Radiofluorination of 2-fluoropyridines by isotopic exchange with [18F]fluoride

In homoaromatic systems, isotopic exchange (18F/19F) was previously (J Label Compd Radiopharm 18(12):1721-1730 [2], J Chem Soc Perkin Trans 1(3):295-298 [3]) proven to be advantageous, yet in general specific activity is thought to be low. For heteroaromatic systems, in particular, very few examples are published regarding the 18F-labelling of 2-substituted pyridines (J Label Compd Radiopharm 42:975-985 [9]). Therefore, in 2-fluoropyridines, we decided to study the 18F labelling by isotopic exchange (18F/19F). The radiochemical yield for 2-fluoropyridine was 90 ± 2%. Even if 2-fluoropyridine was substituted by an electron-donating group such as a methyl or a methoxy group, radiochemical yields were 80 ± 1 and 78 ± 1%, respectively. Although in benzenes, these substituents are known to decrease nucleophilic substitutions by 18F-Fluoride significantly. Moreover, by choosing appropriate concentrations of 2-fluoropyridines, reasonably high specific activities up to 10 GBq/μmol were obtained. (author)