[11C]Cyanation of arylboronic acids in aqueous solutions

Recent investigations into deborylative (thio-/seleno-) cyanation of aryl boronic acids

In this review, we intend to summarize the most important discoveries in the deborylative (thio-/seleno-) cyanation of aryl boronic acids from 2006 to the end of 2023. Thus, the review is divided into three parts. The first section focuses exclusively on cyanation of aryl boronic acids into aryl nitriles. The second section covers the available literature on the synthesis of aryl thiocyanates through thiocyanation of respective aryl boronic acids. The third will discuss selenocyanation of aryl boronic acids into aryl selenocyanates.

Effect of Water on Direct Radioiodination of Small Molecules/Peptides Using Copper-Mediated Iododeboronation in Water-Alcohol Solvent

Direct radioiodination of peptides using copper-mediated iododeboronation is a promising radiosynthetic method for solving issues of classical direct radiolabeling, such as toxicity of the organotin precursor (iododestannylation) or formation of radio byproducts (by electrophilic iodination of a tyrosine residue). However, the parameters for optimizing the reaction conditions for various peptides are not completely understood. In particular, considering peptide solubility, the effects of water-containing solvents on labeling efficiency should be thoroughly investigated. Herein, we describe the effect of water on copper-mediated radioiododeboronation and the key factors for ensuring the successful radiolabeling of small molecules and peptides in water-organic solvents. ¹²⁵I-labeled substrates containing peptides ([¹²⁵I]m/p-IBTA) were obtained with high radiochemical conversions (RCCs: >95%) using an alcohol solvent, and a decrease in these RCCs was observed with increasing water content in the methanol solvent. Additionally, when using water-methanol solvents, a difference in RCC due to the substituent effect was also observed. However, the RCCs can be improved without the use of other additives by adjusting the copper catalyst and time of the labeling reaction or by utilizing substituent effects. This study contributes to the improvement of the design of boronic peptide precursors and radiolabeling protocols using copper-mediated iododeboronation.

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.

Room-Temperature Copper-Mediated Radiocyanation of Aryldiazonium Salts and Aryl Iodides via Aryl Radical Intermediates

Radiocyanation is an attractive strategy for incorporating carbon-11 into radiotracer targets, particularly given the broad scope of acyl moieties accessible from nitriles. Most existing methods for aromatic radiocyanation require elevated temperatures (Cu-mediated reactions of aryl halides or organometallics) or involve expensive and toxic palladium complexes (Pd-mediated reactions of aryl halides). The current report discloses a complementary approach that leverages the capture of aryl radical intermediates by a Cu-11CN complex to achieve rapid and mild (5 min, room temperature) radiocyanation. In a first example, aryl radicals are generated via the reaction of a CuI mediator with an aryldiazonium salt (a Sandmeyer-type reaction) followed by radiocyanation with Cu-11CN. In a second example, aryl radicals are formed from aryl iodides via visible-light photocatalysis and then captured by a Cu-11CN species to achieve aryl-11CN coupling. This approach provides access to radiocyanated products that are challenging to access using other methods (e.g., ortho-disubstituted aryl nitriles).

Evaluation and improvement of CuI-mediated 11 C-cyanation

CuI-mediated ¹¹ C-cyanation was evaluated by synthesizing [¹¹ C]perampanel ([¹¹ C]5) as a model compound and compared with previous reports. To a DMF solution with 5'-(2-bromophenyl)-1'-phenyl-[2,3'-bipyridin]-6'(1'H)-one (4) and CuI, [¹¹ C]NH₄ CN in a stream of ammonia/nitrogen (5:95, v/v) gas was bubbled. Subsequently, the reaction mixture was heated at 180°C for 5 min. After HPLC purification, [¹¹ C]5 was obtained in 7.2 ± 1.0% (n = 4) non-decay corrected radiochemical yield with >99% radiochemical purity and a molar activity of 98 ± 28 GBq/μmol. In vivo evaluations of [¹¹ C]5 were performed using small animals. PET scans to check the kinetics of [¹¹ C]5 in the whole body of mice suggested that [¹¹ C]5 spreads rapidly into the brain, heart, and lungs and then accumulates in the small intestine. To evaluate the performance of CuI-mediated ¹¹ C-cyanation reaction, bromobenzene (6a) was selected as the model compound; however, it failed. Therefore, optimization of the reaction conditions has been performed, and consequently, the addition of K₂ CO₃ and prolonging the reaction time improved the radiochemical yield about double. With this improved method, CuI-mediated ¹¹ C-cyanation of various (hetero)aromatic bromides was performed to exhibit the tolerance of most functional groups and to provide ¹¹ C-cyanated products in good to moderate radiochemical yields.

