A Universal Procedure for the [18F]Trifluoromethylation of Aryl Iodides and Aryl Boronic Acids with Highly Improved Specific Activity

Isotopologues of potassium 2,2,2-trifluoroethoxide for applications in positron emission tomography and beyond

The 2,2,2-trifluoroethoxy group increasingly features in drugs and potential tracers for biomedical imaging with positron emission tomography (PET). Herein, we describe a rapid and transition metal-free conversion of fluoroform with paraformaldehyde into highly reactive potassium 2,2,2-trifluoroethoxide (CF3CH2OK) and demonstrate robust applications of this synthon in one-pot, two-stage 2,2,2-trifluoroethoxylations of both aromatic and aliphatic precursors. Moreover, we show that these transformations translate easily to fluoroform that has been labeled with either carbon-11 (t1/2 = 20.4 min) or fluorine-18 (t1/2 = 109.8 min), so allowing the appendage of complex molecules with a no-carrier-added 11C- or 18F- 2,2,2-trifluoroethoxy group. This provides scope to create candidate PET tracers with radioactive and metabolically stable 2,2,2-trifluoroethoxy moieties. We also exemplify syntheses of isotopologues of potassium 2,2,2-trifluoroethoxide and show their utility for stable isotopic labeling which can be of further benefit for drug discovery and development.

Synthesis of 18F-labeled Aryl Trifluoromethyl Sulfones, -Sulfoxides, and -Sulfides for Positron Emission Tomography

Positron emission tomography (PET) is becoming increasingly important in nuclear medicine and drug discovery. To date, the development of many potential PET tracers is hampered by the lack of suitable synthetic pathways for their preparation. This is particularly true for the highly desired radiolabeling of compounds bearing [18F]CF3-groups. For instance, S(O)nCF3-groups (n=0, 1, 2) serve as structural motif in a range of biologically active compounds, but their radiosynthesis remains largely unprecedented (for n=1, 2). Herein, we describe general methods for the radiosynthesis of 18F-labeled aryl trifluoromethyl sulfones, -sulfoxides, and -sulfides. All three methods are operationally straightforward, start from widely available precursors, i.e., sulfonyl fluorides and thiophenols, and make use of the recently established [18F]Ruppert-Prakash reagent. Further, the syntheses display good functional group tolerance as demonstrated by the 18F-labeling of more than 40 compounds. The applicability of the new method is demonstrated by the radiolabeling of three bioactive molecules, optionally to be used as PET tracers. In a broader context, this work presents a substantial expansion of the chemical space of radiofluorinated structural motifs to be used for the development of new PET tracers.

Synthesis of 18F-labelled aryl trifluoromethyl ketones with improved molar activity

A method for the radiosynthesis of 18F-labelled aryl trifluoromethyl ketones starting from widely available Weinreb amides using [18F]fluoroform is presented. The method uses potassium hexamethyldisilazane as base and delivers products in high molar activity (up to 24 GBq μmol-1) and excellent radiochemical conversions. The applicability for PET tracer synthesis is demonstrated by the radiosynthesis of ten (hetero)aryl trifluoromethylketones, bearing electron-withdrawing and -donating substituents including a derivative of bioactive probenecid.

One-Pot Formal Carboradiofluorination of Alkenes: A Toolkit for Positron Emission Tomography Imaging Probe Development

We report the first one-pot formal alkene carboradiofluorination reaction employing easily accessible alkenes as both prosthetic group precursors and coupling partners. The methodology features rapid sequential Markovnikov-selective iodofluorination and photoinduced Pd(0/I/II)-catalyzed alkyl Heck reaction as a mild and robust fluorine-18 (18F) radiochemical approach for positron emission tomography (PET) imaging probe development. A new class of prosthetic groups for PET imaging probe synthesis was isolated as iodofluorinated intermediates in moderate to excellent yields. The one-pot formal alkenylfluorination reaction was carried out to produce over 30 analogues of a wide range of bioactive molecules. Further application of the Pd(0/I/II) manifold in PET probe development was illustrated by the direct carbo(radio)fluorination of electron-rich alkenes. The methods were successfully translated to radiolabel a broad scope of medicinally relevant small molecules in generally good radiochemical conversion. The protocol was further optimized to accommodate no-carrier-added conditions with similar efficiency for future (pre)clinical translation. Moreover, the radiosynthesis of prosthetic groups was automated in a radiochemistry module to facilitate its practical use in multistep radiochemical reactions.

