Synthesis of 5′-Thio-3′‑<i>O</i>‑ribonucleoside Phosphoramidites

The chemical synthesis of phosphoramidite derivatives of all four 5′-deoxy-5′-thioribonucleosides is described. These phosphoramidites contained trityl (A, G, C, and U), dimethoxytrityl (A and G), or <i>tert</i>-butyldisulfanyl (G) as the 5′-<i>S</i>-protecting group. The application of several of these phosphoramidites for solid-phase synthesis of oligoribonucleotides containing a 2′-<i>O</i>-photocaged 5′-<i>S</i>-phosphorothiolate linkage or 5′-thiol-labeled RNAs is also further investigated