Chemistry with [ 18 F]Fluoride Ion

Crystal Structures of Diaryliodonium Fluorides and Their Implications for Fluorination Mechanisms

The radiofluorination of diaryliodonium salts is of value for producing radiotracers for positron emission tomography. We report crystal structures for two diaryliodonium fluorides. Whereas diphenyliodonium fluoride (1 a) exists as a tetramer bridged by four fluoride ions, 2-methylphenyl(phenyl)iodonium fluoride (2 a) forms a fluoride-bridged dimer that is further halogen bonded to two other monomers. We discuss the topological relationships between the two and their implications for fluorination in solution. Both radiofluorination and NMR spectroscopy show that thermolysis of 2 a gives 2-fluorotoluene and fluorobenzene in a 2 to 1 ratio that is in good agreement with the ratio observed from the radiofluorination of 2-methylphenyl(phenyl)iodonium chloride (2 b). The constancy of the product ratio affirms that the fluorinations occur via the same two rapidly interconverting transition states whose energy difference dictates chemoselectivity. From quantum chemical studies with density functional theory we attribute the "ortho-effect" to the favorable electrostatic interaction between the incoming fluoride and the o-methyl in the transition state. By utilizing the crystal structures of 1 a and 2 a, the mechanisms of fluoroarene formation from diaryliodonium fluorides in their monomeric, homodimeric, heterodimeric, and tetrameric states were also investigated. We propose that oligomerization energy dictates whether the fluorination occurs through a monomeric or an oligomeric pathway.

Development of Customized [18F]Fluoride Elution Techniques for the Enhancement of Copper-Mediated Late-Stage Radiofluorination

In a relatively short period of time, transition metal-mediated radiofluorination reactions have changed the PET radiochemistry landscape. These reactions have enabled the radiofluorination of a wide range of substrates, facilitating access to radiopharmaceuticals that were challenging to synthesize using traditional fluorine-18 radiochemistry. However, the process of adapting these new reactions for automated radiopharmaceutical production has revealed limitations in fitting them into the confines of traditional radiochemistry systems. In particular, the presence of bases (e.g. K2CO3) and/or phase transfer catalysts (PTC) (e.g. kryptofix 2.2.2) associated with fluorine-18 preparation has been found to be detrimental to reaction yields. We hypothesized that these limitations could be addressed through the development of alternate techniques for preparing [18F]fluoride. This approach also opens the possibility that an eluent can be individually tailored to meet the specific needs of a metal-catalyzed reaction of interest. In this communication, we demonstrate that various solutions of copper salts, bases, and ancillary ligands can be utilized to elute [18F]fluoride from ion exchange cartridges. The new procedures are effective for fluorine-18 radiochemistry and, as proof of concept, have been used to optimize an otherwise base-sensitive copper-mediated radiofluorination reaction.

Silver-promoted (radio)fluorination of unsaturated carbamates via a radical process

The intramolecular fluorocyclization of unsaturated carbamates is described here using a hypervalent iodine reagent in the presence of a silver catalyst. Both (hetero)aryl-substituted olefins and acrylamides can be utilized as effective substrates. Preliminary mechanistic investigations suggest that the reaction proceeds via a cyclization/1,2-(hetero)aryl migration/fluorination cascade involving an unusual radical process. Furthermore, starting from no-carrier-added [¹⁸F]TBAF, a simple one-pot, two-step cascade method was developed for the generation of ¹⁸F-labeled heterocycles with high radiochemical purity.

Crown Ether Nucleophilic Catalysts (CENCs): Agents for Enhanced Silicon Radiofluorination

New bifunctional phase transfer agents were synthesized and investigated for their abilities to promote rapid fluorination at silicon. These agents, dubbed crown ether nucleophilic catalysts (CENCs), are 18-crown-6 derivatives containing a side-arm and a potentially nucleophilic hydroxyl group. These CENCs proved efficacious in the fluorination of hindered silicon substrates, with fluorination yields dependent on the length of linker connecting the metal chelating unit to the hydroxyl group. The efficacy of these CENCs was also demonstrated for rapid radiofluorination under mild conditions for eventual application in molecular imaging with positron emission tomography (PET). The hydrolysis-resistant aryl silicon fragment is promising as a convenient synthon for labeling potential PET radiotracers.

