作者:Jean-Damien Charrier、James E. S. Duffy、Peter B. Hitchcock、Douglas W. Young
DOI:10.1039/b106451g
日期:——
Previous studies on the alkylation of pyroglutamate ester urethanes have led to a consensus that alkylation at C-4 occurs to give a mixture of diastereoisomers, the major isomer of which usually has the alkyl group trans to the ester group at C-2. We have now discovered that this generalisation is not invariably correct and that, although for SN1-type electrophiles stereoselectivity is in fact trans, SN2-type electrophiles can give the thermodynamically less stable cis compounds as the predominant products. Use of the bulky proton source 2,6-di-tert-butylphenol to quench these reactions yields the cis isomers as the only products in good yield, thus making direct alkylation of pyroglutamic acid derivatives a useful alternative to our hydrogenation approach to these synthons.
先前关于焦谷氨酸盐酯尿烷烷基化的研究已达成共识,即在C-4位烷基化会生成一种非对映异构体混合物,其中主要异构体的烷基团通常与C-2位上的酯团处于反式。我们如今发现这一概括并不总是正确,尽管对于SN1型亲电试剂而言,其立体选择性确实是反式的,但SN2型亲电试剂却能生成热力学上较不稳定的顺式化合物作为主要产物。利用大体积的质子源2,6-二叔丁基苯酚来终止这些反应,可在良好产率下得到顺式异构体作为唯一产物,因此直接烷基化焦谷氨酸衍生物成为我们氢化获取这些合成试剂的有效替代方法。