Phosphorothioate-based oligos

Much more promising seems to be another, so far hypothetical, assumption that an access to stereo-defined oligo (nucleoside phosphoro-thioates) will allow stereospecific conversion of phosphorothioate function to 0-alkyl functions, for example, by selective 5-alkylation and conversion of 5-alkyl phosphates into 0-alkyl phosphates (24). However, one has to take into account potential problems with selective 5-alkylation in the presence of other nucleophilic centers, mostly on the nucleic bases. 

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