Efficient Deracemization of Pipecolic Acid Amides through Enantioselective Protonation of Their Lithium Enolates: Insights into the Origin of the Transferred Proton
作者:Juliette Martin、Jean-Christophe Plaquevent、Jacques Maddaluno、Jacques Rouden、Marie-Claire Lasne
DOI:10.1002/ejoc.200900726
日期:2009.11
alkoxide/nBuLi” were the only bases to allow complete formation of the enolate in conjunction with high stereocontrol of the protonation. Experiments with (+)- or(–)-ephedrine derivatives as chiral sources and deuteriated reagents gave evidence that both the OH and NH protons of ephedrine were involved in the stereoinduction. External delivery of the proton was mainly operative with the aniline derivative (+)-5,
报道了哌可酸酰胺去外消旋的详细研究。使用市售麻黄碱对这些酰胺的烯醇锂进行对映选择性质子化导致对映体过量(ee 值)高于 99%。反应的成功很大程度上取决于以下参数:碱、反应温度、手性源的结构和非手性猝灭剂。sBuLi 和双金属碱“钾醇盐/nBuLi”是唯一允许完全形成烯醇化物并结合质子化的高立体控制的碱。用 (+)- 或 (-)- 麻黄碱衍生物作为手性来源和氘化试剂的实验证明麻黄碱的 OH 和 NH 质子都参与了立体诱导。如氘标记实验所示,质子的外部传递主要通过苯胺衍生物 (+)-5 起作用,而内部淬灭是使用麻黄碱观察到的主要途径 (6)。最后,脱消旋过程成功地用于从外消旋哌可酸制备 N-保护的哌可酸的两种对映异构体(总产率为 51%,ee 为 99%)。(© Wiley-VCH Verlag GmbH & Co. KGaA,69451 Weinheim,德国, 2009)