Ascertaining the suitability of aryl sulfonyl fluorides for [18F]radiochemistry applications: a systematic investigation using microfluidics

Microfluidic synthesis of radiotracers: recent developments and commercialization prospects

Positron emission tomography (PET) is a powerful diagnostic tool that holds incredible potential for clinicians to track a wide variety of biological processes using specialized radiotracers. Currently, however, a single radiotracer accounts for over 95% of procedures, largely due to the cost of radiotracer synthesis. Microfluidic platforms provide a solution to this problem by enabling a dose-on-demand pipeline in which a single benchtop platform would synthesize a wide array of radiotracers. In this review, we will explore the field of microfluidic production of radiotracers from early research to current development. Furthermore, the benefits and drawbacks of different microfluidic reactor designs will be analyzed. Lastly, we will discuss the various engineering considerations that must be addressed to create a fully developed, commercially effective platform that can usher the field from research and development to commercialization.

Recent Advances of S-18F Radiochemistry for Positron Emission Tomography

The click chemistry of sulfur(VI) fluoride exchange (SuFEx) has facilitated the widespread application of sulfur-fluoride compounds such as sulfonyl fluorides, fluorosulfates, and sulfamoyl fluorides in various fields, especially in the development of 18F ligands for PET (positron emission tomography) imaging. In recent years, the prominent progress of sulfur-[18F]fluoride compounds has been achieved through the combination of 18F and sulfur-fluoride chemistry. These compounds serve as potential 18F tracers, 18F synthons, and reagents for 18F-fluorination, thereby complementing the range of 18F ligands, typically C-18F structures, used in PET studies. This review aims to provide an overview of S-18F labeling reactions through examples of relevant 18F compounds and highlight the advancements and breakthroughs achieved in the past decade.

Automated sulfur-[18F]fluoride exchange radiolabelling of a prostate specific membrane antigen (PSMA) targeted ligand using the GE FASTlab™ cassette-based platform

Sulfur-[18F]fluoride exchange radiochemistry is a rapid and convenient method for incorporating fluorine-18 into biologically active molecules. We report a fully automated radiolabelling procedure for the synthesis of a [18F]SO3F-bearing prostate specific membrane antigen (PSMA) targeted ligand ([18F]5) using the GE FASTLab™ cassette-based platform in a 25.0 ± 2.6% radiochemical yield (decay corrected). Uptake in vitro and in vivo correlated with PSMA expression, and the radioligand exhibited favourable biodistribution and pharmacokinetic profiles.

Radiosynthesis of 18F-Labeled Arenesulfonyl Fluorides through Two-Bond Construction with [18F]Fluoride

A novel ¹⁸F labeling strategy was developed to directly construct aryl-SO₂-¹⁸F from arenediazonium tosylates with a SO₂ source and [¹⁸F]fluoride. This approach is compatible with a wide range of substrates and enabled the production of ¹⁸F-labeled drug-like derivatives through late-stage ¹⁸F fluorination, representing a significant advance in the radiosynthesis of ¹⁸F-labeled arenesulfonyl fluorides. A reactive ¹⁸F labeling synthon, bearing a maleimide-based prosthetic group, allowed for the generation of ¹⁸F-labeled temperature-sensitive biomolecules containing cysteine residues via maleimide-cysteine chemistry.

An Investigation into the In Vitro Metabolic Stability of Aryl Sulfonyl Fluorides for their Application in Medicinal Chemistry and Radiochemistry

Molecules that feature a sulfonyl fluoride (SO₂F) moiety have been gaining increasing interest due to their unique reactivity and potential applications in synthetic chemistry, medicinal chemistry, and other biological uses. A particular interest is towards ¹⁸F-radiochemistry where sulfonyl fluorides can be used as a method to radiolabel biomolecules or can be used as radiofluoride relay reagents that facilitate radiolabeling of other molecules. The low metabolic stability of sulfonyl fluoride S-F bonds, however, presents an issue and limits the applicability of sulfonyl fluorides. The aim of this work was to increase understanding of what features contribute to the metabolic instability of the S-F bond in model aryl sulfonyl fluorides and identify approaches to increasing sulfonyl fluoride stability for ¹⁸F-radiochemistry and other medicinal, synthetic chemistry and biological applications. To undertake this, 14 model aryl sulfonyl fluorides compounds with varying functional groups and substitution patterns were investigated, and their stabilities were examined in various media, including phosphate-buffered saline and rat serum as a model for biological conditions. The results indicate that both electronic and steric factors affect the stability of the S-F bond, with the 2,4,6-trisubstituted model aryl sulfonyl fluorides examined displaying the highest in vitro metabolic stability.

