Synthesis of Precursors for 18F-Labeling of Folic Acid for PET Application

Radiolabeled folate derivatives have the potential to target folate receptor-positive tumor cells for noninvasive diagnosis via positron emission tomography (PET) and single photon emission computed tomography (SPECT). We report the 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 F-18. A modular synthetic strategy was applied: Fmoc-and Boc-protected 4-amino-2-nitrobenzoic acid were linked via amide bonds to di-tert-butyl L-glutamate and dimethyl L-glutamate, respectively, to form building blocks 10 and 19. After Fmoc and Boc removal, both compounds were coupled to 6-(bromomethyl)pterin hydrobromide to give crude 2'-nitrofolic acid di-tert-butyl ester and 2'-nitrofolic acid dimethyl ester. After formylation of 2'-nitrofolic acid dimethyl ester at N-10 and the introduction of an N,N-dimethylaminomethylene group at N-2, precursor 25 was obtained in an overall yield of 3%. The analogous 2'-fluoroderivative 28 was obtained in 7% overall yield from 4-amino-2-fluorobenzoic acid. Precursor 13 was obtained from 2'-nitrofolic acid di-tert-butyl ester in 6% yield after N-2-protection. The synthesis of the reference materials 2'-nitro- and 2'-fluorofolic acid was achieved by the reaction of N-(4-amino-2-nitrobenzoyl)- and N-(4-amino-2-fluorobenzoyl)-L-glutamic acid with 6-(bromomethyl)pterin hydrobromide, giving 7% and 14% overall yield, respectively.