Radiochemistry for positron emission tomography

Positron emission tomography (PET) constitutes a functional imaging technique that is harnessed to probe biological processes in vivo. PET imaging has been used to diagnose and monitor the progression of diseases, as well as to facilitate drug development efforts at both preclinical and clinical stages. The wide applications and rapid development of PET have ultimately led to an increasing demand for new methods in radiochemistry, with the aim to expand the scope of synthons amenable for radiolabeling. In this work, we provide an overview of commonly used chemical transformations for the syntheses of PET tracers in all aspects of radiochemistry, thereby highlighting recent breakthrough discoveries and contemporary challenges in the field. We discuss the use of biologicals for PET imaging and highlight general examples of successful probe discoveries for molecular imaging with PET - with a particular focus on translational and scalable radiochemistry concepts that have been entered to clinical use.

Recent Advances in 18F-Labeled Amino Acids Synthesis and Application

Radiolabeled amino acids are an important class of agents for positron emission tomography imaging that target amino acid transporters in many tumor types. Traditional ¹⁸F-labeled amino acid synthesis strategies are always based on nucleophilic aromatic substitution reactions with multistep radiosynthesis and low radiochemical yields. In recent years, new ¹⁸F-labeling methodologies such as metal-catalyzed radiofluorination and heteroatom (B, P, S, Si, etc.)-¹⁸F bond formation are being effectively used to synthesize radiopharmaceuticals. This review focuses on recent advances in the synthesis, radiolabeling, and application of a series of ¹⁸F-labeled amino acid analogs using new ¹⁸F-labeling strategies.

[18F]Difluorocarbene for positron emission tomography

The advent of total-body positron emission tomography (PET) has vastly broadened the range of research and clinical applications of this powerful molecular imaging technology1. Such possibilities have accelerated progress in fluorine-18 (18F) radiochemistry with numerous methods available to 18F-label (hetero)arenes and alkanes2. However, access to 18F-difluoromethylated molecules in high molar activity is mostly an unsolved problem, despite the indispensability of the difluoromethyl group for pharmaceutical drug discovery3. Here we report a general solution by introducing carbene chemistry to the field of nuclear imaging with a [18F]difluorocarbene reagent capable of a myriad of 18F-difluoromethylation processes. In contrast to the tens of known difluorocarbene reagents, this 18F-reagent is carefully designed for facile accessibility, high molar activity and versatility. The issue of molar activity is solved using an assay examining the likelihood of isotopic dilution on variation of the electronics of the difluorocarbene precursor. Versatility is demonstrated with multiple [18F]difluorocarbene-based reactions including O-H, S-H and N-H insertions, and cross-couplings that harness the reactivity of ubiquitous functional groups such as (thio)phenols, N-heteroarenes and aryl boronic acids that are easy to install. The impact is illustrated with the labelling of highly complex and functionalized biologically relevant molecules and radiotracers.

Mild, Organo-Catalysed Borono-Deamination as a Key to Late-Stage Pharmaceutical Precursors and 18F-Labelled Radiotracers

A tris(pentafluorophenyl)borane catalysed method for the synthesis of boronic acid esters from aromatic amines in yields of up to 93% was devised. Mild conditions, benign reagents, short reaction times, low temperatures and a wide substrate scope characterize the method. The reaction was found applicable to the synthesis of boronic acid ester derivatives of complex drug molecules in up to 86% isolated yield and high purity suitable for labelling. These boronates were subsequently labelled with [¹⁸F]fluoride ion in radiochemical yields of up to 55% with and even without isolation of the boronate-intermediate.

Synthesis of ArCF2X and [18F]Ar-CF3 via Cleavage of the Trifluoromethylsulfonyl Group

A versatile synthesis of ArCF₂X and [¹⁸F]Ar-CF₃ type compounds from readily available ArCF₂SO₂CF₃ has been developed. Diverse nucleophiles, including weak nucleophiles such as halides (¹⁸F^-, Cl^-, Br^-, and I^-), RSH, and ROH, could react with ArCF₂SO₂CF₃ efficiently to give the corresponding difluoromethylene products. The control experiments and the Hammett plot indicated that the reaction might proceed through a difluorocarbocation intermediate generated from the steric hindrance-assisted cleavage of the trifluoromethylsulfonyl group.

