Leaving group effects in reductively triggered fragmentation of 4-nitrobenzyl carbamates †
作者:Bridget M. Sykes、Michael P. Hay、Dubravka Bohinc-Herceg、Nuala A. Helsby、Charmian J. O’Connor、William A. Denny
DOI:10.1039/b000135j
日期:——
The rates and extent of release of a series of substituted anilines from 4-nitrobenzyl carbamates, following nitro group reduction by radiolytic, enzymic and chemical methods, are reported. The yield of released anilines decreased over the pH range 4–7, but was independent of the basicity of the leaving aniline. Detailed studies of the fragmentation of one example identified the 4-hydroxylamine as the key intermediate. At pH greater than 5 the released aniline 3b condenses with a reactive 4-iminoquinomethane intermediate 4a to give amine 26, thus depleting the measurable amount of aniline 3b released. At pH less than 5 the release of amine proceeds to completion. The efficiency of reductively triggered release of anilines 7 varied with small changes in the leaving group, but this was not uniformly related to aniline basicity. The competing reaction of the released aniline 3b to form amine 26 lowers the efficiency of release of 3b. This reaction occurs at the relatively high concentrations (50 μM) used in the study and indicates the released effector amine should be toxic at concentrations considerably lower than 50 μM. This highlights the need for prodrugs of very potent cytotoxic effectors to be used in tumour-directed nitroreductase enzyme-prodrug therapy.
报告了通过放射性、酶和化学方法将硝基还原后,一系列取代苯胺从 4-硝基苄基氨基甲酸酯中释放出来的速率和程度。释放出的苯胺的产率在 pH 值为 4-7 的范围内下降,但与离开苯胺的碱性无关。对一个例子的碎裂过程进行的详细研究发现,4-羟胺是关键的中间体。当 pH 值大于 5 时,释放出的苯胺 3b 与反应性 4- 亚氨基喹啉甲烷中间体 4a 缩合,生成胺 26,从而消耗了可测量的苯胺 3b 释放量。当 pH 值小于 5 时,胺的释放会进行到底。还原触发释放苯胺 7 的效率随离去基团的微小变化而变化,但这与苯胺的碱性并不一致。释放出的苯胺 3b 发生竞争反应生成胺 26,从而降低了 3b 的释放效率。这种反应发生在研究中使用的相对较高的浓度(50 μM)下,表明释放的效应胺在浓度大大低于 50 μM 时就会产生毒性。这凸显了在肿瘤定向硝基还原酶酶促药物疗法中使用强效细胞毒性效应物质原药的必要性。