Synthesis of stereoregulated backbone-modified oligodeoxyribonucleotides via the corresponding diastereopure H-phosphonate intermediates is described. The diastereopure H-phosphonate intermediates were synthesized by using the nucleoside 3'-oxazaphospholidine monomers, and were stereospecifically converted into a variety of P-stereoregulated backbone-modified oligonucleotides, such as phosphorothioates, boranophosphates, and phosphoramidates, on a solid-support.