Selective Fluorosulfonylation of Thianthrenium Salts Enabled by Electrochemistry

A practical electrochemically driven method for fluorosulfonylation of both aryl and alkyl thianthrenium salts has been disclosed. The strategy does not need external redox reagents or metal catalysts. In combination with C-H thianthrenation of aromatics, this method provides a new tool for the site-selective fluorosulfonylation of drugs.

Isotope Exchange-Based 18F-Labeling Methods

¹⁸F-Labeling methods for the preparation of ¹⁸F-labeled molecular probes can be classified into electrophilic fluorination, nucleophilic fluorination, metal-F coordination, and ¹⁸F/¹⁹F isotope exchange. Isotope exchange-based ¹⁸F-labeling methods demonstrate mild conditions featuring water resistance and facile high-performance liquid chromatography-free purification in direct ¹⁸F-labeling of substrates. This paper systematically reviews isotope exchange-based ¹⁸F-labeling methods sorted by the adjacent atom bonding with F, i.e., carbon and noncarbon atoms (Si, B, P, S, Ga, Fe, etc.). The respective isotope exchange mechanism, radiolabeling condition, radiochemical yield, molar activity, and stability of the ¹⁸F-product are mainly discussed for each isotope exchange-based ¹⁸F-labeling method as well as the cutting-edge application of the corresponding ¹⁸F-labeled molecular probes.

Synthesis and 18F Labeling of Alkenyl Sulfonyl Fluorides via an Unconventional Elimination Pathway

A successful Cu-catalyzed addition of both Cl and SO₂OCF₂H groups into alkenes allows us to discover the unusual reactivity of the SO₂OCF₂H group. As opposed to common sulfonic esters (RSO₂-O-R'), in which the R' group is highly electrophilic, the SO₂ moiety demonstrates higher electrophilicity in RSO₂-OCF₂H. The unexpected reactivity is further developed not only as a synthetic tool for well-functionalized alkenyl sulfonyl fluorides but also for the first ¹⁸F labeling of alkenyl sulfonyl fluorides.

Sulfonyl fluorides as targets and substrates in the development of new synthetic methods

The advent of sulfur(VI)-fluoride exchange (SuFEx) processes as transformations with click-like reactivity has invigorated research into electrophilic species featuring a sulfur-fluorine bond. Among these, sulfonyl fluorides have emerged as the workhorse functional group, with diverse applications being reported. Sulfonyl fluorides are used as electrophilic warheads by both medicinal chemists and chemical biologists. The balance of reactivity and stability that is so attractive for these applications, particularly the resistance of sulfonyl fluorides to hydrolysis under physiological conditions, has provided opportunities for synthetic chemists. New synthetic approaches that start with sulfur-containing substrates include the activation of sulfonamides using pyrilium salts, the deoxygenation of sulfonic acids, and the electrochemical oxidation of thiols. Employing non-sulfur-containing substrates has led to the development of transition-metal-catalysed processes based on palladium, copper and nickel, as well as the use of SO₂F₂ gas as an electrophilic hub. Selectively manipulating molecules that already contain a sulfonyl fluoride group has also proved to be a popular tactic, with metal-catalysed processes again at the fore. Finally, coaxing sulfonyl fluorides to engage with nucleophiles, when required, and under suitable reaction conditions, has led to new activation methods. This Review provides an overview of the challenges in the efficient synthesis and manipulation of these intriguing functional groups.

Electrochemical Synthesis of Sulfonyl Fluorides from Sulfonyl Hydrazides

The synthesis of sulfonyl fluorides via the reaction of sulfonyl hydrazides and Et3N3HF under electrochemical conditions is reported. Various sulfonyl fluorides were obtained in good yields under a constant current...

