Synthesis of <i>P</i>-Modified Chimeric Oligonucleotides via Chemoselective Condensation of Nucleoside 3′-<i>H</i>-Phosphonothioate

Nucleoside 3'-<i>H</i>-phosphonothioate monoesters possess two nucleophilic atoms, i.e., oxygen and sulfur atoms; therefore, their chemoselectivity must be controlled during condensation with a 5'-hydroxy group for the synthesis of oligonucleotides. However, the selective activation of a sulfur atom to form <i>H</i>-phosphonate diesters has been scarcely investigated. In this study, we examined the chemoselective condensation of <i>H</i>-phosphonothioate monoesters and developed a reaction to form two types of internucleotidic linkages, i.e., <i>H</i>-phosphonate and <i>H</i>-phosphonothioate diesters, from a single monomer. Using this approach, we synthesized a chimeric oligonucleotide bearing both <i>H</i>-phosphonate and <i>H</i>-phosphonothioate diester linkages on a solid support. By performing a global conversion reaction, phosphorodithioate/phosphorothioate chimeric dodecamers and phosphorothioamidate/phosphoramidate chimeric pentamers were successfully synthesized using only <i>H</i>-phosphonothioate monomers. The method developed herein offers a versatile tool for the synthesis of a wide variety of <i>P</i>-modified chimeric oligonucleotides, thus overcoming the difficulties in synthesizing these oligonucleotides by using existing techniques.