The potential of many F-labeled (hetero)aromatics for applications in positron emission tomography remains underexplored because convenient procedures for their radiosynthesis are lacking. Consequently, simple methods to prepare radiofluorinated (hetero)arenes are highly sought after. Herein, we report the beneficial effect of primary and secondary alcohols on Cu-mediated F-labeling. This observation contradicts the assumption that such alcohols are inappropriate solvents for aromatic fluorination. Therefore, we developed a protocol for rapid radiolabeling of an extraordinarily broad scope of boronic and stannyl substrates under general reaction conditions. Notably, radiofluorinated indoles, phenols, and anilines were synthesized directly from the corresponding unprotected precursors. Furthermore, the novel method enabled the preparation of radiofluorinated tryptophans, [ F]F-DPA, [ F]DAA1106, 6-[ F]FDA, and 6-[ F]FDOPA.
In the era of personalized precision medicine, positron emission tomography (PET) and related hybrid methods like PET/CT and PET/MRI gain recognition as indispensable tools of clinical diagnostics. A broader implementation of these imaging modalities in clinical routine is closely dependent on the increased availability of established and emerging PET-tracers, which in turn could be accessible by the development of simple, reliable, and efficient radiolabeling procedures. A further requirement is a cGMP production of imaging probes in automated synthesis modules. Herein, a novel protocol for the efficient preparation of 18F-labeled aromatics via Cu-mediated radiofluorination of (aryl)(mesityl)iodonium salts without the need of evaporation steps is described. Labeled aromatics were prepared in high radiochemical yields simply by heating of iodonium [18F]fluorides with the Cu-mediator in methanolic DMF. The iodonium [18F]fluorides were prepared by direct elution of 18F− from an anion exchange resin with solutions of the corresponding precursors in MeOH/DMF. The practicality of the novel method was confirmed by the racemization-free production of radiolabeled fluorophenylalanines, including hitherto unknown 3-[18F]FPhe, in 22–69% isolated radiochemical yields as well as its direct implementation into a remote-controlled synthesis unit.
2-[18F]Fluorophenylalanine (2-[18F]FPhe), a promising PET tracer for imaging of cerebral infarction and tumors, was efficiently prepared from an easily accessible iodonium salt precursor using Cu-mediated radiofluorination under ‘low base’ or ‘minimalist’ conditions. Whereas significant racemization was initially observed if the ‘minimalist’ protocol was applied for radiolabeling, it was completely suppressed by the careful adjustment of 18F– preprocessing. The initial biological study revealed a higher uptake of 2-[18F]FPhe in different tumor cells in comparison to that of [18F]FET. In contrast to 4-[18F]FPhe, which suffered from rapid defluorination in vivo, 2-[18F]FPhe demonstrated a sufficient in vivo stability. Conclusively, 2-[18F]FPhe is a promising PET probe that is now readily available using Cu-mediated radiofluorination under ‘minimalist’ or ‘low base’ conditions. The simplicity of the translation of the proposed procedures to automated synthesis modules allows a broad biological evaluation of 2-[18F]FPhe. Notably, a novel protocol for the preparation of N-Boc protected amino acids from the respective Ni-Schiff base complexes was developed that avoided application of strongly acidic conditions.
A reaction pathway via oxidation of [ 18 F]fluorobenzaldehydes offers a very useful tool for the no-carrier-added radiosynthesis of [ 18 F]fluorophenols, a structural motive of several potential radiopharmaceuticals. A considerably improved chemoselectivity of the Baeyer-Villiger oxidation (BVO) towards phenols was achieved, employing 2,2,2-trifluoroethanol as reaction solvent in combination with Oxone or m-CPBA as oxidation agent. The studies showed the necessity of H 2 SO 4 addition, which appears to have a dual effect, acting as catalyst and desiccant. For example, 2-[ 18 F]fluorophenol was obtained with a RCY of 97% under optimised conditions of 80°C and 30-minute reaction time.The changed performance of the BVO, which is in agreement with known reaction mechanisms via Criegee intermediates, provided the best results with regard to radiochemical yield (RCY) and chemoselectivity, i.e. formation of [ 18 F]fluorophenols rather than [ 18 F]fluorobenzoic acids. Thus, after a long history of the BVO, the new modification now allows an almost specific formation of phenols, even from electron-deficient benzaldehydes. Further, the applicability of the tuned, chemoselective BVO to the n.c.a. level and to more complex compounds was demonstrated for the products n.c.a. 4-[ 18 F]fluorophenol (RCY 95%; relating to 4-[ 18 F]fluorobenzaldehyde) and 4-[ 18 F]fluoro-m-tyramine (RCY 32%; relating to [ 18 F]fluoride), respectively.
Alcohol is the solution: The potential of many 18F‐labeled (hetero)aromatics for positron emission tomography (PET) remains underexploited, so the development and optimization of radiolabeling procedures remains a research area of considerable interest. It is shown herein that alcohol‐containing solvent systems give rise to unexpectedly enhanced copper‐mediated aromatic 18F‐labeling, enabling rapid radiolabeling of a broad scope of boronic and stannyl substrates under general reaction conditions. Among others, this allows the direct synthesis of radiofluorinated indoles, phenols, and anilines from unprotected precursors. More information can be found in the Communication by B. Neumaier et al. on page 3251 ff.
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