Metal-Ion-Promoted Cleavage, Isomerization, and Desulfurization of the Diastereomeric Phosphoromonothioate Analogues of Uridylyl(3‘,5‘)uridine

Abstract

Metal-ion-promoted hydrolytic reactions of the S P and R P diastereomers of the phosphoromonothioate analogues of uridylyl(3‘,5‘)uridine [3‘,5‘-Up(s)U] and their cleavage products, diastereomeric uridine 2‘,3‘-cyclic phosphates [2‘,3‘-cUMPS], were followed by HPLC as a function of pH (4.7−5.6) and metal ion concentration (1−10 mmol L -1 ). With 3‘,5‘-Up(s)U, three reactions compete: (i) cleavage to 2‘,3‘-cUMPS, (ii) isomerization to 2‘,5‘-Up(s)U, and (iii) desulfurization to an equilibrium mixture of 2‘,5‘- and 3‘,5‘-UpU. Of these, the cleavage to 2‘,3‘-cUMPS is markedly accelerated by Zn 2+, Cd 2+, and Gd 3+, the rate enhancements observed with the S P isomer at [M z+ ] = 5 mmol L -1 and pH 5.6 ( T = 363.2 K) being 410-, 3600-, and 2000-fold, respectively. The effect of Mn 2+ and Mg 2+ on the cleavage rate is, in turn, modest (6- and 1.7-fold acceleration, respectively). The rate-accelerations are almost equal with the S P and R P diastereomers. The metal-ion-promoted reaction is first-order in both the hydroxide and metal ion concentration, and it proceeds by inversion of configuration at phosphorus, consistent with an in-line displacement mechanism. The isomerization and desulfurization are much less susceptible to metal ion catalysis: 6.4- and 7.7-fold accelerations were observed with Zn 2+, respectively. Gd 3+ does not promote these reactions at all. The isomerization proceeds by retention of configuration at phosphorus, consistent with formation of a pentacoordinated thiophosphorane intermediate having the entering 2‘-hydroxy group apical and the leaving 3‘-hydroxy equatorial, and subsequent pseudorotation posing the leaving group apical. The hydrolysis of 2‘,3‘-cUMPS is accelerated by metal ions slightly more efficiently than the cleavage of 3‘,5‘-Up(s)U to 2‘,3‘-cUMPS. In striking contrast to the reactions of 3‘,5‘-Up(s)U, the hydrolytic desulfurization to 2‘,3‘-cUMP is accelerated as efficiently as its endocyclic hydrolysis to uridine 2‘- and 3‘-phosphoromonothioates [2‘- and 3‘-UMPS]. Somewhat unexpectedly, the latter compounds were observed to undergo metal-ion-promoted cyclization/desulfurization to 2‘,3‘-cUMP. The hydrolysis of 2‘- or 3‘-UMPS to uridine was, in turn, observed to be retarded by metal ions. The mechanisms of the partial reactions are discussed.