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Practical Synthesis of 4′‐Thioribonucleosides Starting from D‐Ribose

A practical synthesis of 4′‐thioribonucleosides, i.e., 4′‐thiouridine, ‐cytidine, ‐adenosine, and ‐guanosine, which are versatile units for nucleic acids–based therapeutics, is described. Large‐scale synthesis of 4‐thiosugar starting from D‐ribose was achieved (33%) in eight steps and with only three chromatographic purifications. After the appropriate chemical conversion of the 4‐thiosugar, the resulting sulfoxide was subjected to the Pummerer reaction in the presence of silylated nucleobases. In reactions with silylated pyrimidine bases, the desired 4′‐thioribonucleoside derivatives were obtained in good yield and β‐selectively. On the other hand, N‐7 isomers were obtained mainly in the Pummerer reaction with purine bases under the same conditions. However, the desired N‐9 isomers were obtained in moderate yields when the reaction mixtures were subsequently heated under reflux. As a result, effective synthesis of 4′‐thioribonucleosides was accomplished. © 2014 by John Wiley & Sons, Inc.