4'-thiouridine 14 was also converted to its C2' epimer 18 via an intramolecular cyclisation of the C4-carbonyl, displacing a C2'-carbonate leaving group. This inverted the C2' stereochemistry to give β-1',2'-syn-bicycle 17 in 92% yield, with C NMR confirming a chemical shift change of δC 152.3 ppm to δC 175.4 ppm for C2' in 17. Subsequent ring opening of 17 was achieved by stirring with KOH in EtOH/H2O, to give C2'-arabino configured 18 in almost quantitative yield. The observed 1 H NMR coupling constant between H2' and H3' for 18 ( 3 JH2'-H3' = 9.1 Hz) was significantly different than that observed for 14 ( 3 JH2'-H3' = 3.7 Hz), supporting inversion of stereochemistry at C2'. 4'-thioarabinouridine 18 was also converted to its cytosine form using the method previously established for 15, delivering 4'-thioarabinocytidine 19 in 52% yield over three steps from 18. Earlier efforts to directly convert 15 to 19 via a β-1',2'-syn-intermediate bi-cycle were unsuccessful; multiple reaction products were observed so this conversion was abandoned. Finally, the corresponding sulfoxides of 15 and 19 were prepared via treatment with 1.5 equivalents of m-CPBA, to afford novel 16 and 20 in 90% and 78% yields respectively. C NMR analysis confirmed a significant downfield shift of C1' and C4' from δC 65.1 and 51.7 ppm in 16 to δC 73.1 and 65.7 ppm in 20 respectively. Attempted separation of the Sdiastereoisomers by crystallisation and reverse phase preparative HPLC were unsuccessful.