4-Thio oxathiaphosphepane nucleosides 2–4 undergo a rearrangement in pyridine that leads selectively to the β anomer of the 2′-deoxy-5,6-dihydro-4-thiouridine derivative 5. This diastereoselective reaction proceeds through a multistep mechanism initiated by the addition of pyridine at the C1′ position of 2–4 and concomitant opening of the oxathiaphosphepane. This was confirmed by the trapping of the corresponding intermediate in the closely related DMAP series. In contrast, LR thiation of 1 in pyridine leads to a new class of modified nucleosides 12 containing an oxathiaphospholane moiety. The quantitative conversion of 5 into the corresponding 5,6-dihydrocytosine derivative with NH3–MeOH is also reported.