11C, 12C and 13C-Cyanation of Electron-Rich Arenes via Organic Photoredox Catalysis

As a non-invasive imaging technology, positron emission tomography (PET) plays a crucial role in personalized medicine, including early diagnosis, patient screening, and treatment monitoring. The advancement of PET research depends on the discovery of new PET agents, which requires the development of simple and efficient radiolabeling methods in many cases. As bioisosteres for halogen and carbonyl moieties, nitriles are important functional groups in pharmaceutical and agrochemical compounds. Here, we disclose a mild organophotoredox-catalyzed method for efficient cyanation of a broad spectrum of electron-rich arenes, including abundant and readily available veratroles and pyrogallol trimethyl ethers. Notably, the transformations not only are compatible with various affordable 12C and 13C-cyanide sources, but also could be applied to carbon-11 synthons to incorporate [11C]nitriles into arenes. The aryl [11C]nitriles can be further derivatized to [11C]carboxylic acids, [11C]amides, and [11C]alkyl amines. The newly developed reaction can serve as a powerful tool for generating new PET agents.

Oxidative Radical Transnitrilation of Arylboronic Acids with Trityl Isocyanide

We report a radical transnitrilation of arylboronic acids with trityl isocyanide in the presence of manganese(III) acetate. Many functional groups can be tolerated in this transformation, and a special positive effect of benzoic acid in this reaction has been observed.

Direct C-H Radiocyanation of Arenes via Organic Photoredox Catalysis

Innovative labeling methods to incorporate the short-lived positron emitter carbon-11(¹¹C) into bioactive molecules are attractive for positron emission tomography (PET) tracer discovery. Herein, we report a direct C-H radiocyanation method that incorporates [¹¹C]cyanide (¹¹CN^-) to a series of functional electron-rich arenes via photoredox catalysis. This photoredox-mediated radiocyanation can proceed in an aerobic environment and is not moisture sensitive, which allows for ease of reaction setup and for scalable synthesis of ¹¹C-aryl nitriles from readily available precursors.

Copper-Mediated Radiocyanation of Unprotected Amino Acids and Peptides

This report describes a copper-mediated radiocyanation of aryl halides that is applicable to complex molecules. This transformation tolerates an exceptionally wide range of functional groups, including unprotected amino acids. As such, it enables the site-specific introduction of [¹¹C]CN into peptides at an iodophenylalanine residue. The use of a diamine-ligated copper(I) mediator is crucial for achieving high radiochemical yield under relatively mild conditions, thus limiting racemization and competing side reactions of other amino acid side chains. The reaction has been scaled and automated to deliver radiolabeled peptides, including analogues of adrenocorticotropic hormone 1-27 (ACTH) and nociceptin (NOP). For instance, this Cu-mediated radiocyanation was leveraged to prepare >40 mCi of [¹¹C]cyano-NOP to evaluate biodistribution in a primate using positron emission tomography. This investigation provides preliminary evidence that nociceptin crosses the blood-brain barrier and shows uptake across all brain regions (SUV > 1 at 60 min post injection), consistent with the known distribution of NOP receptors in the rhesus brain.

