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

Palladium-Mediated S-Arylation of Cysteine Residues with 4-[18F]Fluoroiodobenzene ([18F]FIB)

Transition-metal-mediated bioconjugation chemistry has been used extensively to design and synthesize molecular probes to visualize, characterize, and quantify biological processes within intact living organisms at the cellular and subcellular levels. We demonstrate the development and validation of chemoselective [18F]fluoro-arylation chemistry of cysteine residues using Pd-mediated S-arylation chemistry with 4-[18F]fluoroiodobenzene ([18F]FIB) as an aryl electrophile. The novel bioconjugation technique proceeded in excellent radiochemical yields of 73-96% within 15 min under ambient and aqueous reaction mixture conditions, representing a versatile novel tool for decorating peptides and peptidomimetics with short-lived positron emitter 18F. The chemoselective S-arylation of several peptides and peptidomimetics containing multiple reactive functional groups confirmed the versatility and functional group compatibility. The synthesis and radiolabeling of a novel prostate-specific membrane antigen (PSMA) binding radioligand [18F]6 was accomplished using the novel labeling protocol. The validation of radioligand [18F]6 in a preclinical prostate cancer model with PET resulted in favorable accumulation and retention in PSMA-expressing LNCaP tumors. At the same time, a significantly lower salivary gland uptake was observed compared to clinical PSMA radioligand [18F]PSMA-1007. This finding coincides with ongoing discussions about the molecular basis of the off-target accumulation of PSMA radioligands currently used for clinical imaging and therapy of prostate cancer.

Spirocyclic Iodonium Ylides for Fluorine-18 Radiolabeling of Non-Activated Arenes: From Concept to Clinical Research

Positron emission tomography (PET) is a powerful imaging tool for drug discovery, clinical diagnosis, and monitoring of disease progression. Fluorine-18 is the most common radionuclide used for PET, but advances in radiotracer development have been limited by the historical lack of methodologies and precursors amenable to radiolabeling with fluorine-18. Radiolabeling of electron-rich (hetero)aromatic rings remains a long-standing challenge in the production of PET radiopharmaceuticals. In this personal account, we discuss the history of spirocyclic iodonium ylide precursors, from inception to applications in clinical research, for the incorporation of fluorine-18 into complex non-activated (hetero)aromatic rings.

Rapid 18F-labeling via Pd-catalyzed S-arylation in aqueous medium

We report radiolabeling of thiol-containing substrates via Pd-catalyzed S-arylation with 2-[¹⁸F]fluoro-5-iodopyridine, which is readily accessible using the "minimalist" radiofluorination method. The practicality of the procedure was confirmed by preparation of a novel PSMA-specific PET-tracer as well as labeling of glutathione, Aβ oligomer-binding RD2 peptide, bovine serum albumin and PSMA I&S.

Interaction of Ligands for PET with the Dopamine D3 Receptor: In Silico and In Vitro Methods

[¹⁸F]Fallypride and [¹⁸F]Fluortriopride (FTP) are two different PET radiotracers that bind with sub-nanomolar affinity to the dopamine D3 receptor (D₃R). In spite of their similar D₃ affinities, the two PET ligands display very different properties for labeling the D₃R in vivo: [¹⁸F]Fallypride is capable of binding to D₃R under "baseline" conditions, whereas [¹⁸F]FTP requires the depletion of synaptic dopamine in order to image the receptor in vivo. These data suggest that [¹⁸F]Fallypride is able to compete with synaptic dopamine for binding to the D₃R, whereas [¹⁸F]FTP is not. The goal of this study was to conduct a series of docking and molecular dynamic simulation studies to identify differences in the ability of each molecule to interact with the D₃R that could explain these differences with respect to competition with synaptic dopamine. Competition studies measuring the ability of each ligand to compete with dopamine in the β-arrestin assay were also conducted. The results of the in silico studies indicate that FTP has a weaker interaction with the orthosteric binding site of the D₃R versus that of Fallypride. The results of the in silico studies were also consistent with the IC50 values of each compound in the dopamine β-arrestin competition assays. The results of this study indicate that in silico methods may be able to predict the ability of a small molecule to compete with synaptic dopamine for binding to the D₃R.

The first radiosynthesis of 2-amino-5-[18F]fluoropyridines via a "minimalist" radiofluorination/palladium-catalyzed amination sequence from anisyl(2-bromopyridinyl)iodonium triflate

The synthesis of 2-amino-5-[¹⁸F]fluoropyridines was achieved in 8-85% yields by palladium-catalyzed reaction of 2-bromo-5-[¹⁸F]fluoropyridine with piperidine, dimethylamine, butylamine, methylpiperazine, benzylamine, aniline and 3-aminopyridine. 2-Bromo-5-[¹⁸F]fluoropyridine was obtained by radiofluorination of anisyl(2-bromopyridinyl-5)iodonium triflate (88% yield). The radiofluorination step was performed under "minimalist" conditions to guarantee a successful subsequent amination reaction.

