Application of Methyl Bisphosphine-Ligated Palladium Complexes for Low Pressure N -11 C-Acetylation of Peptides

Enantioselective Synthesis of Carbon-11-Labeled Amino Acids and Peptides

Radiolabeled amino acids (AAs), their derivatives, and peptides are essential radiotracers in nuclear imaging. Despite its potential, the preparation of enantiopure radiopharmaceuticals poses several challenges, demanding a great need for rapid and stereocontrolled reactions. This chapter describes a highly stereoselective carbon-11 alkylation of Schiff bases, to obtain radiolabeled AAs and small peptides. The method uses chiral quaternary ammonium salt phase-transfer catalyst with two alkylating agents, namely, [11C]methyl iodide and [11C]benzyl iodide. This methodology allows the radiolabeling of AAs and peptides with excellent regioselectivity and enantiomeric or diastereomeric excess.

Reactive Palladium-Ligand Complexes for 11C-Carbonylation at Ambient Pressure: A Breakthrough in Carbon-11 Chemistry

The Pd-Xantphos-mediated ¹¹C-carbonylation protocol (also known as the "Xantphos- method"), due to its simplistic and convenient nature, has facilitated researchers in meeting a longstanding need for preparing ¹¹C-carbonyl-labeled radiopharmaceuticals at ambient pressure for positron emission tomography (PET) imaging and drug discovery. This development could be viewed as a breakthrough in carbon-11 chemistry, as evidenced by the rapid global adoption of the method by the pharmaceutical industry and academic laboratories worldwide. The method has been fully automated for the good manufacturing practice (GMP)-compliant production of novel radiopharmaceuticals for human use, and it has been adapted for "in-loop" reactions and microwave technology; an impressive number of ¹¹C-labeled compounds (>100) have been synthesized. Given the simplicity and efficiency of the method, as well as the abundance of carbonyl groups in bioactive drug molecules, we expect that this methodology will be even more widely adopted in future PET radiopharmaceutical research and drug development.

Development of a Novel [11C]CO-Labeled Positron Emission Tomography Radioligand [11C]BIO-1819578 for the Detection of O-GlcNAcase Enzyme Activity

Imaging O-GlcNAcase OGA by positron emission tomography (PET) could provide information on the pathophysiological pathway of neurodegenerative diseases and important information on drug-target engagement and be helpful in dose selection of therapeutic drugs. Our aim was to develop an efficient synthetic method for labeling BIO-1819578 with carbon-11 using ¹¹CO for evaluation of its potential to measure levels of OGA enzyme in non-human primate (NHP) brain using PET. Radiolabeling was achieved in one-pot via a carbon-11 carbonylation reaction using [¹¹C]CO. The detailed regional brain distribution of [¹¹C]BIO-1819578 binding was evaluated using PET measurements in NHPs. Brain radioactivity was measured for 93 min using a high-resolution PET system, and radiometabolites were measured in monkey plasma using gradient radio HPLC. Radiolabeling of [¹¹C]BIO-1819578 was successfully accomplished, and the product was found to be stable at 1 h after formulation. [¹¹C]BIO-1819578 was characterized in the cynomolgus monkey brain where a high brain uptake was found (7 SUV at 4 min). A pronounced pretreatment effect was found, indicating specific binding to OGA enzyme. Radiolabeling of [¹¹C]BIO-1819578 with [¹¹C]CO was successfully accomplished. [¹¹C]BIO-1819578 binds specifically to OGA enzyme. The results suggest that [¹¹C]BIO-1819578 is a potential radioligand for imaging and for measuring target engagement of OGA in the human brain.

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.

