Synthesis of antisense oligonucleotides

It is demonstrated that mixed-sequence phosphorothioate oligodeoxyribonucleotides can be synthesized on scales up to 80 mmol without using chlorinated solvents like dichloromethane, while preserving both high yield and purity of the product. A solution of dichloroacetic acid in toluene cleanly and efficiently removes 4,4′-dimethoxytrityl groups from the 5′-terminus of the growing oligonucleotide chain during synthesis on solid support. Ammonium hydroxide treatment at room temperature at atmospheric pressure furnishes deprotected oligonucleotides reducing the risk that pressurized reaction glass vessels pose. To ensure facile separation of polymer beads (Primer HL 30) and oligonucleotide solution, minimum agitation of the reaction mixture is applied.