Stereochemical assignment and stereoselective synthesis of 3'-C-P-N-5 ' R-P-ethyl phosphonamidate modified nucleic acid

The configuration of the duplex stabilising 3'-C-P-N-5' ethyl phosphonamida te nucleic acid backbone modification has been identified as R-p following the preparation of cyclic nucleoside analogues and NOE studies. Complimenta ry routes for formation of the key phosphonamidate nitrogen to phosphorus b ond from stereochemically defined H-phosphinate intermediates, involving bo th a retention and inversion at the phosphorus centre, have been identified .