The H-phosphonate approach to oligonucleotide synthesis. An investigation on the mechanism of the coupling step
The H-phosphonate approach to oligonucleotide synthesis. An investigation on the mechanism of the coupling step
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H-膦酸寡核苷酸合成方法。
DOI：
10.1039/b205142g
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发表时间：
2002
期刊：
影响因子：
--
通讯作者：
R. Strömberg
中科院分区：
文献类型：
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作者：
S. Sigurdsson;R. Strömberg
对不同取代吡啶存在下特戊酰氯促进的H-膦酸酯缩合反应进行了动力学研究。当3′-H-膦酸核苷与核苷组分（含游离5-OH）的比例为1 ∶ 1时，该体系的动力学符合二级动力学。无论使用何种吡啶衍生物，反应速率都取决于其浓度以及羟基组分和H-膦酸酯的浓度。从动力学证据中间接鉴定出反应性中间体。该中间体被认为是通过吡啶衍生物对最初形成的混合膦酸羧酸酐的攻击而形成的吡啶鎓加合物。在空间位阻吡啶存在下的相当低的缩合速率进一步支持在与非位阻吡啶的反应中存在亲核催化。此外，在pKa值高于1.4.8的吡啶存在下，新戊酰氯浓度的增加会提高反应速率。这种增强的主要原因很可能是新戊酰氯除去新戊酸根离子的事实，新戊酸根离子通过影响混合酸酐和吡啶中间体之间的平衡而阻碍反应。虽然所观察到的速率常数是由几个常数，它们的温度依赖性给出了一些指示的性质的过渡态的限速步骤。活化熵估计是稍微积极的，这表明过渡状态所产生的攻击的羟基组分上的吡啶中间体，但涉及一个公平的程度的键断裂的离去基团，即吡啶。
A kinetic study on the pivaloyl chloride promoted H-phosphonate condensation step in the presence of differently substituted pyridines has been carried out. The kinetics of the system follow second order kinetics with a 1 ∶ 1 ratio of nucleoside 3′-H-phosphonate and nucleoside component (with free 5-OH). Irrespective of the pyridine derivative used, the reaction rate is dependent on the concentration of this as well as of the hydroxy component and H-phosphonate. A reactive intermediate is indirectly identified from the kinetic evidence. This intermediate is suggested to be a pyridinium adduct formed by attack of the pyridine derivative on the initially formed mixed phosphonic carboxylic anhydride. Considerably lower rates of condensation in the presence of sterically hindered pyridines further support the existence of nucleophilic catalysis in the reactions with non-hindered pyridines. In addition, the rate of reaction in the presence of pyridines with pKa values above ∼4.8 is enhanced by an increase in pivaloyl chloride concentration. The main reason for this enhancement is most likely the fact that pivaloyl chloride removes pivalate ion, which retards the reaction by influencing the equilibrium between the mixed anhydride and the pyridinium intermediate. Although the observed rate constants are composed of several constants, their temperature dependence gives some indication of the nature of the transition-state of the rate-limiting step. Entropies of activation are estimated to be slightly positive, suggesting a transition state arising from attack of the hydroxy component on the pyridinium intermediate but involving a fair degree of bond breakage to the leaving group, i.e. the pyridine.