The LDA-induced coupling of 2',3',5'-O-protected uridine with diethyl chlorophosphate, during the synthesis of 6-phosphonouridine, is accompanied by the formation of an unexpected side product. LDA adds slowly to the C4 position of the 2',3',5'-O-protected uridine after the initial reaction with diethyl chlorophosphate. The presence of the phosphorochloridate facilitates the side reaction. This observation accounts for the previously reported low yield when conducting the coupling reaction for longer durations and suggests a new route for the synthesis of N-alkylated 6-phosphonocytidine analogues.