Thiation of 2-deoxy-5,6-dihydropyrimidine nucleosides with Lawessons reagent: Characterisation of oxathiaphosphepane intermediates

Treatment of 2ʹ-deoxy-3ʹ,5ʹ-dithexyldimethylsilyl-5,6-dihydrouridine with Lawessonʹs reagent led to the expected C4-thiolated derivative together with a number of oxathiaphosphepane isomers which resulted from the heat reversible incorporation of an AnPS2 unit within the 2ʹ-deoxyribose moiety explaining the subsequent anomerisation of the 5,6-dihydropyrimidine nucleosides.