2020 年 78 巻 5 号 p. 446-455
To date, a number of chemically modified oligonucleotides (ONs) have been designed for use in nucleic acids-based therapeutics. In our group, we have been intensely working on the synthesis of 4’-thio and 4’-seleno ONs having sulfur or selenium atoms in place of furanose ring oxygen. To prepare 4’-thio and 4’-seleno ribonucleosides, we applied the Pummerer reaction of 4-thio or 4-seleno sugar with nucleobases, and the resulting 4’-thio and 4’-seleno ribonucleoside units were incorporated into ONs. Under the standard phosphoramidite conditions, 4’-thioRNA was obtained in good yields. While 4’-selnoRNA was given in very low yields. We carefully investigated ON synthesis containing 4’-selenoribonucleosides under standard phosphoramidite conditions. As a result, we found the unexpected strand-break occurred during oxidation step using I2. On the basis of this finding, we succeeded in the first synthesis of a fully modified 4’-selenoRNA by using tert-butyl hydroperoxide as an alternative oxidant.