Palladium-catalyzed nucleomethylation of alkynes for synthesis of methylated heteroaromatic compounds

Herein, we disclosed a novel and efficient palladium-catalyzed nucleomethylation of alkynes for the simultaneous construction of the heteroaromatic ring and methyl group. The 3-methylindoles, 3-methylbenzofurans and 4-methylisoquinolines were obtained in moderate to excellent yields. Notably, this methodology was employed as a key step for synthesis of a pregnane X receptor antagonist, zindoxifene, bazedoxifene and AFN-1252. The kinetic studies revealed that reductive elimination might be the rate-determining step.

A novel palladium-catalyzed nucleomethylation of alkynes is developed, affording 3-methylindoles, 3-methylbenzofurans and 4-methylisoquinolines in moderate to excellent yields.

Synthesis and [*C]CO-labelling of (C,N) gem-dimethylbenzylamine-palladium complexes for potential applications in positron emission tomography

Various aryl-palladium complexes were synthesised from gem-dimethylbenzylamine derivatives by C-H activation under extremely mild conditions. Interestingly, these highly stable structures reacted with [13C]carbon monoxide to produce the desired labelled lactams in 29% to 51% yields over the C-H activation/carbonylation steps. As representative examples, a non-natural amino acid and an estradiol-based conjugate were prepared and labelled in model experiments with [13C]CO in homogeneous or heterogeneous conditions. Especially, the latter was radiolabelled with [11C]CO using a convenient procedure from the resin-supported palladium complex precursor. Thus, these results strongly suggest that cyclometallated palladium complexes obtained from gem-dimethylbenzylamine moieties are promising precursors for the practical synthesis of new [11C]tracers for Positron Emission Tomography.

A Nickel(II)-Mediated Thiocarbonylation Strategy for Carbon Isotope Labeling of Aliphatic Carboxamides

A series of pharmaceutically relevant small molecules and biopharmaceuticals bearing aliphatic carboxamides have been successfully labeled with carbon-13. Key to the success of this novel carbon isotope labeling technique is the observation that ¹³ C-labeled Ni^II -acyl complexes, formed from a ¹³ CO insertion step with Ni^II -alkyl intermediates, rapidly react in less than one minute with 2,2'-dipyridyl disulfide to quantitatively form the corresponding 2-pyridyl thioesters. Either the use of ¹³ C-SilaCOgen or ¹³ C-COgen allows for the stoichiometric addition of isotopically labeled carbon monoxide. Subsequent one-pot acylation of a series of structurally diverse amines provides the desired ¹³ C-labeled carboxamides in good yields. A single electron transfer pathway is proposed between the Ni^II -acyl complexes and the disulfide providing a reactive Ni^III -acyl sulfide intermediate, which rapidly undergoes reductive elimination to the desired thioester. By further optimization of the reaction parameters, reaction times down to only 11 min were identified, opening up the possibility of exploring this chemistry for carbon-11 isotope labeling. Finally, this isotope labeling strategy could be adapted to the synthesis of ¹³ C-labeled liraglutide and insulin degludec, representing two antidiabetic drugs.

The development of 11C-carbonylation chemistry: A systematic view

The prospects for using carbon-11 labelled compounds in molecular imaging has improved with the development of diverse synthesis methods, including 11C-carbonylations and refined techniques to handle [11C]carbon monoxide at a nanomole scale. Facilitating biological research and molecular imaging was the driving force when [11C]carbon monoxide was used in the first in vivo application with carbon-11 in human (1945) and when [11C]carbon monoxide was used for the first time as a chemical reagent in the synthesis of [11C]phosgene (1978). This review examines a rich plethora of labelled compounds synthesized from [11C]carbon monoxide, their chemistry and use in molecular imaging. While the strong development of the 11C-carbonylation chemistry has expanded the carbon-11 domain considerably, it could be argued that the number of 11C-carbonyl compounds entering biological investigations should be higher. The reason for this may partly be the lack of commercially available synthesis instruments designed for 11C-carbonylations. But as this review shows, novel and greatly simplified methods to handle [11C]carbon monoxide have been developed. The next important challenge is to make full use of these technologies and synthesis methods in PET research. When there is a PET-tracer that meets a more general need, the incentive to implement 11C-carbonylation protocols will increase.