Copper-catalyzed three-component reaction of arylhydrazine hydrochloride, DABSO, and NFSI for the synthesis of arenesulfonyl fluorides

This paper reports a convenient copper-catalyzed three-component conversion of arylhydrazine hydrochlorides to arenesulfonyl fluorides in good yields under mild conditions, using 1,4-diazabicyclo [2.2.2]octane bis(sulfur dioxide) (DABSO) as a sulfonyl source and N-fluorobenzenesulfonimide (NFSI) as a fluorine source based on a radical sulfur dioxide insertion and fluorination strategy. Notably, arylhydrazine hydrochloride is used as a safe precursor of aryl radicals.

A general approach to nitrile- and sulfonyl fluoride-substituted cyclopropanes

Both cis and trans relative configurations of functionalized cyano cyclopropane bearing sulfonyl fluoride moiety were accessed by Corey-Chaykovsky cyclopropanation reactions. This protocol used mild conditions, and obtained good yields with excellent functional group compatibility. Further application of this class of compounds in SuFEx reactions and cyano reductions were also successfully achieved in good yields.

Synthetic Routes to Arylsulfonyl Fluorides

The goal of this mini-review is to shed the light on the existing methodologies to access arylsulfonyl fluorides. Today, a plethora of methods making use of a different pool of starting materials and in the presence of catalyst or under catalyst free conditions are disclosed in the literature.

Reaction of [18F]Fluoride at Heteroatoms and Metals for Imaging of Peptides and Proteins by Positron Emission Tomography

The ability to radiolabel proteins with [18F]fluoride enables the use of positron emission tomography (PET) for the early detection, staging and diagnosis of disease. The direct fluorination of native proteins through C-F bond formation is, however, a difficult task. The aqueous environments required by proteins severely hampers fluorination yields while the dry, organic solvents that promote nucleophilic fluorination can denature proteins. To circumvent these issues, indirect fluorination methods making use of prosthetic groups that are first fluorinated and then conjugated to a protein have become commonplace. But, when it comes to the radiofluorination of proteins, these indirect methods are not always suited to the short half-life of the fluorine-18 radionuclide (110 min). This review explores radiofluorination through bond formation with fluoride at boron, metal complexes, silicon, phosphorus and sulfur. The potential for these techniques to be used for the direct, aqueous radiolabeling of proteins with [18F]fluoride is discussed.

Recent Advances in the Electrochemical Synthesis of Organosulfur Compounds

Organosulfur compounds are being widely used in medicinal chemistry, as well as in organic transformations and in synthetic applications. Because of their interest in many areas, the development of sustainable and green synthetic methods to access various organosulfur compounds has a high influence on the chemistry community. Electroorganic synthesis has become a very valuable methodology for the synthesis of organosulfur compounds during the last decade. The use of electrochemical technology offers a green, sustainable and safe alternative to prepare and modify such compounds. This review summarises recent developments in the preparation of organosulfur compounds such as sulfoxides, sulfones, sulfinic esters, sulfonamides, thiosulfonates, sulfonyl fluorides and sulfoximines under electrochemical reaction conditions.

Design, synthesis, and evaluation of positron emission tomography/fluorescence dual imaging probes for targeting facilitated glucose transporter 1 (GLUT1)

Increased energy metabolism followed by enhanced glucose consumption is a hallmark of cancer. Most cancer cells show overexpression of facilitated hexose transporter GLUT1, including breast cancer. GLUT1 is the main transporter for 2-deoxy-2-[¹⁸F]fluoro-d-glucose (2-[¹⁸F]FDG), the gold standard of positron emission tomography (PET) imaging in oncology. The present study's goal was to develop novel glucose-based dual imaging probes for their use in tandem PET and fluorescence (Fl) imaging. A glucosamine scaffold tagged with a fluorophore and an ¹⁸F-label should confer selectivity to GLUT1. Out of five different compounds, 2-deoxy-2-((7-sulfonylfluoro-2,1,3-benzoxadiazol-4-yl)amino)-d-glucose (2-FBDG) possessed favorable fluorescent properties and a similar potency as 2-deoxy-2-((7-nitro-2,1,3-benzoxadiazol-4-yl)amino)-d-glucose (2-NBDG) in competing for GLUT1 transport against 2-[¹⁸F]FDG in breast cancer cells. Radiolabeling with ¹⁸F was achieved through the synthesis of prosthetic group 7-fluoro-2,1,3-benzoxadiazole-4-sulfonyl [¹⁸F]fluoride ([¹⁸F]FBDF) followed by the reaction with glucosamine. The radiotracer was finally analyzed in vivo in a breast cancer xenograft model and compared to 2-[¹⁸F]FDG. Despite favourable in vitro fluorescence imaging properties, 2-[¹⁸F]FBDG was found to lack metabolic stability in vivo, resulting in radiodefluorination. Glucose-based 2-[¹⁸F]FBDG represents a novel dual-probe for GLUT1 imaging using FI and PET with the potential for further structural optimization for improved metabolic stability in vivo.

