Thiation of 2'-deoxy-5,6-dihydropyrimidine nucleosides with Lawesson's reagent: characterisation of oxathiaphosphepane intermediates.

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