Synthesis and Screening in Mice of Fluorine-Containing PET Radioligands for TSPO: Discovery of a Promising 18F-Labeled Ligand

Translocator protein 18 kDa (TSPO) is a biomarker of neuroinflammation. [11C]ER176 robustly quantifies TSPO in the human brain with positron emission tomography (PET), irrespective of subject genotype. We aimed to develop an ER176 analog with potential for labeling with longer-lived fluorine-18 (t1/2 = 109.8 min). New fluoro and trifluoromethyl analogs of ER176 were prepared through a concise synthetic strategy. These ligands showed high TSPO affinity and low human genotype sensitivity. Each ligand was initially labeled by a generic 11C-methylation procedure, thereby enabling speedy screening in mice. Each radioligand was rapidly taken up and well retained in the mouse brain at baseline after intravenous injection. Preblocking of TSPO showed that high proportions of brain uptake were specifically bound to TSPO at baseline. Overall, the 3-fluoro analog of [11C]ER176 ([11C]3b) displayed the most promising imaging properties. Therefore, a method was developed to label 3b with [18F]fluoride ion. [18F]3b gave similarly promising PET imaging results and deserves evaluation in higher species.

Synthesis and evaluation of [18F]cinacalcet for the imaging of parathyroid hyperplasia

INTRODUCTION

Parathyroid hyperplasia is a disease characterized by overactive parathyroid glands secreting increased levels of parathyroid hormone. Surgical removal of the parathyroid glands is the standard treatment but requires precise pre-operative localization of the glands. However, currently available imaging modalities show limited sensitivity. Since positron emission tomography (PET) is a molecular imaging technique with high accuracy and sensitivity, our aim was to develop a new PET tracer for overactive parathyroid glands imaging by radiolabelling cinacalcet, a drug binding to the calcium-sensing receptor of the parathyroid glands.

METHODS

[18F]Cinacalcet was synthesized by copper-catalysed [18F]trifluoromethylation of a boronic acid precursor using high molar activity [18F]fluoroform. Ex vivo biodistribution and metabolism were evaluated in 12 healthy male Wistar rats at 5, 15, 45 and 90 min. PET scans were performed at baseline and after blocking with NPS R-568.

RESULTS

[18F]Cinacalcet was obtained in an overall radiosynthesis time of 1 h with a radiochemical purity of 98 ± 1%, a radiochemical yield of 8 ± 4% (overall, n = 7, corrected for decay) and a molar activity of 40 ± 11 GBq/μmol (n = 7, at EOS). The ex vivo biodistribution showed uptake in the thyroid and parathyroid glands as well as in other glands such as adrenals, salivary glands and pancreas. The tracer was rapidly cleared from the blood via liver and kidneys and showed fast metabolism. PET images confirmed uptake in the target organ. However, in a blocking study with NPS R-568 specific binding of [18F]cinacalcet to the CaSR could not be confirmed.

CONCLUSIONS

[18F]Cinacalcet was successfully synthesized. First in vivo experiments in healthy rats showed uptake of the tracer in the target organ and fast metabolism, encouraging further in vivo evaluation of this tracer.

Evaluating N-difluoromethyltriazolium triflate as a precursor for the synthesis of high molar activity [18 F]fluoroform

The trifluoromethyl group is a prominent motif in biologically active compounds and therefore of great interest for the labeling with the positron emitter fluorine-18 for positron emission tomography (PET) imaging. Multiple labeling strategies have been explored in the past; however, most of them suffer from low molar activity due to precursor degradation. In this study, the potential of 1-(difluoromethyl)-3-methyl-4-phenyl-1H-1,2,3-triazol-3-ium triflate as precursor for the synthesis of the [18 F]trifluoromethylation building block [18 F]fluoroform with high molar activity was investigated. The triazolium precursor was reacted under various conditions with [18 F]fluoride, providing [18 F]fluoroform with radiochemical yields (RCY) and molar activities (Am ) comparable and even superior with already existing methods. Highest molar activities (Am  = 153 ± 14 GBq/μmol, dc, EOS) were observed for the automated procedure on the Neptis® perform module. Due to its easy handling and good RCY and Am in the [18 F]fluoroform synthesis, the triazolium precursor is a valuable alternative to already known precursors.

