Stereocontrolled Synthesis of Dimethylamino Phosphorochloridate Monomers toward Stereopure Phosphorodiamidate Morpholino Oligonucleotides

As antisense oligonucleotides, phosphorodiamidate morpholino oligonucleotides (PMOs) exhibit excellent properties. However, they possess two stereoisomers for each phosphorus atom, and these stereoisomers exhibit different physicochemical and biological properties. In this study, we developed a stereocontrolled synthesis method of dimethylamino phosphorochloridate monomers, which was used for a practical synthesis method of PMOs, from oxazaphospholidine derivatives. However, the condensation of a 5′-oxazaphospholidine derivative with an amine under acidic conditions is challenging because the resulting phosphoramidite intermediate can be activated under such conditions. To address this challenge, in the proposed synthesis method, a morpholino nucleoside 5′-oxazaphospholidine derivative was condensed with a phenol derivative with a low pKa value under acidic conditions. Subsequently, the resulting aryl phosphite was reacted with dimethylamine to yield a phosphoramidite, thereby liberating the phenol derivative as a leaving group. Chlorination of the phosphoramidite yielded a phosphorochloridate monomer in a highly stereoselective manner (dr = 93:7–97:3). Subsequently, the resulting chloridate monomer was stereospecifically condensed with the amino group of the morpholino nucleoside. The stereochemistry of the phosphorodiamidate morpholino dimers was unambiguously determined by nuclear magnetic resonance analysis. The results of this study facilitate the synthesis of stereocontrolled PMOs and the elucidation of their properties.