regiospecific synthesis of N ²-(N,N-dimethylaminomethylene)-2′-nitrofolic acid di-tert-butyl ester (13) and N ²-(N,N-dimethylaminomethylene)-N ¹0-formyl-2′-nitrofolic acid dimethyl ester (25), which are precursors for the radiolabeling of folic acid with the PET isotope ¹8F. A modular synthetic strategy was applied: Fmoc- and Boc-protected 4-amino-2-nitrobenzoic acid were linked via amide bonds to di-tert-butyl