Closing the gap between 19F and 18F chemistry

Positron emission tomography (PET) has become an invaluable tool for drug discovery and diagnosis. The positron-emitting radionuclide fluorine-18 is frequently used in PET radiopharmaceuticals due to its advantageous characteristics; hence, methods streamlining access to 18F-labelled radiotracers can make a direct impact in medicine. For many years, access to 18F-labelled radiotracers was limited by the paucity of methodologies available, and the poor diversity of precursors amenable to 18F-incorporation. During the last two decades, 18F-radiochemistry has progressed at a fast pace with the appearance of numerous methodologies for late-stage 18F-incorporation onto complex molecules from a range of readily available precursors including those that do not require pre-functionalisation. Key to these advances is the inclusion of new activation modes to facilitate 18F-incorporation. Specifically, new advances in late-stage 19F-fluorination under transition metal catalysis, photoredox catalysis, and organocatalysis combined with the availability of novel 18F-labelled fluorination reagents have enabled the invention of novel processes for 18F-incorporation onto complex (bio)molecules. This review describes these major breakthroughs with a focus on methodologies for C-18F bond formation. This reinvigorated interest in 18F-radiochemistry that we have witnessed in recent years has made a direct impact on 19F-chemistry with many laboratories refocusing their efforts on the development of methods using nucleophilic fluoride instead of fluorination reagents derived from molecular fluorine gas.

Fluorine-18 labelled Ruppert-Prakash reagent ([18F]Me3SiCF3) for the synthesis of 18F-trifluoromethylated compounds

This article describes the first synthesis and application of fluorine-18 labelled Ruppert-Prakash reagent [18F]Me3SiCF3. [18F]Me3SiCF3 was synthesized from [18F]fluoroform with radiochemical yields of 85-95% and radiochemical purities of >95% within 20 minutes. 18F-trifluoromethylated compounds were successfully prepared by reaction of [18F]Me3SiCF3 with benzaldehydes, acetophenones and benzophenones.

Mechanistic study of nucleophilic fluorination for the synthesis of fluorine-18 labeled fluoroform with high molar activity from N-difluoromethyltriazolium triflate

The synthesis of fluorine-18 labeled fluoroform with high molar activity has grown in importance for the development of fluorine-18 labeled aryl-CF₃ radiopharmaceuticals that are useful as diagnostic radiotracers for the powerful technique of positron emission tomography (PET). We designed a strategy of synthesizing fluorine-18 labeled fluoroform from N1-difluoromethyl-N3-methyltriazolium triflate (1) via S_N2 fluorination without stable fluorine isotope scrambling. Fluoroform was generated at rt in 10 min by fluorination of the triazolium precursor with TBAF (6 equiv.). We propose three routes (a), (b), and (c) for this fluorination. Quantum chemical calculations have been carried out to elucidate the mechanism of experimentally observed nucleophilic attack of fluoride at difluoromethyl group via route (a), not N3-methyl via route (b). ¹H and ¹⁹F NMR studies using deuterium source have been performed to examine the competition between S_N2 fluorination (route (a)) and the formation of difluorocarbene (route (c)). The observed superiority of S_N2 pathway to formation of difluorocarbene in the reaction of the precursor using CsF in (CD₃CN/(CD₃)₃COD (17.8 : 1)) gives the possibility of preparing the fluorine-18 labeled fluoroform in high molar activity.

Route a: desired S_N2 reaction of fluoride to form fluoroform with high molar activity; route b: side reaction to form methyl fluoride; route c: side reaction to form difluorocarbene to give fluoroform with lower molar activity.

