the preparation of the 5'-phosphordiamidate of adenosine (3a), cytidine (3b), guanosine (3c), uridine (3d) and thymidine (3e), and also for that of adenosine 5'-(N,N'-dimethyl) phosphordiamidate (4a), adenosine 5'-(N,N,N',N'-tetramethyl) phosphordiamidate (5a) and adenosine 5'-phosphor-dimorpholidate (6a). The method is based on the aminolysis of nucleoside 5'-phosphordichloridates performed insitu by