Synthesis and conformation of 3′,4′-BNA monomers, 3′-O,4′-C-methyleneribonucleosides

In order to develop novel 2′,5′-linked oligonucleotide analogues aimed for antivirus reagents and antisense/antigene oligonucleotides, novel nucleoside analogues, 3′-O,4′-C-methyleneribonucleosides (3′,4′-BNA monomers) were synthesized via two synthetic routes. The first route starting from uridine utilized a regioselective ring-closure reaction of the 4′-C-(p-toluenesulfonyl)oxymethyluridine derivative. The second route involved a coupling reaction of 1,2,3-tri-O-acetyl-4-C-(p-toluenesulfonyl)oxymethylribofuranose derivative with nucleobases followed by oxetan-ring formation to afford the 3′,4′-BNA monomers bearing all four nucleobases. By means of 1H NMR, X-ray crystallography and computational analysis, the sugar puckering of the 3′,4′-BNA monomers was found to be restricted in S-conformation (C1′-exo–C2′-endo puckering mode).