Synthesis of P-Modified Chimeric Oligonucleotides via Chemoselective Condensation of Nucleoside 3′-H-Phosphonothioate

Nucleoside 3′-H-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 H-phosphonate diesters has been scarcely investigated. In this study, we examined the chemoselective condensation of H-phosphonothioate monoesters and developed a reaction to form two types of internucleotidic linkages, i.e., H-phosphonate and H-phosphonothioate diesters, from a single monomer. Using this approach, we synthesized a chimeric oligonucleotide bearing both H-phosphonate and H-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 H-phosphonothioate monomers. The method developed herein offers a versatile tool for the synthesis of a wide variety of P-modified chimeric oligonucleotides, thus overcoming the difficulties in synthesizing these oligonucleotides by using existing techniques.