Some novel pyrimidine nucleoside rearrangements effected by diethylaminosulfur trifluoride (DAST).

Pyrimidine nucleosides (or their 5'-aldehydes) when treated with DAST give O2,5'-(fluoro)-anhydronucleosides. If this is prevented by blocking N-3 or O4, the desired 5'-deoxy-5'-(di)-fluoronucleoside is accompanied by the production of a compound resulting from migration of the base following scission of the N-1-->C-1' bond and formation of O2-->C-5'. This is a particular example of a much more general phenomenon, seen when suitably substituted ribofuranoses are treated with DAST.