Radiolabeling with [11C]HCN for Positron emission tomography

Hydrogen cyanide (HCN) is a versatile synthon for generating carbon‑carbon and carbon-heteroatom bonds. Unlike other one-carbon synthons (i.e., CO, CO2), HCN can function as a nucleophile (as in potassium cyanide, KCN) and an electrophile (as in cyanogen bromide, (CN)Br). The incorporation of the CN motif into organic molecules generates nitriles, hydantoins and (thio)cyanates, which can be converted to carboxylic acids, aldehydes, amides and amines. Such versatile chemistry is particularly attractive in PET radiochemistry where diverse bioactive small molecules incorporating carbon-11 in different positions need to be produced. The first examples of making [11C]HCN for radiolabeling date back to the 1960s. During the ensuing decades, [11C]cyanide labeling was popular for producing biologically important molecules including 11C-labeled α-amino acids, sugars and neurotransmitters. [11C]cyanation is now reemerging in many PET centers due to its versatility for making novel tracers. Here, we summarize the chemistry of [11C]HCN, review the methods to make [11C]HCN past and present, describe methods for labeling different types of molecules with [11C]HCN, and provide an overview of the reactions available to convert nitriles into other functional groups. Finally, we discuss some of the challenges and opportunities in [11C]HCN labeling such as developing more robust methods to produce [11C]HCN and developing rapid and selective methods to convert nitriles into other functional groups in complex molecules.

Copper-mediated radioiodination reaction through aryl boronic acid or ester precursor and its application to direct radiolabeling of a cyclic peptide

A copper-mediated radioiodination using aryl boronic precursors is attracting attention as a solution to oxidative iododestannylation and nickel-mediated radioiodination drawbacks. The copper-mediated radiolabeling method allows radioiodination at room temperature with stable aryl boronic precursors without preparing complex starting materials or reagents and can be performed in a reaction vessel exposed to air. This method has good potential in radiochemistry; however, studies on the scope of copper-mediated radioiodination through boronic precursors are insufficient. In particular, few reports have demonstrated the effect of protecting groups on radiolabeling efficiency. Therefore, the effect of the protecting group of aryl boronic acids on the copper-mediated radioiodination was investigated. In addition, this method, which does not require heating, is expected to be useful for direct radiolabeling of peptides. Thus, we attempted direct radioiodination of c(RGDyk) as an example. The resulting radioiodination method was well tolerated in various substrates and was unaffected by the pinacol ester-type protecting group. Also, c(RGDyk) was labeled with 125 I via copper-mediated radioiodination using an aryl boronic acid precursor. The reaction time and yield were improved, compared with the indirect method. Furthermore, the large difference in polarity between the boronic acid precursor and the radiolabeled compound facilitated purification.

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.

Copper(I)-Mediated 11C-Carboxylation of (Hetero)arylstannanes

A novel copper-mediated carboxylation strategy of aryl- and heteroaryl-stannanes is described. The method serves as a mild (i.e., 1 atm) carboxylation method using stable carbon dioxide and is transferable as a radiosynthetic approach for carbon-11-labeled aromatic and heteroaromatic carboxylic acids using sub-stoichiometric quantities of [¹¹C]CO₂. The methodology was applied to the radiosynthesis of the retinoid X receptor agonist, [¹¹C]bexarotene, with a decay-corrected radiochemical yield of 32 ± 5% and molar activity of 38 ± 23 GBq/μmol (n = 3).

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.

Cu(OAc)2-Mediated benzimidazole-directed C–H cyanation using 2-(4-methylpiperazin-1-yl)acetonitrile as the cyano source

The C–H activation catalytic system was originally applied to the C(sp²)–H cyanation of aryls/heteroaryls to synthesize 2-(1H-benzo[d]imidazol-2-yl)aryl nitriles.

Radiosynthesis of [11C]LY2795050 for Preclinical and Clinical PET Imaging Using Cu(II)-Mediated Cyanation

Copper-mediated ¹¹C-cyanation reactions have enabled the synthesis of PET radiotracers from a range of readily available precursors and avoid the need to use more toxic Pd catalysts. In this work we adapt our recently developed ¹¹C-cyanation of arylpinacolboronate (BPin) esters for the cGMP synthesis of [¹¹C]LY2795050, a selective antagonist radiotracer for the kappa opioid receptor (KOR). [¹¹C]LY2795050 was synthesized in 6 ± 1% noncorrected radiochemical yield (based on [¹¹C]HCN, n = 3) using an automated synthesis module. Quality control testing confirmed the suitability of doses for preclinical and clinical PET imaging (radiochemical purity >99%; specific activity >900 mCi/μmol; residual Cu < 0.1 μg/mL). PET imaging was conducted in rodent and nonhuman primates, showing good brain uptake of [¹¹C]LY2795050 and the expected distribution of KOR. Analogous imaging with [¹¹C]carfentanil (a selective mu opioid receptor (MOR) radiotracer) revealed the anticipated regional differences in MOR and KOR distribution in the primate brain.