A Unified Strategy for Arylsulfur(VI) Fluorides from Aryl Halides: Access to Ar-SOF3 Compounds

A convenient protocol to selectively access various arylsulfur(VI) fluorides from commercially available aryl halides in a divergent fashion is presented. Firstly, a novel sulfenylation reaction with the electrophilic N-(chlorothio)phthalimide (Cl-S-Phth) and arylzinc reagents afforded the corresponding Ar-S-Phth compounds. Subsequently, the S(II) atom was selectively oxidized to distinct fluorinated sulfur(VI) compounds under mild conditions. Slight modifications on the oxidation protocol permit the chemoselective installation of 1, 3, or 4 fluorine atoms at the S(VI) center, affording the corresponding Ar-SO2 F, Ar-SOF3 , and Ar-SF4 Cl. Of notice, this strategy enables the effective introduction of the rare and underexplored -SOF3 moiety into various (hetero)aryl groups. Reactivity studies demonstrate that such elusive Ar-SOF3 can be utilized as a linchpin for the synthesis of highly coveted aryl sulfonimidoyl fluorides (Ar-SO(NR)F).

Clickable Transformation of Nitriles (RCN) to Oxazolyl Sulfonyl Fluoride Warheads

The protocol for simple, efficient, and mild synthesis of oxazolyl sulfonyl fluorides was developed through Rh₂(OAc)₄-catalyzed annulation of methyl-2-diazo-2-(fluorosulfonyl)acetate (MDF) or its ethyl ester derivative with nitriles. This practical method provides a general and direct route to a unique class of highly functionalized oxazolyl-decorated sulfonyl fluoride warheads with great potential in medicinal chemistry, chemical biology, and drug discovery.

Synthesis of 18F-Labeled Aryl Fluorosulfates via Nucleophilic Radiofluorination

Sulfuryl fluoride gas is a key reagent for SO₂F transfer. However, conventional SO₂F transfer reactions have limited ¹⁸F-radiochemistry translation, due to the inaccessibility of gaseous [¹⁸F]SO₂F₂. Herein, we report the first SO₂F₂-free synthesis of aryl [¹⁸F]fluorosulfates from both phenolic and isolated aryl imidazylate precursors with cyclotron-produced ¹⁸F^-. The radiochemical yields ranged from moderate to good with excellent functional group tolerance. The reliability of our approach was validated by the automated radiosynthesis of 4-acetamidophenyl [¹⁸F]fluorosulfate.

Light-induced [2 + 2] cycloadditions for the construction of cyclobutane-fused pyridinyl sulfonyl fluorides

A photocatalytic [2 + 2] cycloaddition between pyridones or isoquinolones and ethenesulfonyl fluoride was achieved, generating unique cyclobutane-fused pyridinyl sulfonyl fluoride compounds.

Sulfonyl Fluoride Synthesis through Electrochemical Oxidative Coupling of Thiols and Potassium Fluoride

Sulfonyl fluorides are valuable synthetic motifs for a variety of applications, among which sulfur(VI) fluoride exchange-based “click chemistry” is currently the most prominent. Consequently, the development of novel and efficient synthetic methods to access these functional groups is of great interest. Herein, we report a mild and environmentally benign electrochemical approach to prepare sulfonyl fluorides using thiols or disulfides, as widely available starting materials, in combination with KF, as an inexpensive, abundant and safe fluoride source. No additional oxidants nor additional catalysts are required and, due to mild reaction conditions, the reaction displays a broad substrate scope, including a variety of alkyl, benzyl, aryl and heteroaryl thiols or disulfides.

