NMR studies on 4‐thio‐5‐furan‐modified and 4‐thio‐5‐thiophene‐modified nucleosides

Systematic NMR characterization of 4-thio-5-furan-pyrimidine nucleosides or 4-thio-5-thiophene-pyrimidine nucleosides (ribonucleosides and 2′-deoxynucleosides) was performed. All proton and carbon signals of 4-thio-5-thiophene-ribouridine and related analogues were unambiguously assigned. The orientations of the base (4-thiouridine or its deoxy analogue) relative to the ring (furan or thiophene) are explored by a NMR approach and further supported by X-ray crystallographic studies. The procedures presented here would be applicable to other modified nucleosides and nucleotides.