Acceleration of H-phosphonate diester oxidation with iodine accompanied by a thermolabile protecting group (TPG) is presented. It is shown that the intermediate product of this reaction is an oxazaphospholidine oxide which forms a phosphate diester only when a 2-pyridyl TPG is applied. The intermediate product is formed with exocyclic nitrogen. The absolute configurations of phosphorothioate diesters, H-phosphonate diesters, and oxazaphospholidine oxides were determined. (31)P NMR spectroscopy was used to evaluate the relationship between chemical shift and absolute configuration at the phosphorus center of H-phosphonate diesters and oxazaphospholidine oxides.