Formation of an unexpected 3,3-diphenyl-3H-indazole through a facile intramolecular [2 + 3] cycloaddition of the diazo intermediate

The one-pot reaction of 2,6-bis(diphenylmethyl)-4-methoxyaniline with tert-butylnitrite, BTEAC and DABSO in the presence of CuCl2 provided an unexpected 3H-indazole product 8. The structure of the compound was determined by HRMS, IR, NMR and further confirmed by single crystal X-ray crystallography. The compound crystallises in the triclinic P-1 space group, with unit cell parameters a = 9.2107 (4), b = 10.0413 (5), c = 14.4363 (6) Å, α = 78.183 (2), β = 87.625 (2), γ = 71.975 (2)°. The formation of 8 proceeded through a facile intramolecular [2 + 3] cycloaddition of the diazo intermediate 9. The molecules of 8 are organised by edge-face Ar-H···π, face-face π···π, and bifurcated OCH2-H···N interactions. In addition to these, there are Ar-H···H-Ar close contacts, (edge-edge and surrounding inversion centres) arranged as infinite tapes along the a direction.

Facile one-pot synthesis of sulfonyl fluorides from sulfonates or sulfonic acids

A facile cascade process for directly transforming the abundant and inexpensive sulfonates (or sulfonic acids) to the highly valuable sulfonyl fluorides was developed. This new protocol features mild reaction conditions using readily available and easy-to-operate reagents. A diverse set of sulfonyl fluorides was prepared facilitating the enrichment of the sulfonyl fluoride library.

A mild one-pot protocol for directly converting sulfonates or sulfonic acids into sulfonyl fluorides was developed.

Regioselective installation of fluorosulfate (–OSO2F) functionality into aromatic C(sp2)–H bonds for the construction of para-amino-arylfluorosulfates

The synthesis of novel para-amino-arylfluorosulfates was achieved through installing fluorosulfate functionality into aromatic C(sp²)–H bonds mediated by sulfuryl fluoride.

One-pot fluorosulfurylation of Grignard reagents using sulfuryl fluoride

Herein, we report a new method for the one-pot syntheses of sulfonyl fluorides.

Synthesis of Arenesulfonyl Fluorides via Sulfuryl Fluoride Incorporation from Arynes

Transition-metal-free multicomponent reactions involving aryne precursors, secondary amines, and sulfuryl fluoride are reported herein. Zwitterionic intermediates formed from the reaction of arynes with amine nucleophiles can capture SO₂F₂ under mild conditions, offering a novel and practical protocol for the synthesis of 2-dialkyl-, 2-alkylaryl-, or 2-diarylamino-substituted arenesulfonyl fluoride derivatives in good to excellent yields.

18F-AlF Labeled Peptide and Protein Conjugates as Positron Emission Tomography Imaging Pharmaceuticals

The clinical applications of positron emission tomography (PET) imaging pharmaceuticals have increased tremendously over the past several years since the approval of ¹⁸fluorine-fluorodeoxyglucose (¹⁸F-FDG) by the Food and Drug Administration (FDA). Numerous ¹⁸F-labeled target-specific potential imaging pharmaceuticals, based on small and large molecules, have been evaluated in preclinical and clinical settings. ¹⁸F-labeling of organic moieties involves the introduction of the radioisotope by C-¹⁸F bond formation via a nucleophilic or an electrophilic substitution reaction. However, biomolecules, such as peptides, proteins, and oligonucleotides, cannot be radiolabeled via a C-¹⁸F bond formation as these reactions involve harsh conditions, including organic solvents, high temperature, and nonphysiological conditions. Several approaches, including ¹⁸F-labeled prosthetic groups, silicon, boron, and aluminum fluoride acceptor chemistry, and click chemistry have been developed, in the past, for ¹⁸F labeling of biomolecules. Linear and macrocyclic polyaminocarboxylates and their analogs and derivatives form thermodynamically stable and kinetically inert aluminum chelates. Hence, macrocyclic polyaminocarboxylates have been used for conjugation with biomolecules, such as folate, peptides, affibodies, and protein fragments, followed by ¹⁸F-AlF chelation, and evaluation of their targeting abilities in preclinical and clinical environments. The goal of this report is to provide an overview of the ¹⁸F radiochemistry and ¹⁸F-labeling methodologies for small molecules and target-specific biomolecules, a comprehensive review of coordination chemistry of Al³⁺, ¹⁸F-AlF labeling of peptide and protein conjugates, and evaluation of ¹⁸F-labeled biomolecule conjugates as potential imaging pharmaceuticals.

