Acid-catalyzed solvolysis of CMP-N-acetyl neuraminate: Evidence for a sialyl cation with a finite lifetime
Acid-catalyzed solvolysis of CMP-N-acetyl neuraminate: Evidence for a sialyl cation with a finite lifetime
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DOI：
10.1021/ja961811z
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发表时间：
1996-10-30
影响因子：
15
通讯作者：
Bruner, M
中科院分区：
文献类型：
--
作者：
Horenstein, BA;Bruner, M
本文介绍了对胞苷一磷酸 - N - 乙酰神经氨酸（CMP - NeuAc）溶剂解机制的研究，包括一系列同位素标记的CMP - NeuAc的合成，用于测量CMP - NeuAc酸催化溶剂解的多种动力学同位素效应的新方法的使用，以及在甲醇水溶液中通过溶剂解对反应进行的立体化学分析。合成的CMP - NeuAc同位素异构体带有以下标记：[9 - H - 3]、[1 - C - 14 - N - 乙酰基]、[2 - C - 14]、[1 - C - 14]和[9 - H - 3；3,3' - H - 2]，产率分别为78%、86%、76%、85%和35%。在pH 5.0、37℃下溶剂解的β - 氘动力学同位素效应为1.276 ± 0.008；在异头碳C2处的一级C - 14同位素效应为1.030 ± 0.004；在羧基碳C1处观察到异常大的二级C - 14动力学同位素效应，为1.037 ± 0.004。对pH与速率数据以及速率与缓冲液浓度数据的分析确定，溶剂解反应是特异性酸催化的。在甲醇/水混合物中，pH为5或6时CMP - NeuAc的溶剂解产生了NeuAc、等量的NeuAc的α - 和β - 甲基糖苷以及少量的消除产物2,3 - 脱氢 - N - 乙酰神经氨酸。非常大的β - H - 2动力学同位素效应、小的一级C - 14动力学同位素效应以及在羧基碳处大的二级C - 14动力学同位素效应，与一种非常晚期的类似氧鎓离子的过渡态相符，在该过渡态中羧基碳所处的环境比基态更宽松。在溶剂解反应中观察到的外消旋化现象支持一种在过渡态之后形成唾液酸阳离子的反应途径。
An investigation of the mechanism of solvolysis of CMP-N-acetyl neuraminate (CMP-NeuAc) is presented that includes synthesis of a family of isotopically labeled CMP-NeuAc's, use of new methodology for measurement of multiple kinetic isotope effects for acid-catalyzed solvolysis of CMP-NeuAc, and a stereochemical analysis of the reaction by solvolysis in aqueous methanol. The CMP-NeuAc isotopomers were synthesized with the following labels: [9-H-3], [1-C-14-N-acetyl], [2-C-14], [1-C-14], and [9-H-3;3,3'-H-2] in yields of 78%, 86%, 76%, 85%, and 35%, respectively. The beta-dideuterium kinetic isotope effect for solvolysis at pH 5.0, 37 degrees C, was 1.276 +/- 0.008; the primary C-14 isotope effect at C2, the anomeric carbon, was 1.030 +/- 0.004; and an unusually large secondary C-14 KIE was observed at C1, the carboxylate carbon, of 1.037 +/- 0.004. Analysis of pH versus rate data and rate versus buffer concentration data establish that the solvolytic reaction is specific acid-catalyzed. Solvolysis of CMP-NeuAc at pH 5 or pH 6 in methanol/water mixtures afforded NeuAc, equal quantities of the alpha- and beta-methyl glycosides of NeuAc, and small amounts of the elimination product 2,3-dehydro-N-acetyl neuraminic acid. The very large beta-H-2 KIE, small primary C-14 KIE, and the large secondary C-14 KIE at the carboxylate carbon are consistent with a very late oxacarbenium ion-like transition state in which the carboxylate carbon is in a looser environment than in the ground state. The observation of racemization in the solvolysis reaction supports a reaction pathway that proceeds with the formation of a sialyl cation after the transition state.