Hydrolysis and desulfurization of the diastereomeric phosphoromonothioate analogs of uridine 2',3'-cyclic monophosphate

Hydrolyses of the two diastereomeric phosphoromonothioate analogs of uridine 2',3'-cyclic monophosphate [(R(p))- and (S-p)-2',3'-cUMPS] at 363.2 K have been followed by HPLC over ps-range 0-12. In aqueous alkali (pH > 9) only base-catalyzed endocyclic phosphoester hydrolysis to a nearly equimolar mixture of uridine 2'- and 3'-phosphoromonothioates (2'- and 3'-UMPS) takes place, analogously to the hydrolysis of uridine 2',3'-cyclic monophosphate (2',3'-cUMP). The (R(p))- and (S-p)-2',3'-cUMPS are hydrolyzed 50 and 30%, respectively, more slowly than 2',3'-cUMP. Under neutral and acidic conditions, desulfurization of the cyclic thiophosphates to 2',3'-cUMP competes with the phophoester hydrolysis, both reactions being acid-catalyzed at pH < 5. The desulfurization is most pronounced in strongly acidic solutions ([HCl] > 0.1 mol L(-1)), where more than 90% of the starting material is degraded via this route. At pH < 2, the thioates are considerably, i.e., more than 1 order of magnitude, more stable than 2',3'-cUMP. While the hydrolysis of 2',3'-cUMP is second-order in hydronium-ion concentration, that of 2',3'-cUMPS exhibits a first-order dependence. The reactivities of the two diastereomers are comparable with each other over the entire pH-range studied, the most significant difference being that the pH-independent desulfurization at pH > 5 is with the R(p)-isomer 5-fold faster than with the S-p-isomer. In contrast to 2',3'-cUMP, depyrimidination of the starting material (i.e., release of the uracil base) competes with the hydrolysis of the thiophosphate moiety under neutral conditions (pH 6-8).