HYDROLYSIS AND DESULFURIZATION OF THE DIASTEREOMERIC PHOSPHOROMONOTHIOATE ANALOGS OF URIDINE 2',3'-CYCLIC MONOPHOSPHATE

Hydrolyses of the two diastereomeric phosphoromonothioate analogs of u ridine 2',3'-cyclic monophosphate [(R(p))- and (S-p)-2',3'-cUMPS] at 3 63.2 K have been followed by HPLC over ps-range 0-12. In aqueous alkal i (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 uridin e 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 cyc lic thiophosphates to 2',3'-cUMP competes with the phophoester hydroly sis, both reactions being acid-catalyzed at pH < 5. The desulfurizatio n 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 or der 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 t wo diastereomers are comparable with each other over the entire pH-ran ge studied, the most significant difference being that the pH-independ ent desulfurization at pH > 5 is with the R(p)-isomer 5-fold faster th an with the S-p-isomer. In contrast to 2',3'-cUMP, depyrimidination of the starting material (i.e., release of the uracil base) competes wit h the hydrolysis of the thiophosphate moiety under neutral conditions (pH 6-8).