HYDROLYSIS OF THE CIS-PHENYL ESTER OF THYMIDINE 3',5'-CYCLIC-MONOPHOSPHATE - PH-DEPENDENT COMPETITION BETWEEN DEPYRIMIDINATION AND PHOSPHOTRIESTER HYDROLYSIS VIA CO AND PO BOND RUPTURES

Hydrolytic reactions of the cis-phenyl ester of thymidine 3',5'-cyclic monophosphate (In) have been followed by HPLC over a wide pH range. U nder acidic conditions (pH < 4) two reactions compete: depyrimidinatio n (cleavage of the N-glycosidic bond) and phosphotriester hydrolysis t o a mixture of three phosphodiesters, viz. thymidine 3',5'-cyclic mono phosphate (2) and thymidine 3'- and 5'-(phenyl phosphates) (3 and 4). Depyrimidination predominates (>80%) at pH < 1 and shows first-order d ependence on acidity. The reaction is 4.5 to 5 times slower than with 2. The phosphotriester hydrolysis of 1a is acid catalyzed at pH < 2, g iving all three phosphodiester products (2-4). Over a broad acidity ra nge from pH 2 to 7, the reaction is pH-independent. In this pH region, the predominant product is 3 (up to 85%). At pH > 10, the hydrolysis is hydroxide-ion-catalyzed, yielding the three phosphodiester products in a 42:42:16 ratio ([2]:[3]:[4]). From pH 7 to 10, the pH-rate profi le is nonlinear, possibly due to N3H deprotonation of the thymine moie ty. In the same pH range, the site of bond cleavage appears to be chan ged. The product analyses of the corresponding methanolysis reactions suggest that the pH-independent reaction predominantly takes place via cleavage of the C5'O bond, while the alkaline reaction proceeds by ru pture of one of the PO bonds. Consistent with this proposal, the pH-in dependent hydrolysis yields at high concentrations of sodium chloride 5'-chloro-5'-deoxythymidine 3'-(phenyl phosphate) and in concentrated acetate buffers 5'-O-acetylthymidin 3'-(phenyl phosphate). Accordingly , the hydrolytic reactions of 1a markedly differ from those of more si mple 2-aryloxy-2-oxo-1,3,2-dioxaphosphorinanes.