Synthesis of high-quality antisense drugs. Addition of acrylonitrile to phosphorothioate oligonucleotides: Adduct characterization and avoidance

It is demonstrated that the acrylonitrile (AN) generated during the ammonolysis step of oligonucleotide manufacture selectively adds to thymine residue present in ISIS 2302 to give a full-length oligonucleotide in which thymine is replaced by an N-3-cyanoethylthymine residue. Treatment of support-bound ISIS 2302 with a solution of triethylamine in CH3CN before ammonolysis is sufficient to prevent formation of this class of impurity.