Large-Scale Synthesis of Antisense Oligonucleotides without Chlorinated Solvents

It is demonstrated that mixed-sequence phosphorothioate oligodeoxyribonucleotides can be synthesized on scales from 1 μmol up to 80 mmol without using chlorinated solvents such as dichloromethane or dichloroethane, while preserving both high yield and purity of the product. A solution of dichloroacetic acid in organic solvents (e.g., toluene, xylenes, benzotrifluoride) cleanly and efficiently removes the 4,4'-dimethoxytrityl (DMTr) group from the 5'-terminus of the growing oligonucleotide chain during synthesis on solid support. We have therefore replaced hazardous dichloromethane, formerly used in oligonucleotide synthesis, as the solvent for DMTr-removal, with toluene.