Diastereoisomeric isopropyl-, 2-ethoxyethyl-, 2,2-dichloroethyl- and 2,2,2-trichloroethyl uridine 3'-thiomonophosphates, 1a-1d, respectively, have been synthesized, and their hydrolyses in aqueous alkali at 25 degrees have been followed by HPLC. According to the time-dependent product distributions obtained, the alkyl phosphorothioates 1a-1d undergo cleavage to uridine 2'- and 3'-thiophosphates, 7a and 7b, respectively, via a uridine 2',3'-cyclic thiophosphate (6). The rate of the hydroxide ion-catalyzed cyclization of both (R(P))- and (S(P))-diastereoisomer is highly dependent on the polar nature of the leaving group, the beta(lg) values being -1.23 + 0.03 and -1.24 + 0.03, respectively. Bronsted dependence of the second-order rate constants (k(OH) [dm(3) mol(-1) s(-1)]) on the pK(a) values of the leaving alcohols shows a convex breakpoint on going from alkyl esters 1a-1d to aryl esters studied earlier. This may be considered as a strong evidence for a stepwise mechanism, where the formation and breakdown of the phosphorane intermediate is the rate-limiting step with aryl and alkyl esters, respectively.