Automated, Resin-Based Method to Enhance the Specific Activity of Fluorine-18 Clicked PET Radiotracers

Radiolabeling of substrates with 2-[¹⁸F]fluoroethylazide exploits the rapid kinetics, chemical selectivity, and mild conditions of the copper-catalyzed azide-alkyne cycloaddition reaction. While this methodology has proven to result in near-quantitative labeling of alkyne-tagged precursors, the relatively small size of the fluoroethylazide group makes separation of the ¹⁸F-labeled radiotracer and the unreacted precursor challenging, particularly with precursors >500 Da (e.g., peptides). We have developed an inexpensive azide-functionalized resin to rapidly remove unreacted alkyne precursor following the fluoroethylazide labeling reaction and integrated it into a fully automated radiosynthesis platform. We have carried out 2-[¹⁸F]fluoroethylazide labeling of four different alkynes ranging from <300 Da to >1700 Da and found that >98% of the unreacted alkyne was removed in less than 20 min at room temperature to afford the final radiotracers at >99% radiochemical purity with specific activities up to >200 GBq/μmol. We have applied this technique to label a novel cyclic peptide previously evolved to bind the Her2 receptor with high affinity, and demonstrated tumor-specific uptake and low nonspecific background by PET/CT. This resin-based methodology is automated, rapid, mild, and general allowing peptide-based fluorine-18 radiotracers to be obtained with clinically relevant specific activities without chromatographic separation and with only a minimal increase in total synthesis time.

18F-Labelling of electron rich iodonium ylides: application to the radiosynthesis of potential 5-HT2A receptor PET ligands

Nucleophilic ¹⁸F-labelling of electron aromatic systems.

Thermolysis and radiofluorination of diaryliodonium salts derived from anilines

Mechanistic and theoretical studies reveal new reactions of Ar₂I salts that can interfere with radiolabeling of these substrates.

18F-Fluoroform: a 18F-trifluoromethylating agent for the synthesis of SCF218F-aromatic derivatives

A new protocol for the synthesis of SCF₂¹⁸F-aromatic derivatives from ¹⁸F-fluoroform is described.

Facile room temperature synthesis of fluorine-18 labeled fluoronicotinic acid-2,3,5,6-tetrafluorophenyl ester without azeotropic drying of fluorine-18

Fluorine-18 labeled fluoronicotinic acid-2,3,5,6-tetrafluorophenyl ester has been successfully synthesized in an unprecedented way by flowing an acetonitrile solution of its quaternary ammonium salt precursor (N,N,N-trimethyl-5-((2,3,5,6-tetrafluorophenoxy)carbonyl)pyridin-2-aminium trifluoromethanesulfonate, 1) through an anion exchange cartridge. The fluorination reaction proceeded at room temperature without azeotropic drying of the fluoride. Over 75% conversion was observed with 10mg of precursor in 2:8, acetonitrile: t-butanol in 1min. The total synthesis time was 5min which is ~30min shorter than the current literature method.

Translocator protein ligands based on N-methyl-(quinolin-4-yl)oxypropanamides with properties suitable for PET radioligand development

Modifications to an N-methyl-(quinolin-4-yl)oxypropanamide scaffold were explored to discover leads for developing new radioligands for PET imaging of brain TSPO (translocator protein), a biomarker of neuroinflammation. Whereas contraction of the quinolinyl portion of the scaffold or cyclization of the tertiary amido group abolished high TSPO affinity, insertion of an extra nitrogen atom into the 2-arylquinolinyl portion was effective in retaining sub-nanomolar affinity for rat TSPO, while also decreasing lipophilicity to within the moderate range deemed preferable for a PET radioligand. Replacement of a phenyl group on the amido nitrogen with an isopropyl group was similarly effective. Among others, compound 20 (N-methyl-N-phenyl-2-[2-(pyridin-2-yl)-1,8-naphthyridin-4-yloxy]propanamide) appears especially appealing for PET radioligand development, based on high selectivity and high affinity (K_i = 0.5 nM) for rat TSPO, moderate lipophilicity (logD = 2.48), and demonstrated amenability to labeling with carbon-11.

Synthesis, dynamic NMR characterization and XRD studies of novel N,N'-substituted piperazines for bioorthogonal labeling

