一系列选定的双环氨基甲酸酯，其中羰基与在锂化反应中去除的质子之间的可达角范围和距离在结构上是确定的。恶唑烷酮7-10通过立体选择性锂化取代反应生成了顺式18-27和顺式31-35作为主要的非对映体。两个系列的竞争实验表明，构象受限氨基甲酸酯7、10、11和15比Boc胺4-6更有效地通过配合物进行锂化。这些结果以及半经验计算表明，氨基甲酸酯羰基氧与待去除质子之间较小的二面角和计算出的2.78埃的距离有利于氨基甲酸酯定向锂化。在顺式、反式18和(S)-39上进行的一系列锡锂交换实验表明，氨基甲酸酯稳定的有机锂可以通过限制几何结构来增强其构型稳定性。
A series of selected bicyclic carbamates in which the range of accessible angles and distances between the carbonyl group and the proton removed in an a-lithiation reaction are structurally defined have been investigated. Oxazolidinones 7-10 undergo stereoselective lithiation-substitution reactions to provide cis-18-27 and cis-31-35 as the major diastereomers. Two series of competition experiments show that the conformationally restricted carbamates 7, 10, 11, and 15 undergo lithiation via complexes more efficiently than Boc amines 4-6. These results along with semiempirical calculations suggest that a small dihedral angle and a calculated distance of 2.78 Angstrom between the carbamate carbonyl oxygen and the proton to be removed are favorable for a carbamate-directed lithiation. A series of tin-lithium exchange experiments on cis- and trans-18 and (S)-39 indicate that the configurational stability of a carbamate-stabilized organolithium species may be enhanced by restrictive geometry.