Stereoselective synthesis of 2,4,5-trisubstituted piperidines by carbonyl ene and Prins cyclisations
作者:Claire A. M. Cariou、Benson M. Kariuki、John S. Snaith
DOI:10.1039/b808644c
日期:——
An approach to 2,4,5-trisubstituted piperidines is reported, in which the key step is the Prins or carbonyl ene cyclisation of aldehydes of the type 1. Prins cyclisation catalysed by concentrated hydrochloric acid in CH2Cl2 at −78 °C afforded good yields of two of the four possible diastereomeric piperidines, with the 4,5-cis product 7 predominating in a diastereomeric ratio of up to 94 : 6. The diastereoselectivity of the cyclisation decreased as the 2-substituent increased in size, becoming unselective for very bulky 2-substituents. In contrast, cyclisation catalysed by MeAlCl2 in CH2Cl2 or CHCl3 at temperatures of between 20–60 °C, favoured the 4,5-trans diastereomer 8, in a diastereomeric ratio of up to 99 : 1. The low-temperature cyclisations catalysed by HCl proceed under kinetic control via a mechanism involving the development of significant carbocationic character, in which the 4,5-cis cation is more stable than the 4,5-trans cation as a result of overlap with the neighbouring oxygen. The cyclisations catalysed by MeAlCl2 proceed under thermodynamic control, affording the product in which both the 4- and 5-substituents are equatorial.
报告了一种合成2,4,5-三取代哌啶的方法,其中关键步骤是醛类化合物1的Prins或羰基烯环化反应。在-78°C下,采用浓盐酸催化的CH2Cl2中进行的Prins环化反应,获得了四种可能的立体异构体中的两种良好产率,其中4,5-顺式产物7的产率在立体异构体比率上高达94:6。随着2-取代基体积的增大,环化反应的立体选择性降低,对于体积非常大的2-取代基体则变得无选择性。相比之下,在20–60°C的CH2Cl2或CHCl3中,MeAlCl2催化的环化反应则偏向于4,5-反式立体异构体8,其立体异构体比率高达99:1。由HCl催化的低温环化反应在动力学控制下进行,机制涉及显著的碳正离子特性,其中,4,5-顺式阳离子由于与相邻氧的重叠而比4,5-反式阳离子更稳定。而由MeAlCl2催化的环化反应则在热力学控制下进行,生成的产物中4-和5-取代基均为赤面配置。