Novel, functionalized piperazine derivatives were successfully synthesized and fully characterized by ¹H/¹³C/¹⁹F NMR, MS, elemental analysis and lipophilicity. All piperazine compounds occur as conformers resulting from the partial amide double bond. Furthermore, a second conformational shape was observed for all nitro derivatives due to the limited change of the piperazine chair conformation. Therefore, two coalescence points were determined and their resulting activation energy barriers were calculated using ¹H NMR. To support this result, single crystals of 1-(4-nitrobenzoyl)piperazine (3a, monoclinic, space group C2/c, a = 24.587(2), b = 7.0726(6), c = 14.171(1) Å, β = 119.257(8)°, V = 2149.9(4) ų, Z = 4, D_obs = 1.454 g/cm³) and the alkyne derivative 4-(but-3-yn-1-yl)-1-(4-fluorobenzoyl)piperazine (4b, monoclinic, space group P2₁/n, a = 10.5982(2), b = 8.4705(1), c = 14.8929(3) Å, β = 97.430(1)°, V = 1325.74(4) ų, Z = 4, D_obs = 1.304 g/cm³) were obtained from a saturated ethyl acetate solution. The rotational conformation of these compounds was also verified by XRD. As proof of concept for future labeling purposes, both nitropiperazines were reacted with [¹⁸F]F^–. To test the applicability of these compounds as possible ¹⁸F-building blocks, two biomolecules were modified and chosen for conjugation either using the Huisgen-click reaction or the traceless Staudinger ligation.

Efficient Pathway for the Preparation of Aryl(isoquinoline)iodonium(III) Salts and Synthesis of Radiofluorinated Isoquinolines

Iodonium compounds play a pivotal role in (18) F-fluorination of radiopharmaceuticals containing non-activated arenes. However, preparation of these species is limited to oxidation conditions or exchange with organometallics that are prepared from aryl halides. Herein we describe a novel "one-pot" process to assemble aryl(isoquinoline)iodonium salts in 40-94 % yields from mesoionic carbene silver complex and Aryl-I-Py2 (OTf)2 . The method is general, practical, and compatible with well-functionalized molecules as well as useful for the preparation of a wide range of (18) F-labeled isoquinolines resulting in up to 92 % radiochemical conversion. As proof of concept, a fluorinated isoquinoline alkaloid, (18) F-aspergillitine is prepared in 10 % isolated radiochemical yield from the corresponding phenyl(aspergillitine)iodonium salt.

(18)F-Labeling of Mannan for Inflammation Research with Positron Emission Tomography

Recently mannan from Saccharomyces cerevisiae has been shown to be able to induce psoriasis and psoriatic arthritis in mice, and the phenotypes resemble the corresponding human diseases. To investigate the pathological processes, we set out to label mannan with fluorine-18 ((18)F) and study the (18)F-labeled mannan in vitro and in vivo with positron emission tomography (PET). Accordingly, mannan has been transformed into (18)F-fluoromannan with (18)F-bicyclo[6.1.0]nonyne. In mouse aorta, the binding of [(18)F]fluoromannan to the atherosclerotic lesions was clearly visualized and was significantly higher compared to blocking assays (P < 0.001) or healthy mouse aorta (P < 0.001). In healthy rats the [(18)F]fluoromannan radioactivity accumulated largely in the macrophage-rich organs such as liver, spleen, and bone marrow and the excess excreted in urine. Furthermore, the corresponding (19)F-labeled mannan has been used to induce psoriasis and psoriatic arthritis in mice, which indicates that the biological function of mannan is preserved after the chemical modifications.

Nucleophilic (Radio)Fluorination of α-Diazocarbonyl Compounds Enabled by Copper-Catalyzed H-F Insertion

The copper-catalyzed H-F insertion into α-diazocarbonyl compounds is described using potassium fluoride (KF) and hexafluoroisopropanol. Access to complex α-fluorocarbonyl derivatives is achieved under mild conditions, and the method is readily adapted to radiofluorination with [(18)F]KF. This late-stage strategy provides an attractive route to (18)F-labeled biomolecules.

In Vitro and In Vivo Characterization of Selected Fluorine-18 Labeled Radioligands for PET Imaging of the Dopamine D3 Receptor

Cerebral dopamine D3 receptors seem to play a key role in the control of drug-seeking behavior. The imaging of their regional density with positron emission tomography (PET) could thus help in the exploration of the molecular basis of drug addiction. A fluorine-18 labeled D3 subtype selective radioligand would be beneficial for this purpose; however, as yet, there is no such tracer available. The three candidates [¹⁸F]1, [¹⁸F]2a and [¹⁸F]2b were chosen for in vitro and in vivo characterization as radioligands suitable for selective PET imaging of the D3 receptor. Their evaluation included the analysis of radiometabolites and the assessment of non-specific binding by in vitro rat brain autoradiography. While [¹⁸F]1 and [¹⁸F]2a revealed high non-specific uptake in in vitro rat brain autoradiography, the D3 receptor density was successfully determined on rat brain sections (n = 4) with the candidate [¹⁸F]2b offering a B_max of 20.38 ± 2.67 pmol/g for the islands of Calleja, 19.54 ± 1.85 pmol/g for the nucleus accumbens and 16.58 ± 1.63 pmol/g for the caudate putamen. In PET imaging studies, the carboxamide 1 revealed low signal/background ratios in the rat brain and relatively low uptake in the pituitary gland, while the azocarboxamides [¹⁸F]2a and [¹⁸F]2b showed binding that was blockable by the D3 receptor ligand BP897 in the ventricular system and the pituitary gland in PET imaging studies in living rats.

