Hydrolysis of the <i>cis</i>-Phenyl Ester of Thymidine 3‘,5‘-Cyclic Monophosphate: pH-Dependent Competition between Depyrimidination and Phosphotriester Hydrolysis <i>via</i> CO and PO Bond Ruptures
作者:Jaana Varila、Teemu Hankamäki、Mikko Oivanen、Leo H. Koole、Harri Lönnberg
DOI:10.1021/jo960693v
日期:1997.2.1
Hydrolytic reactions of the cis-phenyl ester of thymidine 3',5'-cyclic monophosphate (In) have been followed by HPLC over a wide pH range. Under acidic conditions (pH < 4) two reactions compete: depyrimidination (cleavage of the N-glycosidic bond) and phosphotriester hydrolysis to a mixture of three phosphodiesters, viz. thymidine 3',5'-cyclic monophosphate (2) and thymidine 3'- and 5'-(phenyl phosphates) (3 and 4). Depyrimidination predominates (>80%) at pH < 1 and shows first-order dependence 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, giving all three phosphodiester products (2-4). Over a broad acidity range 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 profile is nonlinear, possibly due to N3H deprotonation of the thymine moiety. In the same pH range, the site of bond cleavage appears to be changed. 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 rupture of one of the PO bonds. Consistent with this proposal, the pH-independent 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 simple 2-aryloxy-2-oxo-1,3,2-dioxaphosphorinanes.