The present invention relates to methods of synthesizing 18F-FMAU. In particular, 18F-FMAU is synthesized using one-pot reaction conditions in the presence of Friedel-Crafts catalysts. The one-pot reaction conditions are incorporated into a fully automated cGMP-compliant radiosynthesis module, which results in a reduction in synthesis time and simplifies reaction conditions. The one-pot reaction conditions are also suitable for the production of 5-substitued thymidine or cytidine analogues. The products from the one-pot reaction (e.g. the labeled thymidine or cytidine analogues) can be used as probes for imaging tumor proliferative activity. More specifically, these [18F]-labeled thymidine or cytidine analogues can be used as a PET tracer for certain medical conditions, including, but not limited to, cancer disease, autoimmunity inflammation, and bone marrow transplant.