Synthesis of (18) F-Difluoromethylarenes from Aryl (Pseudo) Halides

A general method for the synthesis of [(18) F]difluoromethylarenes from [(18) F]fluoride for radiopharmaceutical discovery is reported. The method is practical, operationally simple, tolerates a wide scope of functional groups, and enables the labeling of a variety of arenes and heteroarenes with radiochemical yields (RCYs, not decay-corrected) from 10 to 60 %. The (18) F-fluorination precursors are readily prepared from aryl chlorides, bromides, iodides, and triflates. Seven (18) F-difluoromethylarene drug analogues and radiopharmaceuticals including Claritin, fluoxetine (Prozac), and [(18) F]DAA1106 were synthesized to show the potential of the method for applications in PET radiopharmaceutical design.

[(18)F]-Organotrifluoroborates as Radioprosthetic Groups for PET Imaging: From Design Principles to Preclinical Applications

Positron emission tomography (PET) is revolutionizing our ability to visualize in vivo targets for target validation and personalized medicine. Of several classes of imaging agents, peptides afford high affinity and high specificity to distinguish pathologically distinct cell types by the presence of specific molecular targets. Of various available PET isotopes, [(18)F]-fluoride ion is preferred because of its excellent nuclear properties and on-demand production in hospitals at Curie levels. However, the short half-life of (18)F and its lack of reactivity in water continue to challenge peptide labeling. Hence, peptides are often conjugated to a metal chelator for late-stage, one-step labeling. Yet radiometals, while effective, are neither as desirable nor as available as [(18)F]-fluoride ion. Despite considerable past success in identifying semifeasible radiosyntheses, significant challenges continue to confound tracer development. These interrelated challenges relate to (1) isotope/prosthetic choice; (2) bioconjugation for high affinity; (3) high radiochemical yields, (4) specific activities of >1 Ci/μmol to meet FDA microdose requirements; and (5) rapid clearance and in vivo stability. These enduring challenges have been extensively highlighted, while a single-step, operationally simple, and generally applicable means of labeling a peptide with [(18)F]-fluoride ion in good yield and high specific activity has eluded radiochemists and nuclear medicine practitioners for decades. Radiosynthetic ease is of primordial importance since multistep labeling reactions challenge clinical tracer production. In the past decade, as we sought to meet this challenge, appreciation of reactions with aqueous fluoride led us to consider organotrifluoroborate (RBF3(-)) synthesis as a means of rapid aqueous peptide labeling. We have applied principles of mechanistic chemistry, knowledge of chemical reactivity, and synthetic chemistry to design stable RBF3(-)s. Over the past 10 years, we have developed several new [(18)F]-RBF3(-) radioprosthetic groups, all of which guarantee radiosynthetic ease while in most cases providing high tumor:nontumor (T:NT) ratios and moderate-to-high tumor uptake. Although others have developed methods for labeling of peptides with [(18)F]-silylfluorides or [(18)F]-Al-NOTA chelates, this Account focuses on the synthesis of [(18)F]-organotrifluoroborates. In this Account, I detail mechanistic, kinetic, thermodynamic, synthetic, and radiosynthetic approaches that enabled the translation of fundamental principles regarding the chemistry of RBF3(-)s into a tantalizingly close realization of a clinical application of an [(18)F]-organotrifluoroborate-peptide conjugate for imaging of neuroendocrine tumors and the generalization of this method for labeling of several other peptides.

Discovery of Phosphodiesterase 10A (PDE10A) PET Tracer AMG 580 to Support Clinical Studies

We report the discovery of PDE10A PET tracer AMG 580 developed to support proof of concept studies with PDE10A inhibitors in the clinic. To find a tracer with higher binding potential (BPND) in NHP than our previously reported tracer 1, we implemented a surface plasmon resonance assay to measure the binding off-rate to identify candidates with slower washout rate in vivo. Five candidates (2-6) from two structurally distinct scaffolds were identified that possessed both the in vitro characteristics that would favor central penetration and the structural features necessary for PET isotope radiolabeling. Two cinnolines (2, 3) and one keto-benzimidazole (5) exhibited PDE10A target specificity and brain uptake comparable to or better than 1 in the in vivo LC-MS/MS kinetics distribution study in SD rats. In NHP PET imaging study, [(18)F]-5 produced a significantly improved BPND of 3.1 and was nominated as PDE10A PET tracer clinical candidate for further studies.

