Synthesis and Purification of Phosphorodithioate DNA

Oligonucleotide analogs bearing modified phosphodiester linkages have been the focus of considerable interest in the antisense field, with methylphosphonates and phosphorothioates being the most extensively studied derivatives to date (1 –3 ). These modifications, whereby a single nonbridging oxygen atom is replaced with either a methyl group or sulfur atom, generate certain very desirable oligomer characteristics (4 –7 ), such as resistance toward nucleases, retention of the ability to form duplexes with natural DNA or RNA, and, relative to the phosphorothioate derivative, an ability to stimulate RNase H activity. Unfortunately, and in contrast to natural DNA, the phosphorus center is rendered chiral by these substitutions. Thus, for a deoxyoligonucleotide with n phosphodiester linkages, there can be 2 ⁿ stereo-isomers with perhaps only one being the most active. Recent attempts to overcome this problem have focused on developing chemistries that either are stereoselective and generate oligomers enriched in one diastereomer (8 ,9 ) or are designed to synthesize new, phosphorus achiral analogs. In addition to being phosphorus achiral, perhaps new analogs may also have unique and attractive properties relative to the antisense area.