Direct Carbon Isotope Exchange through Decarboxylative Carboxylation

Integrating Umpolung and CO2 Shuttling Strategies for the Synthesis of 12C- and 13C-α-Ketoacids from Aldehydes

The direct carboxylation of aldehydes with CO2 is rare due to the polarity mismatch between these two electrophilic substrates. To address this challenge, we propose a sequential approach for synthesizing α-ketoacids from commercially available aldehydes by integrating umpolung and CO2 shuttling strategies. This transition metal-free shuttle carboxylation method enables the transfer of CO2 from triphenylacetic acid potassium salt to thioacetal, eliminating the need for handling pressurized CO2 gas or using specialized equipment, while also enhancing the reaction's functional group tolerance. Furthermore, the use of stoichiometric or slightly excess amounts of triphenylacetic acid potassium salt as a formal CO2 donor makes it suitable for complete 13C labeling of α-ketoacids.

Combining Umpolung and Carbon Isotope Exchange Strategies for Accessing Isotopically Labeled α-Keto Acids

The integration of umpolung and carbon isotope exchange for accessing isotopically labeled α-keto acids through photoredox catalysis is elucidated. This process involves the carbonyl umpolung of C(sp2)-α-keto acids to yield C(sp3)-α-thioketal acids, followed by the carbon isotope exchange of C(sp3)-α-thioketal acids, and ultimately, deprotection to generate carbon-labeled α-keto acids.

Enantioselective Carbon Isotope Exchange

The synthesis of isotopically labeled organic molecules is vital for drug and agrochemical discovery and development. Carbon isotope exchange is emerging as a leading method to generate carbon-labeled targets, which are sought over hydrogen-based labels due to their enhanced stability in biological systems. While many bioactive small molecules bear carbon-containing stereocenters, direct enantioselective carbon isotope exchange reactions have not been established. We describe the first example of an enantioselective carbon isotope exchange reaction, where (radio)labeled α-amino acids can be generated from their unlabeled precursors using a stoichiometric chiral aldehyde receptor with isotopically labeled CO2 followed by imine hydrolysis. Many proteinogenic and non-natural derivatives undergo enantioselective labeling, including the late-stage radiolabeling of complex drug targets.

Catalytic Regio- and Enantioselective Remote Hydrocarboxylation of Unactivated Alkenes with CO2

The catalytic regio- and enantioselective hydrocarboxylation of alkenes with carbon dioxide is a straightforward strategy to construct enantioenriched α-chiral carboxylic acids but remains a big challenge. Herein we report the first example of catalytic highly enantio- and site-selective remote hydrocarboxylation of a wide range of readily available unactivated alkenes with abundant and renewable CO2 under mild conditions enabled by the SaBOX/Ni catalyst. The key to this success is utilizing the chiral SaBOX ligand, which combines with nickel to simultaneously control both chain-walking and the enantioselectivity of carboxylation. This process directly furnishes a range of different alkyl-chain-substituted or benzo-fused α-chiral carboxylic acids bearing various functional groups in high yields and regio- and enantioselectivities. Furthermore, the synthetic utility of this methodology was demonstrated by the concise synthesis of the antiplatelet aggregation drug (R)-indobufen from commercial starting materials.

Effect of 6,6'-Substituents on Bipyridine-Ligated Ni Catalysts for Cross-Electrophile Coupling

A family of 4,4'-tBu2-2,2'-bipyridine (tBubpy) ligands with substituents in either the 6-position, 4,4'-tBu2-6-Me-bpy (tBubpyMe), or 6 and 6'-positions, 4,4'-tBu2-6,6'-R2-bpy (tBubpyR2; R = Me, iPr, sBu, Ph, or Mes), was synthesized. These ligands were used to prepare Ni complexes in the 0, I, and II oxidation states. We observed that the substituents in the 6 and 6'-positions of the tBubpy ligand impact the properties of the Ni complexes. For example, bulkier substituents in the 6,6'-positions of tBubpy better stabilized (tBubpyR2)NiICl species and resulted in cleaner reduction from (tBubpyR2)NiIICl2. However, bulkier substituents hindered or prevented coordination of tBubpyR2 ligands to Ni0(cod)2. In addition, by using complexes of the type (tBubpyMe)NiCl2 and (tBubpyR2)NiCl2 as precatalysts for different XEC reactions, we demonstrated that the 6 or 6,6' substituents lead to major differences in catalytic performance. Specifically, while (tBubpyMe)NiIICl2 is one of the most active catalysts reported to date for XEC and can facilitate XEC reactions at room temperature, lower turnover frequencies were observed for catalysts containing tBubpyR2 ligands. A detailed study on the catalytic intermediates (tBubpy)Ni(Ar)I and (tBubpyMe2)Ni(Ar)I revealed several factors that likely contributed to the differences in catalytic activity. For example, whereas complexes of the type (tBubpy)Ni(Ar)I are low spin and relatively stable, complexes of the type (tBubpyMe2)Ni(Ar)I are high-spin and less stable. Further, (tBubpyMe2)Ni(Ar)I captures primary and benzylic alkyl radicals more slowly than (tBubpy)Ni(Ar)I, consistent with the lower activity of the former in catalysis. Our findings will assist in the design of tailor-made ligands for Ni-catalyzed transformations.