Carbon Isotope Labeling Strategy for β-Amino Acid Derivatives via Carbonylation of Azanickellacycles

A series of 4-membered azametallacycles have been prepared by the oxidative addition of Ni(0) with aziridines. Stoichiometric ¹³C-labeled carbon monoxide could be efficiently incorporated via Ni-C bond insertion to generate air stable and isolable cyclic Ni-acyl complexes. Upon subjection to a range of C-, N-, O-, and S-nucleophiles, ¹³C-labeled β-amino acids and derivatives thereof, as well as β-aminoketones, could be rapidly accessed. The methodology proved highly adaptable for the synthesis of the antidiabetic drug, sitagliptin, with a single carbon isotope label.

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.

Bioconjugated arylpalladium complexes on solid supports for a convenient last-step synthesis of 11C-labelled tracers for positron emission tomography

Bioconjugated arylpalladium complexes anchored onto polystyrene beads provided [¹¹C]CO-labelled compounds with excellent radiochemical purities after a simple filtration.

Site-Selective, Late-Stage C-H 18 F-Fluorination on Unprotected Peptides for Positron Emission Tomography Imaging

Peptides are often ideal ligands for diagnostic molecular imaging due to their ease of synthesis and tuneable targeting properties. However, labelling unmodified peptides with ¹⁸ F for positron emission tomography (PET) imaging presents a number of challenges. Here we show the combination of photoactivated sodium decatungstate and [¹⁸ F]-N-fluorobenzenesulfonimide effects site-selective ¹⁸ F-fluorination at the branched position in leucine residues in unprotected and unaltered peptides. This streamlined process provides a means to directly convert native peptides into PET imaging agents under mild aqueous conditions, enabling rapid discovery and development of peptide-based molecular imaging tools.

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.

18F-Trifluoromethylation of Unmodified Peptides with 5-18F-(Trifluoromethyl)dibenzothiophenium Trifluoromethanesulfonate

The 18F-labeling of 5-(trifluoromethyl)-dibenzothiophenium trifluoromethanesulfonate, commonly referred to as the Umemoto reagent, has been accomplished applying a halogen exchange 18F-fluorination with 18F-fluoride, followed by oxidative cyclization with Oxone and trifluoromethanesulfonic anhydride. This new 18F-reagent allows for the direct chemoselective 18F-labeling of unmodified peptides at the thiol cysteine residue.

Featuring Xantphos

This review highlights the use of the bisphosphine ligand group in homogeneous catalysis.

Direct 11CN-Labeling of Unprotected Peptides via Palladium-Mediated Sequential Cross-Coupling Reactions

A practical procedure for ¹¹CN-labeling of unprotected peptides has been developed. The method was shown to be highly chemoselective for cysteine over other potentially nucleophilic residues, and the radiolabeled products were synthesized and purified in less than 15 min. Appropriate for biomedical applications, the method could be used on an extremely small scale (20 nmol) with a high radiochemical yield. The success of the protocol stems from the use of a Pd-reagent based on a dihaloarene, which enables direct "nucleophile-nucleophile" coupling of the peptide and [¹¹C]cyanide by temporal separation of nucleophile addition.

Palladium-Catalyzed Aminocarbonylation in Solid-Phase Peptide Synthesis: A Method for Capping, Cyclization, and Isotope Labeling

A new synthetic approach for introducing N-capping groups onto peptides attached to a solid support, combining aminocarbonylation under mild conditions using a palladacycle precatalyst and solid-phase peptide synthesis, is reported. The use of a silacarboxylic acid as an in situ CO-releasing molecule allowed the reaction to be performed in a single vial. The method also enables versatile substitution of side chains, side-chain-to-side-chain cyclizations, and selective [¹³C] acyl labeling of modified peptides.