NUCLEOSIDES. 145. SYMEEESIS OF 2,5’-AN3YDRD-2-THIOURIDINE AND ITS CONVERSION TO 3iH>ACETYL-2,2,-ANBYDH3-5,-CHII»0-5’-I«KY-2-THIOORIDINE. STUDIES DIRECTS) TOWARD THE SYNTHESIS OF 2‘-DBCKY-2’-SUBSTITUTE) arabino NUCEJ0OSIDES (7).)

In an attempt to introduce a substituent at C-2’ in the “up” arabino configuration directly by nucleophilic displacement reaction of a preformed pyrimidine ribonucleoside, we synthesized 2,5’-anhydro-5 deoxy-2-thiouridine (6) in three steps from uridine. Compound 6 was converted into the 3’-O-acetyl derivative 7. Upon treatment of 7 with triflyl chloride in methylene chloride in the presence of triethylamine and p-dimethylaminopyridine, 2,2’-anhydro-1-(3-0-acetyl-5-chloro-2,5-dideoxy-p-D-arabinofuranosyl)-2-thiouracil (9) was obtained as the only isolable product. Obviously, the intermediate 3'-0-acetyl-2,5'-anhydro-2'-0-triflyl-2-thiouridine (8) was attacked by the chloride nucleophile at C-5’ first giving the 2’-0-triflyl-2-thiouridine intermediate from which 9 was formed by intramolecular nucleophilic reaction.