One-Step, Rapid, 18F-19F Isotopic Exchange Radiolabeling of Difluoro-dioxaborinins: Substituent Effect on Stability and In Vivo Applications

The β-diketone moiety is commonly present in many anticancer drugs, antibiotics, and natural products. We describe a general method for radiolabeling β-diketone-bearing molecules with fluoride-18. Radiolabeling was carried out via ¹⁸F-¹⁹F isotopic exchange on nonradioactive difluoro-dioxaborinins, which were generated by minimally modifying the β-diketone as a difluoroborate. Radiochemistry was one-step, rapid (<10 min), and high-yielding (>80%) and proceeded at room temperature to accommodate the half-life of F-18 (t_1/2 = 110 min). High molar activities (7.4 Ci/μmol) were achieved with relatively low starting activities (16.4 mCi). It was found that substituents affected both the solvolytic stability and fluorescence properties of difluoro-dioxaborinins. An F-18 radiolabeled difluoro-dioxaborinin probe that was simultaneously fluorescent showed sufficient stability for in vivo positron emission tomography (PET)/fluorescence imaging in mice, rabbits, and patients. These findings will guide the design of probes with specific PET/fluorescence properties; the development of new PET/fluorescence dual-modality reporters; and accurate in vivo tracking of β-diketone molecules.

Syntheses of [11C]2- and [11C]3-trifluoromethyl-4-aminopyridine: potential PET radioligands for demyelinating diseases

Trifluoromethyl groups are of great interest in PET radiopharmaceuticals. Radiolabelled 4-aminopyridine (4AP) derivatives have been proposed for imaging demyelinating diseases. Here, we describe methods for producing 11C-trifluoromethylated derivatives of 4AP and present early imaging results with [11C]3-trifluoromethyl-4AP in a rhesus macaque. This study shows the utility of [11C]CuCF3 for labelling pyridines and provides initial evidence for the potential use of [11C]3-trifluoromethyl-4AP as a PET radioligand.

Nickel-Mediated Trifluoromethylation of Phenol Derivatives by Aryl C-O Bond Activation

The increasing pharmaceutical importance of trifluoromethylarenes has stimulated the development of more efficient trifluoromethylation reactions. Tremendous efforts have focused on copper- and palladium-mediated/catalyzed trifluoromethylation of aryl halides. In contrast, no general method exists for the conversion of widely available inert electrophiles, such as phenol derivatives, into the corresponding trifluoromethylated arenes. Reported herein is a practical nickel-mediated trifluoromethylation of phenol derivatives with readily available trimethyl(trifluoromethyl)silane (TMSCF₃ ). The strategy relies on PMe₃ -promoted oxidative addition and transmetalation, and CCl₃ CN-induced reductive elimination. The broad utility of this transformation has been demonstrated through the direct incorporation of trifluoromethyl into aromatic and heteroaromatic systems, including biorelevant compounds.

Rapid Syntheses of [11C]Arylvinyltrifluoromethanes through Treatment of (E)-Arylvinyl(phenyl)iodonium Tosylates with [11C]Trifluoromethylcopper(I)

We report a method for labeling arylvinyltrifluoromethanes with carbon-11 (t_1/2 = 20.4 min) as representatives of a new radiolabeled chemotype that has potential for developing radiotracers for biomedical imaging with positron emission tomography. Treatment of (E)-arylvinyl(phenyl)iodonium tosylates (1a-1k) with [¹¹C[CuCF₃ gave the corresponding [¹¹C]arylvinyltrifluoromethanes ([¹¹C]2a-[¹¹C]2k) in high radiochemical yields (90-97%) under rapid (2 min) and mild (60 °C) conditions.

Copper-Mediated Late-stage Radiofluorination: Five Years of Impact on Pre-clinical and Clinical PET Imaging

PURPOSE

Copper-mediated radiofluorination (CMRF) is emerging as the method of choice for the formation of aromatic C-18F bonds. This minireview examines proof-of-concept, pre-clinical, and in-human imaging studies of new and established imaging agents containing aromatic C-18F bonds synthesized with CMRF. An exhaustive discussion of CMRF methods is not provided, although key developments that have enabled or improved upon the syntheses of fluorine-18 imaging agents are discussed.

METHODS

A comprehensive literature search from April 2014 onwards of the Web of Science and PubMed library databases was performed to find reports that utilize CMRF for the synthesis of fluorine-18 radiopharmaceuticals, and these represent the primary body of research discussed in this minireview. Select conference proceedings, previous reports describing alternative methods for the synthesis of imaging agents, and preceding fluorine-19 methodologies have also been included for discussion.