Facile 18F labeling of non-activated arenes via a spirocyclic iodonium(III) ylide method and its application in the synthesis of the mGluR5 PET radiopharmaceutical [18F]FPEB

Non-activated (electron-rich and/or sterically hindered) arenes are prevalent chemical scaffolds in pharmaceuticals and positron emission tomography (PET) diagnostics. Despite substantial efforts to develop a general method to introduce ¹⁸F into these moieties for molecular imaging by PET, there is an urgent and unmet need for novel radiofluorination strategies that result in sufficiently labeled tracers to enable human imaging. Herein, we describe an efficient method that relies on spirocyclic iodonium ylide (SCIDY) precursors for one-step and regioselective radiofluorination, as well as proof-of-concept translation to the radiosynthesis of a clinically useful PET tracer, 3-[¹⁸F]fluoro-5-[(pyridin-3-yl)ethynyl] benzonitrile ([¹⁸F]FPEB). The protocol begins with the preparation of a SCIDY precursor for FPEB, followed by radiosynthesis of [¹⁸F]FPEB, by either manual operation or an automated synthesis module. [¹⁸F]FPEB can be obtained in quantities >7.4 GBq (200 mCi), ready for injection (20 ± 5%, non-decay corrected), and has excellent chemical and radiochemical purity (>98%) as well as high molar activity (666 ± 51.8 GBq/μmol; 18 ± 1.4 Ci/μmol). The total time for the synthesis and purification of the corresponding labeling SCIDY precursor is 10 h. The subsequent radionuclide production, experimental setup, ¹⁸F labeling, and formulation of a product that is ready for injection require 2 h.

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.

Hypervalent aryliodine compounds as precursors for radiofluorination

Over the last 2 decades or so, hypervalent iodine compounds, such as diaryliodonium salts and aryliodonium ylides, have emerged as useful precursors for labeling homoarenes and heteroarenes with no-carrier-added cyclotron-produced [¹⁸ F]fluoride ion (t_1/2  = 109.8 min). They permit rapid and effective radiofluorination at electron-rich as well as electron-deficient aryl rings, and often with unrestricted choice of ring position. Consequently, hypervalent aryliodine compounds have found special utility as precursors to various small-molecule ¹⁸ F-labeling synthons and to many radiotracers for biomedical imaging with positron emission tomography. This review summarizes this advance in radiofluorination chemistry, with emphasis on precursor synthesis, radiofluorination mechanism, method scope, and method application.

From imine to amine: an unexpected left turn. Cis-β iron(ii) PNNP' precatalysts for the asymmetric transfer hydrogenation of acetophenone

A slight change in the iron catalyst structure (amine arm with PEt₂ to imine arm with PPh₂) results in a complete reversal of the enantioselectivity toward ketone reduction.

A novel PNN ligand bearing an orthophenylene group and a primary amine was synthesized with the aid of a palladium-catalyzed amination and reacted with phosphonium dimers [–PR₂CH₂CH(OH)–]₂[Br]₂ R = Et, iPr, Cy, Ph, xylyl, and o-Tol, and [Fe(OH₂)₆]²⁺ to produce a new series of cis-β iron(ii) PNNP′ precatalysts cis-β-[Fe(Br)(CO)(PNNP′)]BPh₄ as a pair of diastereomers. The more stable orthophenylene amido group was chosen to imitate and replace the enamido moiety of a highly active iron precatalyst for the asymmetric transfer hydrogenation (ATH) of ketones in an attempt to prevent its deactivation caused by reduction of the enamido group. This objective was partially achieved using the complex with a PEt₂ group which catalyzed the transfer hydrogenation in isopropanol of 150 000 equivalents of acetophenone to racemic 1-phenylethanol. With a low acetophenone to catalyst ratio of 500 to 1, the catalytic activity was moderate and the enantiomeric excess (ee) of the product 1-phenylethanol ranged surprisingly from 94% (R) to 95% (S) depending on the nature of PR₂ and whether the precatalyst contained an imine or amine donor. The amine precatalyst with a PEt₂-group produced a more stable hydride species when activated, allowing the reaction mixture to be heated to 75 °C to obtain a TON of 8821 for acetophenone while retaining the high ee of 95% (S). The activation pathway in basic isopropanol (iPrOH) was studied for three precatalysts to elucidate that the cis-β precatalysts rearrange to form trans hydride complexes. The study suggests that the enantioselectivity of these complexes is determined by from which side of the PNNP′ plane the hydride transfer occurs.

Fluorine-18 labelled building blocks for PET tracer synthesis

This review presents a comprehensive overview of the synthesis and application of fluorine-18 labelled building blocks since 2010.