Fluorine-18 Radiolabeled PET Tracers for Imaging Monoamine Transporters: Dopamine, Serotonin, and Norepinephrine

This review focuses on the development of fluorine-18 radiolabeled PET tracers for imaging the dopamine transporter (DAT), serotonin transporter (SERT), and norepinephrine transporter (NET). All successful DAT PET tracers reported to date are members of the 3β-phenyl tropane class and are synthesized from cocaine. Currently available carbon-11 SERT PET tracers come from both the diphenylsulfide and 3β-phenyl nortropane class, but so far only the nortropanes have found success with fluorine-18 derivatives. NET imaging has so far employed carbon-11 and fluorine-18 derivatives of reboxetine but due to defluorination of the fluorine-18 derivatives further research is still necessary.

Synthesis and Biological Evaluation of an (18)Fluorine-Labeled COX Inhibitor--[(18)F]Fluorooctyl Fenbufen Amide--For Imaging of Brain Tumors

Molecular imaging of brain tumors remains a great challenge, despite the advances made in imaging technology. An anti-inflammatory compound may be a useful tool for this purpose because there is evidence of inflammatory processes in brain tumor micro-environments. Fluorooctylfenbufen amide (FOFA) was prepared from 8-chlorooctanol via treatment with potassium phthalimide, tosylation with Ts₂O, fluorination with KF under phase transfer catalyzed conditions, deprotection using aqueous hydrazine, and coupling with fenbufen. The corresponding radiofluoro product [¹⁸F]FOFA, had a final radiochemical yield of 2.81 mCi and was prepared from activated [¹⁸F]F⁻ (212 mCi) via HPLC purification and concentration. The radiochemical purity was determined to be 99%, and the specific activity was shown to exceed 22 GBq/μmol (EOS) based on decay-corrected calculations. Ex-vivo analysis of [¹⁸F]FOFA in plasma using HPLC showed that the agent had a half-life of 15 min. PET scanning showed significant accumulation of [¹⁸F]FOFA over tumor loci with reasonable contrast in C6-glioma bearing rats. These results suggest that this molecule is a promising agent for the visualization of brain tumors. Further investigations should focus on tumor micro-environments.

Synthesis and evaluation of 6-(3-[(18)F]fluoro-2-hydroxypropyl)-substituted 2-pyridylbenzothiophenes and 2-pyridylbenzothiazoles as potential PET tracers for imaging Aβ plaques

3-[(18)F]Fluoro-2-hydroxypropyl substituted compounds were synthesized and evaluated as novel (18)F-labeled PET tracers for imaging Aβ plaque in a living brain. All compounds exhibited high binding affinities toward the synthetic Aβ1-42 aggregate and/or Alzheimer's disease brain homogenate. In the microPET study with normal mice, the 3-[(18)F]fluoro-2-hydroxypropyl substituted compounds resulted in fast brain washout by reducing the lipophilicities of the compounds. Intriguingly, (S)-configured PET tracers, (S)-[(18)F]1b and (S)-[(18)F]1c, exhibited a 2.8 and 4.0-fold faster brain washout rate at a peak/30 min in the mouse brain than the corresponding (R)-configured PET tracers despite there being no meaningful difference in binding affinities toward Aβ plaque. A further evaluation of (S)-[(18)F]1c with healthy rhesus monkeys also revealed excellent clearance from the frontal cortex with ratios of 7.0, 16.0, 30.0 and 49.0 at a peak/30, 60, 90, and 120 min, respectively. These results suggest that (S)-[(18)F]1c may be a potential PET tracer for imaging Aβ plaque in a living brain.

(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.

Synthesis and in vivo stability studies of [18F]-zwitterionic phosphonium aryltrifluoroborate/indomethacin conjugates

Conjugation of ortho-phosphonium phenyltrifluoroborates with indomethacin affords conjugates which have been radiolabeled by ¹⁸F–¹⁹F isotopic exchange in aqueous solutions and imaged by positron emission tomography in mice.