CONCLUSIONS

CMRF has significantly expanded the chemical space that is accessible to fluorine-18 radiolabeling with production methods that can meet the regulatory requirements for use in Nuclear Medicine. Furthermore, it has enabled novel and improved syntheses of radiopharmaceuticals and facilitated subsequent PET imaging studies. The rapid adoption of CMRF will undoubtedly continue to simplify the production of imaging agents and inspire the development of new radiofluorination methodologies.

Electrochemical Synthesis of Biaryls via Oxidative Intramolecular Coupling of Tetra(hetero)arylborates

We report herein versatile, transition metal-free and additive-free (hetero)aryl-aryl coupling reactions promoted by the oxidative electrocoupling of unsymmetrical tetra(hetero)arylborates (TABs) prepared from ligand-exchange reactions on potassium trifluoroarylborates. Exploiting the power of electrochemical oxidations, this method complements the existing organoboron toolbox. We demonstrate the broad scope, scalability, and robustness of this unconventional catalyst-free transformation, leading to functionalized biaryls and ultimately furnishing drug-like small molecules, as well as late stage derivatization of natural compounds. In addition, the observed selectivity of the oxidative coupling reaction is related to the electronic structure of the TABs through quantum-chemical calculations and experimental investigations.

Cu-Catalyzed Generation of Alkyl Radicals from Alkylsilyl Peroxides and Subsequent C(sp3)-C(sp2) Cross-Coupling with Arylboronic Acids

This work describes a novel and practical method for the Cu-catalyzed C(sp³)-C(sp²) cross-coupling of alkylsilyl peroxides with arylboronic acids. The reductive cleavage of the O-O bond of alkylsilyl peroxides and the desired cross-coupling reactions to afford alkyl-substituted aromatic rings proceed smoothly at room temperature promoted by simple Cu-based catalysts and do not require activation by visible light. The results of mechanistic investigations support a radical-mediated C(sp³)-C(sp²) bond formation via β-scission of the alkoxy radicals generated from the alkylsilyl peroxides.

A Gas Phase Route to [18F]fluoroform with Limited Molar Activity Dilution

Positron emission tomography (PET) is an important imaging modality for biomedical research and drug development. PET requires biochemically selective radiotracers to realize full potential. Fluorine-18 (t1/2 = 109.8 min) is a major radionuclide for labeling such radiotracers but is only readily available in high activities from cyclotrons as [18F]fluoride ion. [18F]fluoroform has emerged for labeling tracers in trifluoromethyl groups. Prior methods of [18F]fluoroform synthesis used difluoro precursors in solution and led to high dilution with carrier and low molar activity (Am). We explored a new approach for the synthesis of [18F]fluoroform based on the radiosynthesis of [18F]fluoromethane from [18F]fluoride ion and then cobaltIII fluoride mediated gas phase fluorination. We estimate that carrier dilution in this process is limited to about 3-fold and find that moderate to high Am values can be achieved. We show that [18F]fluoroform so produced is highly versatile for rapidly and efficiently labeling various chemotypes that carry trifluoromethyl groups, thereby expanding prospects for developing new PET radiotracers.

Late-Stage 18 F-Difluoromethyl Labeling of N-Heteroaromatics with High Molar Activity for PET Imaging

Despite a growing interest in CHF2 in medicinal chemistry, there is a lack of efficient methods for the insertion of CHF18 F into druglike compounds. Herein described is a photoredox flow reaction for 18 F-difluoromethylation of N-heteroaromatics that are widely used in medicinal chemistry. Following the two-step synthesis for a new 18 F-difluoromethylation reagent, the photoredox reaction is completed within two minutes and proceeds by C-H activation, circumventing the need for pre-functionalization of the substrate. The method is operationally simple and affords straightforward access to radiolabeled N-heteroaromatics with high molar activity suitable for biological in vivo studies and clinical application.