Microfluidic Radiosynthesis of the Muscarinic M2 Imaging Agent [18F]FP-TZTP

3-(4-(3-[18F]Fluoropropylthio)-1,2,5-thiadiazol-3-yl)-1-methyl-1,2,5,6-tetrahydropyridine ([18F]FP-TZTP) is a selective 18F-radiotracer for the muscarinic acetylcholine receptor subtype M2, which can be used to perform positron emission tomography (PET) scans on patients with neurological disorders such as Alzheimer’s disease. [18F]FP-TZTP was produced using continuous-flow microfluidics, a technique that uses reduced amounts of chemical reagents, shorter reaction times and in general, results in higher radiochemical yields compared to currently used techniques. The optimal 18F-radiolabelling conditions consisted of a total flow rate of 40 µL min−1 and 190°C, which produced [18F]FP-TZTP in 26 ± 10 % radiochemical yield with a molar activity of 182 ± 65 GBq µmol−1 and >99 % radiochemical purity.

Rhodium(iii)-catalyzed Oxidative Coupling of N-Methoxybenzamides and Ethenesulfonyl fluoride: a C–H Bond Activation Strategy for the Preparation of 2-Aryl ethenesulfonyl fluorides and Sulfonyl fluoride Substituted γ-Lactams

The synthesis of a new class of 2-aryl ethenesulfonyl fluorides was achieved via a rhodium(iii)-catalyzed sp² C–H bond activation strategy.

A portal to a class of novel sultone-functionalized pyridines via an annulative SuFEx process employing earth abundant nickel catalysts

An efficient Ni²⁺ promoted process for the synthesis of a class of structurally unique heterocycles containing both pyridines and sultones was developed through a SuFEx annulation. This protocol serves as an irreplaceable asset for medicinal chemistry and drug discovery.

Small molecule inhibitors of anthrax edema factor

Anthrax is a highly lethal disease caused by the Gram-(+) bacteria Bacillus anthracis. Edema toxin (ET) is a major contributor to the pathogenesis of disease in humans exposed to B. anthracis. ET is a bipartite toxin composed of two proteins secreted by the vegetative bacteria, edema factor (EF) and protective antigen (PA). Our work towards identifying a small molecule inhibitor of anthrax edema factor is the subject of this letter. First we demonstrate that the small molecule probe 5'-Fluorosulfonylbenzoyl 5'-adenosine (FSBA) reacts irreversibly with EF and blocks enzymatic activity. We then show that the adenosine portion of FSBA can be replaced to provide more drug-like molecules which are up to 1000-fold more potent against EF relative to FSBA, display low cross reactivity when tested against a panel of kinases, and are nanomolar inhibitors of EF in a cell-based assay of cAMP production.

18 F-Labeling of Sensitive Biomolecules for Positron Emission Tomography

Positron emission tomography (PET) imaging study of fluorine-18 labeled biomolecules is an emerging and rapidly growing area for preclinical and clinical research. The present review focuses on recent advances in radiochemical methods for incorporating fluorine-18 into biomolecules via "direct" or "indirect" bioconjugation. Recently developed prosthetic groups and pre-targeting strategies, as well as representative examples in 18 F-labeling of biomolecules in PET imaging research studies are highlighted.

Sulfur - fluorine bond in PET radiochemistry

The importance of the sulfur-fluorine bond is starting to increase in modern medicinal chemistry literature. This is due to a better understanding of the stability and reactivity of this moiety depending on the various oxidation states of sulfur. Furthermore, several commercial reagents used for mild and selective fluorination of organic molecules are based on the known reactivity of S-F groups. In this review, we will show how these examples are translating into the ¹⁸F field, both for use as stable tags in finished radiopharmaceuticals and as mildly reactive fluoride-relay intermediates. Finally, we also discuss current opportunities where examples of non-radioactive S-F applications/chemistry may be translated into future ¹⁸F radiochemistry applications.