The chemistry and radiochemistry of hypoxia-specific, radiohalogenated nitroaromatic imaging probes

Hypoxia is prevalent in many solid tumors. Hypoxic tumors tend to exhibit rapid growth and aberrant vasculature, which lead to oxygen (O2) depletion and impaired drug delivery. The reductive environment in hypoxic tumors alters cellular metabolism, which can trigger transcriptional responses; induce genetic alterations; promote invasion, metastasis, resistance to radiotherapy and chemotherapy, tumor progression, and recurrence; and leads to poor local control and reduced survival rates. Therefore, exploiting the reductive microenvironment in hypoxic tumors by delivering electron-affinic, O2-mimetic radioactive drugs that bioreductively activate selectively in the hypoxic microenvironment offers a logical approach to molecular imaging of focal hypoxia. Because these agents also radiosensitize hypoxic cells, they provide an innovative approach to the therapy management of such tumors. To date, nuclear imaging of hypoxic tumor has proven to be clinically effective, whereas chemical radiosensitization by these compounds has not been helpful. The current review provides an insight into the chemistry, radiochemistry, and purification strategies for selected nitroaromatics that directly exploit the bioreductive environment in hypoxic cells. Both experimental and calculated single-electron reduction potentials of electron-affinic compounds, nitroimidazoles in particular, correlate with in vitro radiosensitizing properties, making them preferred choices for use as radiopharmaceuticals for diagnostic imaging and as sensitizers to enhance the killing effects of low-energy-transfer x-rays (O2-mimetic radiosensitization). Extensive research and careful drug design have led to the development of several potentially useful hypoxia-targeting drugs, for example, [(18)F]FAZA, [(18)F]FMISO, [(18)F]EF5, and [(123)I]IAZA, that accrue selectively in hypoxic cells. These molecular probes are now globally used in clinical hypoxia imaging, including cancer. Future innovative developments must, however, consider hypoxia-selective molecular processes and the physicochemical properties of the drugs that dictate their biodistribution, hypoxia-selective accumulation, pharmacokinetics, clearance, biochemical behavior, and metabolism. This will facilitate their ultimate transformation to effective molecular theranostics, leading to improved multimodal management of cancer.

Synthesis of [18F]Arenes via the Copper-Mediated [18F]Fluorination of Boronic Acids

A copper-mediated radiofluorination of aryl- and vinylboronic acids with K¹⁸F is described. This method exhibits high functional group tolerance and is effective for the radiofluorination of a range of electron-deficient, -neutral, and -rich aryl-, heteroaryl-, and vinylboronic acids. This method has been applied to the synthesis of [¹⁸F]FPEB, a PET radiotracer for quantifying metabotropic glutamate 5 receptors.

[(18)F]-Group 13 fluoride derivatives as radiotracers for positron emission tomography

The field of (18)F chemistry is rapidly expanding because of the use of this radionuclide in radiotracers for positron emission tomography (PET). Until recently, most [(18)F]-radiotracers were generated by the direct attachment of (18)F to a carbon in the organic backbone of the radiotracer. The past decade has witnessed the emergence of a new strategy based on the formation of an (18)F-group 13 element bond. This approach, which is rooted in the field of fluoride anion complexation/coordination chemistry, has led to the development of a remarkable family of boron, aluminium and gallium [(18)F]-fluoride anion complexing agents which can be conjugated with peptides and small molecules to generate disease specific PET radiotracers. This review is dedicated to the chemistry of these group 13 [(18)F]-fluorides anion complexing agents and their use in PET. Some of the key fluoride-binding motifs covered in this review include the trifluoroborate unit bound to neutral or cationic electron deficient backbones, the BF2 unit of BODIPY dyes, and AlF or GaF3 units coordinated to multidentate Lewis basic ligands. In addition to describing how these moieties can be converted into their [(18)F]-analogs, this review also dicusses their incorporation into bioconjugates for application in PET.

Arylation with Diaryliodonium Salts

This chapter focuses on recent developments in metal-free and metal-catalyzed arylations with diaryliodonium salts (diaryl-λ3-iodanes). Synthetic routes to diaryliodonium salts are briefly described, and chemoselectivity trends with unsymmetric iodonium salts are discussed.

Solid-supported reagents composed of a copolymer possessing 2-O-sulfonyl mannosides and phase-transfer catalysts for the synthesis of 2-fluoroglucose

We described the synthesis of a solid-supported co-polymer possessing mannosides and phase-transfer catalysts and synthesis of 2-fluoroglucoside from it. We first prepared a soluble copolymer from two allene monomers possessing a precursor for the synthesis of 2-fluoroglycose and a crown ether. The copolymerization of the monomers via the π-ally nickel-catalyst smoothly proceeded at room temperature to provide a desired copolymer without decomposition of the sulfonate esters. The copolymer exhibited high reactivity towards fluorination in comparison with a conventional precursor. We next synthesized the solid-supported copolymer by using the solid-supported initiator attached with TentaGel® resins. TentaGel® enabled polymerization under stirring with stirring bar without decomposition. The solid-supported copolymer exhibited comparable reactivity towards fluorination in comparison with the soluble copolymer. In addition, it can be easily separated from the reaction vessel by filtration.

