Phosphoester Hydrolysis and Intramolecular Transesterification of Ribonucleoside 2‘- and 3‘-Phosphoromonothioate Triesters: Kinetics and Mechanisms for the Reactions of 5‘-<i>O</i>-Methyluridine 2‘- and 3‘-Dimethylphosphoromonothioates
作者:Mikko Ora、Mikko Oivanen、Harri Lönnberg
DOI:10.1021/jo962356s
日期:1997.5.1
analogs of 5'-O-methyluridine 2'- and 3'-dimethylphosphates have been followed over a wide acidity range, H(0) = -1.7 ([HCl] = 5 mol L(-)(1)) to pH 9. Two reactions were found to compete: mutual interconversion of the 2'- and 3'-isomers and phosphoester hydrolysis to a mixture of phosphorothioate diesters, viz., the R(P) and S(P) diastereomers of 2',3'-cyclic thiophosphate and 2'/3'-monomethylthiophosphates
5'-O-甲基尿苷2'-和3'-二甲基磷酸酯的单硫酸酯类似物的水解反应已在很宽的酸度范围内进行,H(0)= -1.7([HCl] = 5 mol L(-)( 1))的pH值达到9。两个反应竞争:2'-和3'-异构体相互转化,磷酸酯水解为硫代磷酸二酯的混合物,即R(P)和S(P)非对映异构体2′,3′-环硫代磷酸酯和2′/ 3′-单甲基硫代磷酸酯(即三对非对映异构体)。没有观察到明显的脱硫。在pH <0时,互变和水解均显示出速率对酸度的一级依赖性,异构化速度是磷酸酯水解的3-4倍。在弱酸性条件下,水解过程在pH高达7时仍不受pH影响 当异构化反应已经在pH 2时成为氢氧离子催化的([OH(-)]中的一级反应)。水解易于在羧酸缓冲液中进行一般的碱催化,布朗斯台德的β值为0.8。相比之下,没有获得缓冲催化的异构化的确凿证据。建议所有这些反应都是通过五配位的硫代磷烷中间体进行的,该中间体在pH <