31P NMR AND COMPUTATIONAL STUDIES ON STEREOCHEMISTRY OF CONVERSION OF PHOSPHORAMIDATE DIESTERS INTO THE CORRESPONDING PHOSPHOTRIESTERS

P-31 NMR spectroscopy was used to investigate a stereochemical course of a nitrite-promoted conversion of phosphoramidate diesters into the corresponding phosphotriesters. It was found that this reaction occurred with almost complete epimerization at the phosphorus center and at the C1 atom in the amine moiety. On the basis of the P-31 NMR data, a plausible mechanism for the reaction was proposed. The density functional theory calculation of the key step of the reaction, i.e., breaking of the P-N bond and formation of the P-O bond, suggested a one-step S(N)2(P) process with retention of configuration at the phosphorus center.