Stereocontrolled Synthesis of Phosphate-modified Oligonucleotides

Phosphate-modified oligonucleotides in which one of the two non-bridging oxygen atoms of each phosphodiester linkage is replaced by other kinds of atoms or substituents are useful as the basic structures of therapeutic oligonucleotides. These oligonucleotides have stereogenic phosphorus atoms and the development of an efficient method for the stereocontrolled synthesis is one of the unsolved problems. To solve this problem, we developed a novel method using diastereopure nucleoside 3'-O-oxazaphospholidine derivatives as monomers. Proline-derived bicyclic oxazaphospholidine derivatives were found to be the monomers of choice in that they could be synthesized with diastereoselectivity of >99 : 1 and were stable to epimerization. Various phosphate-modified oligonucleotides were synthesized in a stereocontrolled manner by using these monomers. The resultant stereoregulated oligonucleotides enabled us to demonstrate that the hybridizing affinity for complementary RNA was greatly affected by the configuration of the phosphorus atoms.