Selective radical-chain epimerisation at electron-rich chiral tertiary C–H centres using thiols as protic polarity-reversal catalysts
作者:Hai-Shan Dang、Brian P. Roberts、Derek A. Tocher†
DOI:10.1039/b103558b
日期:——
Radical-chain epimerisation at chiral tertiary CH centres adjacent to ethereal oxygen atoms can be brought about in the presence of thiols, the function of which is to act as protic polarity-reversal catalysts for hydrogen-atom transfer between pairs of nucleophilic α-alkoxyalkyl radicals. The viability of the method is demonstrated by epimerisation of a series of simple molecules that contain two chiral centres and then the procedure is applied to more complex carbohydrate-based systems, where it is possible to convert a readily available diastereoisomer into a rarer one in a straightforward manner. Of necessity, epimerisation always proceeds in the direction of thermodynamic equilibrium and, in general, the results obtained are in accord with the predictions of molecular mechanics calculations using the MMX force-field. When the required isomer is less stable than the starting diastereoisomer, thiol-catalysed epimerisation of a suitable derivative of the parent can provide a means to obtain the desired compound in satisfactory yield, after deprotection of the epimerised derivative. This strategy is demonstrated for the conversion of trans-cyclohexane-1,2-diol into the less stable cis-isomer and for related contra-thermodynamic isomerisation of some carbohydrates, as well as for the conversion of meso-1,2-diphenylethane-1,2-diol into the dl-form. Thiol-catalysed epimerisation at a CH centre adjacent to an ether-oxygen atom is much faster than at a similar centre adjacent to an amido-nitrogen atom, a result that can be understood in terms of the importance of polar effects on the rate of abstraction of hydrogen by electrophilic thiyl radicals.
硫醇的作用是作为亲核δ-烷氧基烷基自由基对之间氢原子转移的原生极性反转催化剂,可以在邻近乙氧基原子的手性三级 CH 中心实现自由基链表并化。通过对一系列含有两个手性中心的简单分子进行外延二聚化,证明了该方法的可行性,然后将该程序应用于更复杂的基于碳水化合物的体系,在这些体系中,可以通过直接的方式将容易获得的非对映异构体转化为较罕见的非对映异构体。必要时,二聚化总是朝着热力学平衡的方向进行,一般来说,所获得的结果与使用 MMX 力场进行分子力学计算的预测结果一致。当所需异构体的稳定性低于起始非对映异构体时,可采用巯基催化母体的适当衍生物进行外延化反应,在对外延化衍生物进行脱保护处理后,以令人满意的收率获得所需化合物。这种策略在反式-1,2-环己烷二醇转化为稳定性较差的顺式异构体、某些碳水化合物的相关逆热力学异构化以及介-1,2-二苯基乙烷-1,2-二醇转化为 dl-形式时得到了验证。邻近醚氧原子的 CH 中心在硫醇催化下的缩合反应要比邻近酰胺氮原子的类似中心快得多,这一结果可以从极性效应对亲电巯基取氢速率的重要性角度来理解。