A1 Refereed original research article in a scientific journal
Phosphoester hydrolysis and intramolecular transesterification of ribonucleoside 2'- and 3'-phosphoromonothioate triesters: Kinetics and mechanisms for the reactions of 5'-O-methyluridine 2'- and 3'-dimethylphosphoromonothioates
Authors: Ora M, Oivanen M, Lonnberg H
Publisher: AMER CHEMICAL SOC
Publication year: 1997
Journal:: Journal of Organic Chemistry
Journal name in source: JOURNAL OF ORGANIC CHEMISTRY
Journal acronym: J ORG CHEM
Volume: 62
Issue: 10
First page : 3246
Last page: 3253
Number of pages: 8
ISSN: 0022-3263
DOI: https://doi.org/10.1021/jo962356s
Abstract
The hydrolytic reactions of the monothioate 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 (i.e., three pairs of diastereomers). No marked desulfurization could be observed. The interconversion and hydrolysis both show first-order dependence of rate on acidity at pH < 0, the isomerization being 3-4 times as fast as the phosphoester hydrolysis. Under less acidic conditions, the hydrolysis remains pH-independent up to pH 7, while the isomerization becomes hydroxide-ion-catalyzed (first-order in [OH-]) already at pH 2. The hydrolysis is susceptible to general base catalysis in carboxylic acid buffers, the Bronsted beta value being 0.8. In contrast, no conclusive evidence for buffer-catalyzed isomerization could be obtained. All these reactions are suggested to proceed via a pentacoordinated thiophosphorane intermediate, obtained at pH < 1 by an attack of the neighboring hydroxy function on a protonated (monocationic) thiophosphate group and at pH > 2 by an attack of a deprotonated hydroxy function (oxyanion) on a neutral thiophosphate. The monocationic intermediate (pH < 1) may collapse to hydrolysis and isomerization products without further catalysis (departure of alcohol). The monoanionic thiophosphorane (pH > 2) also gives isomerization products without catalysis (departure of 2'/3'-oxyanion), whereas breakdown to the hydrolysis products needs either a specific or a general acid catalysis process (departure of methanol). Accordingly, the observed general-base-catalyzed hydrolysis most likely consists of consecutive specific base/general acid catalysis. The phosphorothioate triesters studied are, under very acidic conditions, more than 2 orders of magnitude more stable than their oxyphosphate counterparts, whereas the rate-retarding ''thio effect'' (k(P=O)/K-P=S) is much smaller with the hydroxide ion-catalyzed reactions (ca. ij and almost negligible with the pH-independent hydrolysis.
The hydrolytic reactions of the monothioate 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 (i.e., three pairs of diastereomers). No marked desulfurization could be observed. The interconversion and hydrolysis both show first-order dependence of rate on acidity at pH < 0, the isomerization being 3-4 times as fast as the phosphoester hydrolysis. Under less acidic conditions, the hydrolysis remains pH-independent up to pH 7, while the isomerization becomes hydroxide-ion-catalyzed (first-order in [OH-]) already at pH 2. The hydrolysis is susceptible to general base catalysis in carboxylic acid buffers, the Bronsted beta value being 0.8. In contrast, no conclusive evidence for buffer-catalyzed isomerization could be obtained. All these reactions are suggested to proceed via a pentacoordinated thiophosphorane intermediate, obtained at pH < 1 by an attack of the neighboring hydroxy function on a protonated (monocationic) thiophosphate group and at pH > 2 by an attack of a deprotonated hydroxy function (oxyanion) on a neutral thiophosphate. The monocationic intermediate (pH < 1) may collapse to hydrolysis and isomerization products without further catalysis (departure of alcohol). The monoanionic thiophosphorane (pH > 2) also gives isomerization products without catalysis (departure of 2'/3'-oxyanion), whereas breakdown to the hydrolysis products needs either a specific or a general acid catalysis process (departure of methanol). Accordingly, the observed general-base-catalyzed hydrolysis most likely consists of consecutive specific base/general acid catalysis. The phosphorothioate triesters studied are, under very acidic conditions, more than 2 orders of magnitude more stable than their oxyphosphate counterparts, whereas the rate-retarding ''thio effect'' (k(P=O)/K-P=S) is much smaller with the hydroxide ion-catalyzed reactions (ca. ij and almost negligible with the pH-independent hydrolysis.
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