The 5′-phosphate ester of 5-nitro-2′-deoxyuridine (7c) is a potent mechanism-based inhibitor of thymidylate synthase. 2 Compound 7c3 and 1-(β-D-arabinofuranosyl)-5- nitrouracil4 have antiviral activity, and certain O-nitro esters of pyrimidine5 and purine6 nucleosides also have biological activity. The 5′-O-nitro esters of pyrimidine nucleosides function as good substrates for 5′-modification via SN2 displacement of nitrate and other ...