Ortho-Stabilized (18) F-Azido Click Agents and their Application in PET Imaging with Single-Stranded DNA Aptamers

Azido (18) F-arenes are important and versatile building blocks for the radiolabeling of biomolecules via Huisgen cycloaddition ("click chemistry") for positron emission tomography (PET). However, routine access to such clickable agents is challenged by inefficient and/or poorly defined multistep radiochemical approaches. A high-yielding direct radiofluorination for azido (18) F-arenes was achieved through the development of an ortho-oxygen-stabilized iodonium derivative (OID). This OID strategy addresses an unmet need for a reliable azido (18) F-arene clickable agent for bioconjugation reactions. A ssDNA aptamer was radiolabeled with this agent and visualized in a xenograft mouse model of human colon cancer by PET, which demonstrates that this OID approach is a convenient and highly efficient way of labeling and tracking biomolecules.

Nucleophilic Fluorination and Radiofluorination via Aziridinium Intermediates: N-Substituent Influence, Unexpected Regioselectivity, and Differences between Fluorine-19 and Fluorine-18

The efficient dehydrofluorination and radiofluorination of N,N-disubstituted-β-aminoalcohols through an anchimeric-assisted mechanism was developed. An investigation into the influence of N-substituents on the ring opening of the aziridinium intermediate indicated differences in the isomeric ratio and the yields of fluorinated products obtained from N,N-disubstituted-phenylalaninol. This influence was substantial for (18)F-radiofluorination, with yields varying from 0 to 71% at room temperature (RT). Although no significant effects were observed in the fluorine-19 chemistry when the reaction was heated to 90 °C, considerable changes appeared during radiofluorination. In the latter case, the radiochemical yields increased, and degradation of the 2-fluoro-propan-1-amine isomer (b) occurred, leading to a regiospecific reaction in the radiolabeling of [(18)F]-fluorodeprenyl. This method involving nucleophilic radiofluorination at RT was successfully applied to the radiolabeling of [(18)F]-2-fluoroethylamines in which the influence of the N-substituent was also observed.

Production and Transport of Gaseous 18F-Synthons: 18F-Acyl Fluorides

Fluorine-18 (¹⁸F, T _1/2=109.7 min) is a positron-emitting isotope that has found extensive application as a radiolabel for positron emission tomography (PET). Although gaseous ¹¹C-CO₂ and ¹¹C-CH₄ are practically transported from cyclotron to radiochemistry processes, ¹⁸F-fluoride is routinely transported in aqueous solution because it is commonly produced by proton irradiation of ¹⁸O-enriched water. In most cases, subsequent dry-down steps are necessary to prepare reactive ¹⁸F-fluoride for radiofluorination. In this work, a simple module was designed to generate gaseous ¹⁸F-acyl fluorides from aqueous ¹⁸F-fluoride solution by solid phase ¹⁸F-radiofluorination of acyl anhydride. The gaseous ¹⁸F-acyl fluorides were purified through a column containing Porapak Q/Na₂SO₄, resulting in high yields (>86%), purities (>99%) and specific activities (>1200 GBq/μmol). Prototypic ¹⁸F-acetyl fluoride (¹⁸F-AcF) was readily transported through 15 m of 0.8 mm ID polypropylene tubing with low (0.64 ± 0.12 %) adsorption to the tubing. Following dissolution of ¹⁸F-AcF in solvent containing base, highly reactive ¹⁸F-flouride was generated immediately and used directly for ¹⁸F-labeling reactions. These data indicate that ¹⁸F-acyl fluorides represent a new paradigm for preparation and transport of anhydrous, reactive ¹⁸F-fluoride for radiofluorinations.

