Stereodefined Synthesis of O3‘-Labeled Uracil Nucleosides. 3‘-[17O]-2‘-Azido-2‘-deoxyuridine 5‘-Diphosphate as a Probe for the Mechanism of Inactivation of Ribonucleotide Reductases

Thermolysis of a 2'-[16O]-O-benzoyl-[17O]-5'-O-(tert-butyldimethylsilyl)-O2,3'-cyclouridine derivative gave the more stable 3'-[17O]-O-benzoyl-[16O]- 5'-O-(tert-butyldimethylsilyl)-O2,2'-cyclouridine isomer, which was converted into 3'-[17O]-2'-azido-2'-deoxyuridine by deprotection and nucleophilic ring opening at C2' with lithium azide. The 5'-diphosphate was prepared by nucleophilic displacement of the 5'-O-tosyl group with tris(tetrabutylammonium) hydrogen pyrophosphate. Model reactions gave 16O and 18O isotopomers, and base-promoted hydrolysis of an O2,2'-cyclonucleoside gave stereodefined access to 3'-[18O]-1-(β-d-arabinofuranosyl)uracil. Inactivation of ribonucleoside diphosphate reductase with 2'-azido-2'-deoxynucleotides results in appearance of EPR signals for a nitrogen-centered radical derived from azide, and 3'-[17O]-2'-azido-2'-deoxyuridine 5'-diphosphate provides an isotopomer to perturb EPR spectra in a predictable manner.