Synthesis of a Potent Aminopyridine-Based nNOS-Inhibitor by Two Recent No-Carrier-Added (18)F-Labelling Methods

Nitric oxide (NO), an important multifunctional signaling molecule, is produced by three isoforms of NO-synthase (NOS) and has been associated with neurodegenerative disorders. Selective inhibitors of the subtypes iNOS (inducible) or nNOS (neuronal) are of great interest for decoding neurodestructive key factors, and ¹⁸F-labelled analogues would allow investigating the NOS-function by molecular imaging with positron emission tomography. Especially, the highly selective nNOS inhibitor 6-((3-((3-fluorophenethylamino)methyl)phenoxy)methyl)-4-methylpyridin-2-amine (10) lends itself as suitable compound to be ¹⁸F-labelled in no-carrier-added (n.c.a.) form. For preparation of the ¹⁸F-labelled nNOS-Inhibitor [¹⁸F]10 a “build-up” radiosynthesis was developed based on a corresponding iodonium ylide as labelling precursor. The such activated phenethyl group of the compound was efficiently and regioselectively labelled with n.c.a. [¹⁸F]fluoride in 79% radiochemical yield (RCY). After conversion by reductive amination and microwave assisted displacement of the protecting groups, the desired nNOS-inhibitor was obtained in about 15% total RCY. Alternatively, for a simplified “late-stage” ¹⁸F-labelling procedure a corresponding boronic ester precursor was synthesized and successfully used in a newer, copper(II) mediated n.c.a. ¹⁸F-fluoro-deboroniation reaction, achieving the same total RCY. Thus, both methods proved comparatively suited to provide the highly selective NOS-inhibitor [¹⁸F]10 as probe for preclinical in vivo studies.

Mechanistic Studies and Radiofluorination of Structurally Diverse Pharmaceuticals with Spirocyclic Iodonium(III) Ylides

Theoretical studies provide insight into radiofluorination of non-activated electron-rich and sterically hindered ¹⁸F-arenes using a new class of adamantyl-based spirocyclic iodonium(iii) ylide precursors.

Synthesis of non-activated electron-rich and sterically hindered ¹⁸F-arenes remains a major challenge due to limitations of existing radiofluorination methodologies. Herein, we report on our mechanistic investigations of spirocyclic iodonium(iii) ylide precursors for arene radiofluorination, including their reactivity, selectivity, and stability with no-carrier-added [¹⁸F]fluoride. Benchmark calculations at the G2[ECP] level indicate that pseudorotation and reductive elimination at iodine(iii) can be modeled well by appropriately selected dispersion-corrected density functional methods. Modeling of the reaction pathways show that fluoride–iodonium(iii) adduct intermediates are strongly activated and highly regioselective for reductive elimination of the desired [¹⁸F]fluoroarenes (difference in barriers, ΔΔG^‡ > 25 kcal mol^–1). The advantage of spirocyclic auxiliaries is further supported by NMR spectroscopy studies, which bolster evidence for underlying decomposition processes which can be overcome for radiofluorination of iodonium(iii) precursors. Using a novel adamantyl auxiliary, sterically hindered iodonium ylides have been developed to enable highly efficient radiofluorination of electron-rich arenes, including fragments of pharmaceutically relevant nitrogen-containing heterocycles and tertiary amines. Furthermore, this methodology has been applied for the syntheses of the radiopharmaceuticals 6-[¹⁸F]fluoro-meta-tyrosine ([¹⁸F]FMT, 11 ± 1% isolated radiochemical yield, non-decay-corrected (RCY, n.d.c.), n = 3), and meta-[¹⁸F]fluorobenzylguanidine ([¹⁸F]mFBG, 14 ± 1% isolated RCY, n.d.c., n = 3) which cannot be directly radiolabeled using conventional nucleophilic aromatic substitution with [¹⁸F]fluoride.

Small Molecule Radiopharmaceuticals - A Review of Current Approaches

Radiopharmaceuticals are an integral component of nuclear medicine and are widely applied in diagnostics and therapy. Though widely applied, the development of an “ideal” radiopharmaceutical can be challenging. Issues such as specificity, selectivity, sensitivity, and feasible chemistry challenge the design and synthesis of radiopharmaceuticals. Over time, strategies to address the issues have evolved by making use of new technological advances in the fields of biology and chemistry. This review presents the application of few advances in design and synthesis of radiopharmaceuticals. The topics covered are bivalent ligand approach and lipidization as part of design modifications for enhanced selectivity and sensitivity and novel synthetic strategies for optimized chemistry and radiolabeling of radiopharmaceuticals.

(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.

Highly electron-poor Buchwald-type ligand: application for Pd-catalysed direct arylation of thiophene derivatives and theoretical consideration of the secondary Pd0–arene interaction

Highly electron-poor SPhos ligands stabilised the Pd complex by secondary Pd⁰–arene interaction.