Development of a novel [18 F]fluorobenzyl derivative of the AT1 receptor antagonist Candesartan

Candesartan is a clinically approved angiotensin II type 1 receptor (AT₁ R)-blocker that selectively binds AT₁ Rs in high affinity. We report here the radiosynthesis and automation of the novel [¹⁸ F]fluorobenzyl derivative of Candesartan using the Sonogashira cross-coupling reaction. [¹⁸ F]Fluorobenzyl-Candesartan ([¹⁸ F]7) was developed from 4-[¹⁸ F]fluoroiodobenzene ([¹⁸ F]FIB) that was conjugated with alkyne-trityl-candesartan with the assistance of a Pd (PPh₃ )₄ /CuI catalyst followed by acid deprotection. The three-step two-reactor 2-HPLC purification process was automated resulting in >90% pure [¹⁸ F]7 in a RCY of 4.6 ± 1.1% (decay corrected from EOB) and molar activities of 1,406-5,513 GBq/mmol. [¹⁸ F]FIB was reproducibly obtained by direct radiofluorination of the mono-iodinated triphenylsulfonium salt in the presence of K222/K₂ CO₃ in an ~30% yield (decay-corrected). [¹⁸ F]7 was stable (>97%) up to 4 h in solution and up to 1 h in rat plasma at 37°C. However, the use of Sonogashira cross-coupling reaction to produce [¹⁸ F]7 in high yields and molar activities was found to be challenging for routine use in radiochemistry labs.

(18)F-Labeling of Arenes and Heteroarenes for Applications in Positron Emission Tomography

Diverse radiochemistry is an essential component of nuclear medicine; this includes imaging techniques such as positron emission tomography (PET). As such, PET can track diseases at an early stage of development, help patient care planning through personalized medicine and support drug discovery programs. Fluorine-18 is the most frequently used radioisotope in PET radiopharmaceuticals for both clinical and preclinical research. Its physical and nuclear characteristics (97% β(+) decay, 109.8 min half-life, 635 keV positron energy) and high specific activity make it an attractive nuclide for labeling and molecular imaging. Arenes and heteroarenes are privileged candidates for (18)F-incorporation as they are metabolically robust and therefore widely used by medicinal chemists and radiochemists alike. For many years, the range of (hetero)arenes amenable to (18)F-fluorination was limited by the lack of chemically diverse precursors, and of radiochemical methods allowing (18)F-incorporation in high selectivity and efficiency (radiochemical yield and purity, specific activity, and radio-scalability). The appearance of late-stage fluorination reactions catalyzed by transition metal or small organic molecules (organocatalysis) has encouraged much research on the use of these activation manifolds for (18)F-fluorination. In this piece, we review all of the reactions known to date to install the (18)F substituent and other key (18)F-motifs (e.g., CF3, CHF2, OCF3, SCF3, OCHF2) of medicinal relevance onto (hetero)arenes. The field has changed significantly in the past five years, and the current trend suggests that the radiochemical space available for PET applications will expand rapidly in the near future.

4-[18F]Fluorophenylpiperazines by improved Hartwig-Buchwald N-arylation of 4-[18F]fluoroiodobenzene, formed via hypervalent λ3-iodane precursors: application to build-up of the dopamine D4 ligand [18F]FAUC 316

Substituted phenylpiperazines are often neuropharmacologically active compounds and in many cases are essential pharmacophores of neuroligands for different receptors such as D2-like dopaminergic, serotoninergic and other receptors. Nucleophilic, no-carrier-added (n.c.a.) 18F-labelling of these ligands in an aromatic position is desirable for studying receptors with in vivo molecular imaging. 1-(4-[18F]Fluorophenyl)piperazine was synthesized in two reaction steps starting by 18F-labelling of a iodobenzene-iodonium precursor, followed by Pd-catalyzed N-arylation of the intermediate 4-[18F]fluoro-iodobenzene. Different palladium catalysts and solvents were tested with particular attention to the polar solvents dimethylformamide (DMF) and dimethylsulfoxide (DMSO). Weak inorganic bases like potassium phosphate or cesium carbonate seem to be essential for the arylation step and lead to conversation rates above 70% in DMF which is comparable to those in typically used toluene. In DMSO even quantitative conversation was observed. Overall radiochemical yields of up to 40% and 60% in DMF and DMSO, respectively, were reached depending on the labelling yield of the first step. The fluorophenylpiperazine obtained was coupled in a third reaction step with 2-formyl-1H-indole-5-carbonitrile to yield the highly selective dopamine D4 ligand [18F]FAUC 316.