Palladium(ii)-mediated rapid 11C-cyanation of (hetero)arylborons

A palladium(ii)-mediated rapid ¹¹C-cyanation of (hetero)arylborons with [¹¹C]NH₄CN/NH₃ has been developed using bench-stable and readily available reagents. The method showed excellent functional-group tolerance, and allowed the highly efficient synthesis of a wide range of [¹¹C]cyanoarenes, including PET tracers for aromatase imaging. A mechanistic study of the ¹¹C-cyanation suggests the instantaneous formation of a mono[¹¹C]cyanopalladium(ii) complex that reacts smoothly with arylborons.

Late-Stage Isotopic Carbon Labeling of Pharmaceutically Relevant Cyclic Ureas Directly from CO2

A robust, click-chemistry-inspired procedure for radiolabeling of cyclic ureas was developed. This protocol, suitable for all carbon isotopes (11 C, 13 C, 14 C), is based on the direct functionalization of carbon dioxide: the universal building block for carbon radiolabeling. The strategy is operationally simple and reproducible in different radiochemistry centers, exhibits remarkably wide substrate scope with short reaction times, and demonstrates superior reactivity as compared to previously reported systems. With this procedure, a variety of pharmaceuticals and an unprotected peptide were labeled with high radiochemical efficiency.

New trends and applications in cyanation isotope chemistry

In this review, newly developed cyanation methods are evaluated in regards to their usefulness in synthetic isotope chemistry. In combination with already established protocols in ^13/14 C- or ¹¹ C-isotope chemistry, this manuscript should help isotope scientists to choose the best possible method for their scientific cyanation problem, but with the main focus on ¹⁴ C-applications. Perhaps, most promising of the described novel cyanation reaction is the decarboxylation-cyanation-hydrolysis approach which makes a 1-step late-stage functionalization procedure possible.

Copper-Mediated Nucleophilic Radiobromination of Aryl Boron Precursors: Convenient Preparation of a Radiobrominated PARP-1 Inhibitor

The copper-mediated nucleophilic radiobromination of aryl boron precursors with a radiobromide ion is a novel radiolabeling method that is efficient and robust. High radiochemical conversion (RCC) was observed using a variety of solvents, temperatures and catalysts. The reaction is also clean and is feasible for purification to obtain high chemical and radiochemical purity. This method provides a very useful route for the preparation of radiobrominated pharmaceuticals, including a radiobromine labeled PARP-1 inhibitor, and it is a valuable addition to the family of copper-mediated radiolabeling processes.

Copper(II)-Mediated [11C]Cyanation of Arylboronic Acids and Arylstannanes

A copper-mediated method for the transformation of diverse arylboron compounds and arylstannanes to aryl-[11C]-nitriles is reported. This method is operationally simple, uses commercially available reagents, and is compatible with a wide variety of substituted aryl- and heteroaryl substrates. This method is applied to the automated synthesis of high specific activity [11C]perampanel in 10% nondecay-corrected radiochemical yield (RCY).

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.

Use of carbon-11 labelled tool compounds in support of drug development

The pharmaceutical industry is facing key challenges to improve return on R&D investment. Positron emission tomography (PET), by itself or in combination with complementary technologies such as magnetic resonance imaging (MRI), provides a unique opportunity to confirm a candidate's ability to meet the so-called 'three pillars' of drug development. Positive confirmation provides confidence for go/no-go decision making at an early stage of the development process and enables informed clinical progression. Whereas fluorine-18 has probably gained wider use in the community, there are benefits to using carbon-11 given the greater flexibility the use of this isotope permits in adaptive clinical study design. This review explores the scope of available carbon-11 chemistries and provides clinical examples to highlight its value in PET studies in support of drug development.

A modification to improve the reliability of [11 C]CN- production in the GE radiochemistry system

We report herein a modification to a component of the GE radiochemistry system that increases the yield and reliability of [¹¹ C]CN^- production from [¹¹ C]carbon dioxide.