Solution-Phase Synthesis of Boranophosphate and Boranophosphate/Phosphorothioate/Phosphate Chimeric Oligonucleotides...

Boranophosphate oligodeoxyribonucleotides (PB-ODNs) are promising candidates as antisense oligonucleotides (ASOs) owing to their high nuclease resistance and low cytotoxicity; however, their synthesis via the conventional phosphoramidite method is difficult. To address this issue, we developed an H-boranophosphonate method for the synthesis of PB-ODNs. Since the H-boranophosphonate diester intermediate in the H-boranophosphonate method is unstable during purification on silica gel, the synthesis of PB-ODNs longer than a 3-mer was conducted exclusively via solid-phase synthesis, which hinders the scalability of the method for the large-scale production of PB-ODNs. Herein, we report the first successful application of the H-boranophosphonate method to the solution-phase synthesis of PB-ODNs, overcoming the stability issues by converting an unstable H-boranophosphonate diester into a stable boranophosphotriester in a one-pot reaction. For efficient chain elongation, a block condensation strategy using 4-mer building blocks was employed. The block condensation and oxidative esterification proceeded efficiently to yield PB-ODNs with a length of up to 12-mer. Furthermore, the strategy was applicable to the synthesis of PB/phosphorothioate (PS)/phosphate (PO)-ODNs up to 12-mer using H-phosphonate and H-phosphonothioate derivatives. This advancement enables the scalable production of PB-ODNs and P-modified chimeric ODNs, paving the way for the development of more potent ASOs.