THE CHEMICAL STABILITY OF S-(2-ACYLTHIOETHYL) AND S-ACYLOXYMETHYL PROTECTED THYMIDYLYL-3′,5′-THYMIDINE PHOSPHOROMONOTHIOLATES AND THEIR DEACYLATION PRODUCTS IN AQUEOUS SOLUTION

Abstract

The hydrolytic stability of the S-(2-acetylthioethyl) (1a,b), S-(2-pivaloyl- thioethyl) (2a,b), and S-acetyloxymethyl (3a,b) protected R _P and S _P phosphoromonothiolates of 3′,5′-TpT has been studied. Rather unexpectedly, an intramolecular hydroxide ion catalyzed acetyl migration from the protecting group to the nucleoside 3′- and 5′-hydroxy functions was found to compete with the intermolecular displacement of the AcSCH₂CH₂S- or AcOCH₂S-ligand from the phosphorus atom of 1a,b and 3a,b, respectively. With the S-pivaloylthioethyl derivative 2a,b no such reaction took place. Additionally, the kinetics of the cleavage of the S-(2-mercaptoethyl) group from 4a,b, the products of enzymatic deacylation of 1a,b and 2a,b, were studied as a function of pH.

ACKNOWLEDGMENTS

We wish to thank Dr. Kari Neuvonen and Mr. Kristo Hakala (M.Sc.) for the skilful assistance in NMR and MS analyses.

Notes

^*For the ESI–MS analysis the separations were carried out on a Hypersil ODS 5 column (250–254 mm, 5 μm) eluted with 5 mM aqueous ammonium acetate containing 12% acetonitrile. The retention times (flow rate 1 mL/min) for the intermediates of the alkaline hydrolysis of 3b were 8.7 min (m/z = 605 for [M + 1]⁺), 9.7 min (m/z = 605), 15.3 min (m/z = 529) and 24 min (m/z = 529). The maximum concentration of each of these during the kinetic runs remained below 10% of the initial concentration of 3b.