Hydrolytic Reactions of the cis-methyl ester of 3 '-deoxy-3 '-thiothymidine 3 ',5 '-cyclic(phosphorothiolate)

Hydrolysis of the cis-methyl ester of 3'-deoxy-3'-thiothymidine 3'-S,5'-O-c yclic(phosphorothiolate) (la) has been followed by HPLC and MS. At pH < 2 h ydrolysis of the thiophosphate triester moiety is acid-catalyzed (first ord er), while between pH = 2 and 5 the reaction is pH-independent and at pH > 5 first order in hydroxide ion. The uncatalyzed and acid-catalyzed reaction s yield two thiophosphate diesters, the 3'-S,5'-O-cyclic phosphorothiolate 2 and 3'-S-phosphorothiolate methyl ester 3, in a 9:1 and 1:3 molar ratio, respectively. The hydroxide ion catalyzed reaction gives the endocyclic P-O and P-S bond-cleavage products (3 and 4, respectively) in a 1:2 molar rati o. The pH-independent reaction is suggested to take place by attack of a wa ter molecule on the carbon atom and concomitant C-O bond rupture, whereas t he alkaline and acidic reactions involve attack of the nucleophile on the p hosphorus atom and formation of a pseudorotating thiophosphorane intermedia te. Under acidic conditions, cleavage of the N-glycosidic linkage competes with the phosphoester hydrolysis, corresponding to 20% of the hydrolysis pr oducts at pH < 1.