2,2'-Anhydro-4'-thionucleosides: precursors for 2'-azido- and 2'-chloro-4'-thionucleosides and for a novel thiolane to thietane rearrangement.

2,2'-Anhydro-4'-thio-beta- and alpha-nucleosides 9 and 10 have been prepared by an in situ 4-thio-1,2-glycal addition route. They undergo ring-opening by azide or chloride ion to give, after deprotection, the 2'-substituted-4'-thionucleosides 13 and 14, whereas reactions with cyanide or fluoride sources lead to the unsaturated nucleosides 17 or 18, depending upon conditions. An unexpected and clean rearrangement to the thietane 23 occurs on treatment of uracil derivative 20 with DAST.