Design and synthesis of enantiopure 18 F-labelled [18 F]trifluoromethyltryptophan from 2-halotryptophan derivatives via copper(I)-mediated [18 F]trifluoromethylation and evaluation of its in vitro characterization for the serotonergic system imaging

We synthesized [¹⁸ F]trifluoromethyl-l-tryptophan ([¹⁸ F]CF₃ -l-Trp) using Cu(I)-mediated [¹⁸ F]trifluoromethylation to image serotonergic system. Radiochemical yield was 6 ± 1.5% (n = 9), and radiochemical purity was over 99%. The molar activity was 0.44 to 0.76 GBq/μmol. [¹⁸ F]CF₃ -l-Trp was stable for up to 6 hours in mouse and human sera at 37°C. Protein-binding was 0.26 ± 0.03% and 0.34 ± 0.02% in human and mouse serum at 60 minutes, respectively. In conclusion, enantiopure [¹⁸ F]CF₃ -l-Trp was synthesized as a feasible imaging agent for the serotonergic system.

Synthesis, 3 H-labelling and in vitro evaluation of a substituted dipiperidine alcohol as a potential ligand for chemokine receptor 2

The immune system is implicated in the pathology of neurodegenerative disorders. The C‐C chemokine receptor 2 (CCR2) is one of the key targets involved in the activation of the immune system. A suitable ligand for CCR2 could be a useful tool to study immune activation in central nervous system (CNS) disorders. Herein, we describe the synthesis, tritium radiolabelling, and preliminary in vitro evaluation in post‐mortem human brain tissue of a known potent small molecule antagonist for CCR2. The preparation of a tritium‐labelled analogue for the autoradiography (ARG) study gave rise to an intriguing and unexpected side reaction profile through a novel amination of ethanol and methanol in the presence of tritium. After successful preparation of the tritiated radioligand, in vitro ARG measurements on human brain sections revealed nonspecific binding properties of the selected antagonist in the CNS.

Synthesis of 18F-difluoromethylarenes using aryl boronic acids, ethyl bromofluoroacetate and [18F]fluoride

Herein, we report the radiosynthesis of 18F-difluoromethylarenes via the assembly of three components, a boron reagent, ethyl bromofluoroacetate, and cyclotron-produced non-carrier added [18F]fluoride. The two key steps are a copper-catalysed cross-coupling reaction, and a Mn-mediated 18F-fluorodecarboxylation.

Chemistry for Positron Emission Tomography: Recent Advances in 11 C-, 18 F-, 13 N-, and 15 O-Labeling Reactions

Positron emission tomography (PET) is a molecular imaging technology that provides quantitative information about function and metabolism in biological processes in vivo for disease diagnosis and therapy assessment. The broad application and rapid advances of PET has led to an increased demand for new radiochemical methods to synthesize highly specific molecules bearing positron-emitting radionuclides. This Review provides an overview of commonly used labeling reactions through examples of clinically relevant PET tracers and highlights the most recent developments and breakthroughs over the past decade, with a focus on 11 C, 18 F, 13 N, and 15 O.

Synthesis and Reactivity of α-Cumyl Bromodifluoromethanesulfenate: Application to the Radiosynthesis of [18 F]ArylSCF3

A highly reactive electrophilic bromodifluoromethylthiolating reagent, α-cumyl bromodifluoro-methanesulfenate 1, was prepared to allow for direct bromodifluoromethylthiolation of aryl boron reagents. This coupling reaction takes place under copper catalysis, and affords a large range of bromodifluoromethylthiolated arenes. These compounds are amenable to various transformations including halogen exchange with [18 F]KF/K222  , a process giving access to [18 F]arylSCF3 in two steps from the corresponding aryl boronic pinacol esters.

Unexpected Hydrated Electron Source for Preparative Visible-Light Driven Photoredox Catalysis

The hydrated electron is experiencing a renaissance as a superreductant in lab-scale reductions driven by light, both for the degradation of recalcitrant pollutants and for challenging chemical reactions. However, examples for its sustainable generation under mild conditions are scarce. By combining a water-soluble Ir catalyst with unique photochemical properties and an inexpensive diode laser as light source, we produce hydrated electrons through a two-photon mechanism previously thought to be unimportant for laboratory applications. Adding cheap sacrificial donors turns our new hydrated electron source into a catalytic cycle operating in pure water over a wide pH range. Not only is that catalytic system capable of detoxifying a chlorinated model compound with turnover numbers of up to 200, but it can also be employed for two novel hydrated electron reactions, namely, the decomposition of quaternary ammonium compounds and the conversion of trifluoromethyl to difluoromethyl groups.