Synthesis of 18F-Arenes from Spirocyclic Iodonium(III) Ylides via Continuous-Flow Microfluidics

Spirocyclic hypervalent iodine(III) ylides have proven to be synthetically versatile precursors for efficient radiolabelling of a diverse range of non-activated (hetero)arenes, highly functionalised small molecules, building blocks and radiopharmaceuticals from [18F]fluoride ion. Herein, we report the implementation of these reactions onto a continuous-flow microfluidic platform, thereby offering an alterative and automated synthetic procedure of a radiopharmaceutical, 3-[18F]fluoro-5-[(pyridin-3-yl)ethynyl]benzonitrile ([18F]FPEB) and a routinely used building block for click-radiochemistry, 4-[18F]fluorobenzyl azide. This new protocol was applied to the synthesis of [18F]FPEB (radiochemical conversion (RCC) = 68 ± 5%) and 4-[18F]fluorobenzyl azide (RCC=68 ± 5%; isolated radiochemical yield = 24±0%). We anticipate that the high throughput microfluidic platform will accelerate the discovery and applications of 18F-labelled building blocks and labelled compounds prepared by iodonium ylide precursors as well as the production of radiotracers for preclinical imaging studies.

Improving the (18)F-fluoromethylcholine ((18)F-FCH) radiochemical yield via optimising the azeotropic drying of non-carrier-added (18)F-fluorine

(18)F-Fluoromethylcholine ((18)F-FCH) has been suggested as one of the reputable imaging tracers for diagnosis of prostate tumour in PET/CT examination. Nevertheless, it has never been synthesised in Malaysia. We acknowledged the major problem with (18)F-FCH is due to its relatively low radiochemical yield at the end of synthesis (EOS). Therefore, this technical note presents improved (18)F-FCH radiochemical yields after carrying out optimisation on azeotropic drying of non-carrier-added (18)F-Fluorine.

Difluorocarbene-Derived Trifluoromethylthiolation and [(18)F]Trifluoromethylthiolation of Aliphatic Electrophiles

The first trifluoromethylthiolation and [(18)F]trifluoromethylthiolation of alkyl electrophiles with in situ generated difluorocarbene in the presence of elemental sulfur and external (radioactive) fluoride ion is described. This transition-metal-free approach is high yielding, compatible with a variety of functional groups, and operated under mild reaction conditions. The conceptual advantage of this exogenous-fluoride-mediated transformation enables unprecedented syntheses of [(18)F]CF3S-labeled molecules from most commonly used [(18)F]fluoride ions. The rapid radiochemical reaction time (≤1 min) and high functional-group tolerance allow access to a variety of aliphatic [(18)F]CF3S compounds in high yields.

Iridium-Catalyzed Enantioselective Fluorination of Racemic, Secondary Allylic Trichloroacetimidates

The Ir-catalyzed enantioselective fluorination of racemic, branched allylic trichloroacetimidates with Et3N·3HF is a mild and efficient route for selective incorporation of fluoride ion into allylic systems. We herein describe the asymmetric fluorination of racemic, secondary allylic electrophiles with Et3N·3HF using a chiral-diene-ligated Ir complex. The methodology enables the formation of acyclic fluorine-containing compounds in good yields with excellent levels of asymmetric induction and overcomes the limitations previously associated with the enantioselective construction of secondary allylic fluorides bearing α-linear substituents.

One-step (18)F labeling of biomolecules using organotrifluoroborates

Herein we present a general protocol for the functionalization of biomolecules with an organotrifluoroborate moiety so that they can be radiolabeled with aqueous (18)F fluoride ((18)F(-)) and used for positron emission tomography (PET) imaging. Among the β(+)-emitting radionuclides, fluorine-18 ((18)F) is the isotope of choice for PET, and it is produced, on-demand, in many hospitals worldwide. Organotrifluoroborates can be (18)F-labeled in one step in aqueous conditions via (18)F-(19)F isotope exchange. This protocol features a recently designed ammoniomethyltrifluoroborate, and it describes the following: (i) a synthetic strategy that affords modular synthesis of radiolabeling precursors via a copper-catalyzed 'click' reaction; and (ii) a one-step (18)F-labeling method that obviates the need for HPLC purification. Within 30 min, (18)F-labeled PET imaging probes, such as peptides, can be synthesized in good chemical and radiochemical purity (>98%), satisfactory radiochemical yield of 20-35% (n > 20, non-decay corrected) and high specific activity of 40-111 GBq/μmol (1.1-3.0 Ci/μmol). The entire procedure, including the precursor preparation and (18)F radiolabeling, takes 7-10 d.

A Mild and General One-Pot Synthesis of Densely Functionalized Diaryliodonium Salts

Diaryliodonium salts are powerful and widely used arylating agents in organic chemistry. Here we report a scalable, synthesis of densely functionalized diaryliodonium salts from aryl iodides under mild conditions. This two-step, one-pot process has remarkable functional group tolerance, is compatible with commonly employed acid-labile protective group strategies, avoids heavy metal and transition metal reagents, and provides a direct route to stable precursors to PET imaging agents.