Copper-catalyzed [18F]fluorination of (mesityl)(aryl)iodonium salts

A practical, rapid, and highly regioselective Cu-catalyzed radiofluorination of (mesityl)(aryl)iodonium salts is described. This protocol utilizes [(18)F]KF to access (18)F-labeled electron-rich, -neutral, and -deficient aryl fluorides under a single set of mild conditions. This methodology is applied to the synthesis of protected versions of two important radiotracers: 4-[(18)F]fluorophenylalanine and 6-[(18)F]fluoroDOPA.

Enhanced aqueous Suzuki-Miyaura coupling allows site-specific polypeptide 18F-labeling

The excesses of reagents used in protein chemistry are often incompatible with the reduced or even inverse stoichiometries used for efficient radiolabeling. Analysis and screening of aqueous Pd(0) ligand systems has revealed the importance of a guanidine core and the discovery of 1,1-dimethylguanidine as an enhanced ligand for aqueous Suzuki–Miyaura cross-coupling. This novel Pd catalyst system has now allowed the labeling of small molecules, peptides, and proteins with the fluorine-18 prosthetic [¹⁸F]4-fluorophenylboronic acid. These findings now enable site-specific protein ¹⁸F-labeling under biologically compatible conditions using a metal-triggered reaction.

Selective syntheses of no-carrier-added 2- and 3-[18F]fluorohalopyridines through the radiofluorination of halopyridinyl(4'-methoxyphenyl)iodonium tosylates

No-carrier-added 2- and 3-[(18)F]fluorohalopyridines were readily synthesized as potentially useful labeling synthons for prospective PET radiotracers through the selective radiofluorination of halopyridinyl(4'-methoxyphenyl)iodonium tosylates, themselves conveniently prepared in a single pot from iodohalopyridines.

N-heterocyclic carbene-palladium(II)-1-methylimidazole complex-catalyzed Suzuki-Miyaura coupling of aryl sulfonates with arylboronic acids

A well-defined NHC-Pd(II)-Im complex 1 was found to be an effective catalyst for the Suzuki-Miyaura coupling of aryl sulfonates including tosylates and phenylsulfonates with arylboronic acids, giving the desired coupling products in good to high yields. Acceptable yields can also be achieved even by using the less reactive mesylates as the substrates. It is worthy of noting here that this is the first example of NHC-Pd(II) complex-catalyzed Suzuki-Miyaura coupling of aryl sulfonates with arylboronic acids, enriching an inexpensive, convenient, and alternative method for the synthesis of biaryl compounds.

Cross-coupling reactions as valuable tool for the preparation of PET radiotracers

The increasing application of positron emission tomography (PET) in nuclear medicine has stimulated the extensive development of a multitude of new radiotracers and novel radiolabeling procedures with the most prominent short-lived positron emitters carbon-11 and fluorine-18. Radiolabeling with these radionuclides represents a remarkable challenge. Special attention has to be paid to synthesis time and specific labeling techniques due to the short physical half life of the respective radionuclides ¹¹C (t_1/2 = 20.4 min) and ¹⁸F (t_1/2 = 109.8 min). In the past, numerous transition metal-catalyzed reactions were employed in organic chemistry, even though only a handful of these coupling reactions were adopted in radiochemical practice. Thus, the implementation of modern synthesis methods like cross-coupling reactions offers the possibility to develop a wide variety of novel radiotracers. The introduction of catalysts based on transition metal complexes bears a high potential for rapid, efficient, highly selective and functional group-tolerating incorporation of carbon-11 and fluorine-18 into target molecules. This review deals with design, application and improvement of transition metal-mediated carbon-carbon as well as carbon-heteroatom cross-coupling reactions as a labeling feature with the focus on the preparation of radiolabeled compounds for molecular imaging.