Synthesis and Derivatization of 1,1-[18 F]Difluorinated Alkenes

A general method for the synthesis of 1,1-[¹⁸ F]difluorinated alkenes from [¹⁸ F]fluoride is reported. This transformation is highly regioselective giving the desired ¹⁸ F-fluoroalkenes with radiochemical purities of up to 77 % within 20 minutes and a molar activity (A_m ) of 1 GBq μmol^-1 . The transformations are operationally simple to perform and were readily translated onto a commercial automated synthesis unit. The resultant 1,1-[¹⁸ F]difluorinated alkene motif is prevalent in numerous drug molecules, and this is the first general method to synthesize this motif with fluorine-18. ¹⁸ F-fluorinated alkenes are excellent building blocks and participate in a number of post-labeling transformations to access a range of ¹⁸ F-perfluorinated functional groups that have never before been radiolabeled with non-carrier-added [¹⁸ F]fluoride. This method considerably expands the range of ¹⁸ F-motifs accessible to radiochemists.

Trifluoromethylation of Electron-Rich Alkenyl Iodides with Fluoroform-Derived "Ligandless" CuCF3

We herein present a flexible approach for the incorporation of CF₃ units into a predefined site of electron-rich alkenes that exploits the regiocontrolled introduction of an iodine handle and subsequent trifluoromethylation of the C(sp²)-I bond using fluoroform-derived "ligandless" CuCF₃. The broad substrate scope and functional group tolerance together with the scalability and purity of the resulting products enabled the controlled, late-stage synthesis of single regioisomers of complex CF₃-scaffolds, such as sugars, nucleosides (antivirals), and heterocycles (indoles and chromones), with potential for academic and industrial applications.

Radiosynthesis of the anticancer nucleoside analogue Trifluridine using an automated 18F-trifluoromethylation procedure

Trifluoromethyl groups are widespread in medicinal chemistry, yet there are limited 18F-radiochemistry techniques available for the production of the complementary PET agents. Herein, we report the first radiosynthesis of the anticancer nucleoside analogue trifluridine, using a fully automated, clinically-applicable 18F-trifluoromethylation procedure. [18F]Trifluridine was obtained after two synthetic steps in <2 hours. The isolated radiochemical yield was 3% ± 0.44 (n = 5), with a radiochemical purity >99%, and a molar activity of 0.4 GBq μmol-1 ± 0.05. Biodistribution and PET-imaging data using HCT116 tumour-bearing mice showed a 2.5 %ID g-1 tumour uptake of [18F]trifluridine at 60 minutes post-injection, with bone uptake becoming a prominent feature thereafter. In vivo metabolite analysis of selected tissues revealed the presence of the original radiolabelled nucleoside analogue, together with deglycosylated and phosphorylated [18F]trifluridine as the main metabolites. Our findings suggest a potential role for [18F]trifluridine as a PET radiotracer for elucidation of drug mechanism of action.

A catalytic fluoride-rebound mechanism for C(sp3)-CF3 bond formation

The biological properties of trifluoromethyl compounds have led to their ubiquity in pharmaceuticals, yet their chemical properties have made their preparation a substantial challenge, necessitating innovative chemical solutions. We report the serendipitous discovery of a borane-catalyzed formal C(sp³)-CF₃ reductive elimination from Au(III) that accesses these compounds by a distinct mechanism proceeding via fluoride abstraction, migratory insertion, and C-F reductive elimination to achieve a net C-C bond construction. The parent bis(trifluoromethyl)Au(III) complexes tolerate a surprising breadth of synthetic protocols, enabling the synthesis of complex organic derivatives without cleavage of the Au-C bond. This feature, combined with the "fluoride-rebound" mechanism, was translated into a protocol for the synthesis of ¹⁸F-radiolabeled aliphatic CF₃-containing compounds, enabling the preparation of potential tracers for use in positron emission tomography.

Boron reagents for divergent radiochemistry

This review discusses boron reagents as precursors for divergent radiolabelling with a focus on carbon-11, fluorine-18 and iodine-123, -125, -131.