2′,3′-anhydrouridine.: A useful synthetic intermediate

2,2'-Anhydro-1-(beta-D-arabinofuranosyl)uracil 1 reacts with sodium hydride in dry DMSO to give 2',3'-anhydrouridine 2. When the latter compound 2 is heated below its melting point or treated with triethylamine in methanol, it isomerises back to the 2,2'-anhydronucleoside 1. Treatment of compound 1 with sodium ethanethiolate or the sodium salt of benzyl mercaptan in the presence of an excess of the corresponding thiol in DMA gives 2'-S-ethyl- or 2'-S-benzyl-2'-thiouridine (4 or 11) in high yield; however, treatment of the 2,2'-anhydronucleoside 1 first with sodium hydride in DMA and then with a deficiency (with respect to sodium hydride) of ethanethiol or benzyl mercaptan gives the corresponding 3'-S-ethyl or 3'-S-benzyl derivative (3 or 12) in high yield. When the 2,2'-anhydronucleoside 1 is allowed to react with an excess of potassium tert-butoxide in DMSO, the 3',5'-anhydronucleoside 13 is obtained in good yield. The latter compound 13 undergoes hydrolysis in aqueous trifluoroacetic acid to give 1-(beta-D-xylofuranosyl)uracil 14 in high yield. The 3'-S-benzyl derivative 12 is converted by Raney nickel desulfurisation into 3'-deoxyuridine 15 which, in turn, is converted into 3'-deoxycytidine 17 in good yield. X-Ray crystallographic data relating to compounds 11 and 12 are also reported.