A metal-catalysed functional group metathesis approach to the carbon isotope labelling of carboxylic acids

The distribution, metabolism and ultimate fate of molecules within the body is central to the activity of pharmaceuticals. However, the introduction of radioisotopes into the metabolically stable carbon sites on drugs to probe these features typically requires toxic, radioactive gases such as [14C]CO and [14C]CO2. Here we describe an approach to directly carbon-label carboxylic-acid-containing pharmaceuticals via a metal-catalysed functional group exchange reaction, forming 14C-labelled carboxylic-acid-containing drugs without radioactive gases, in one pot, using an easily available and handled carboxylic acid 14C source. To enable this process, a functional group metathesis of carbon-carbon covalent bonds in acid chloride functionalities is developed, exploiting the ability of nickel catalysts to both reversibly activate carbon-chloride bonds and exchange functionalities between organic molecules. The drug development applicability is illustrated by the direct incorporation of the 14C label or 13C label into an array of complex aryl, alkyl, vinyl and heterocyclic carboxylic acid drugs or drug candidates without gases or a special apparatus, at ambient conditions and without loss of the radiolabel.

Efficient palladium-catalyzed electrocarboxylation enables late-stage carbon isotope labelling

Carbon isotope labelling of bioactive molecules is essential for accessing the pharmacokinetic and pharmacodynamic properties of new drug entities. Aryl carboxylic acids represent an important class of structural motifs ubiquitous in pharmaceutically active molecules and are ideal targets for the installation of a radioactive tag employing isotopically labelled CO2. However, direct isotope incorporation via the reported catalytic reductive carboxylation (CRC) of aryl electrophiles relies on excess CO2, which is incompatible with carbon-14 isotope incorporation. Furthermore, the application of some CRC reactions for late-stage carboxylation is limited because of the low tolerance of molecular complexity by the catalysts. Herein, we report the development of a practical and affordable Pd-catalysed electrocarboxylation setup. This approach enables the use of near-stoichiometric 14CO2 generated from the primary carbon-14 source Ba14CO3, facilitating late-stage and single-step carbon-14 labelling of pharmaceuticals and representative precursors. The proposed isotope-labelling protocol holds significant promise for immediate impact on drug development programmes.

Carbon-Atom Exchange between [MC2]+ (M = Os and Ir) and Methane: on the Thermodynamic and Dynamic Aspects

Gas-phase reactions of [OsC2]+ and [IrC2]+ with methane at ambient temperature have been studied using quadrupole-ion trap mass spectrometry combined with quantum chemical calculations. Both [OsC2]+ and [IrC2]+ undergo carbon-atom exchange reactions with methane. The associated mechanisms for the two systems are found to be similar. The differences in the rates of carbon isotope exchange reactions of methane with [MC2]+ (M = Os and Ir) are explained by several factors like the energy barrier for the initial H3C-H bond breaking processes, the molecular dynamics, orbital interactions, and the H-binding energies of the pivotal steps. Besides, the number of participating valence orbitals might be one of the keys to regulate the rate in the key step. The present findings may provide useful ideas and inspiration for designing similar processes.

The Future of (Radio)-Labeled Compounds in Research and Development within the Life Science Industry

In this review the applications of isotopically labeled compounds are discussed and put into the context of their future impact in the life sciences. Especially discussing their use in the pharma and crop science industries to follow their fate in the environment, in vivo or in complex matrices to understand the potential harm of new chemical structures and to increase the safety of human society.

Core-Labeling (Radio) Synthesis of Phenols

We report a method that enables the fast incorporation of carbon isotopes into the ipso carbon of phenols. Our approach relies on the synthesis of a 1,5-dibromo-1,4-pentadiene precursor, which upon lithium-halogen exchange followed by treatment with carbonate esters results in a formal [5 + 1] cyclization to form the phenol product. Using this strategy, we have prepared 12 1-13C-labeled phenols, show proof-of-concept for the labeling of phenols with carbon-14, and demonstrate phenol synthesis directly from cyclotron-produced [11C]CO2.

Nickel-Catalyzed Asymmetric Decarboxyarylation with NHP Esters of α-Amino Acid to Chiral Benzylamines

A nickel-catalyzed asymmetric decarboxyarylation of NHP esters via reductive cross-coupling has been established. Utilizing the NHP of amino acid esters as radical precursors furnishes a new protocol in which structurally diverse chiral benzylamines could be accessible. This method has demonstrated excellent catalytic efficiency, high enantioselective control, mild conditions, and good functional group tolerance, thus enabling the late-stage modification of bioactive molecules and pharmaceuticals.

Molecular Editing of Pyrroles via a Skeletal Recasting Strategy

Heterocyclic scaffolds are commonly found in numerous biologically active molecules, therapeutic agents, and agrochemicals. To probe chemical space around heterocycles, many powerful molecular editing strategies have been devised. Versatile C-H functionalization strategies allow for peripheral modifications of heterocyclic motifs, often being specific and taking place at multiple sites. The past few years have seen the quick emergence of exciting "single-atom skeletal editing" strategies, through one-atom deletion or addition, enabling ring contraction/expansion and structural diversification, as well as scaffold hopping. The construction of heterocycles via deconstruction of simple heterocycles is unknown. Herein, we disclose a new molecular editing method which we name the skeletal recasting strategy. Specifically, by tapping on the 1,3-dipolar property of azoalkenes, we recast simple pyrroles to fully substituted pyrroles, through a simple phosphoric acid-promoted one-pot reaction consisting of dearomative deconstruction and rearomative reconstruction steps. The reaction allows for easy access to synthetically challenging tetra-substituted pyrroles which are otherwise difficult to synthesize. Furthermore, we construct N-N axial chirality on our pyrrole products, as well as accomplish a facile synthesis of the anticancer drug, Sutent. The potential application of this method to other heterocycles has also been demonstrated.

Modern Strategies for Carbon Isotope Exchange

In contrast to stable and natural abundant carbon-12, the synthesis of organic molecules with carbon (radio)isotopes must be conceived and optimized in order to navigate through the hurdles of radiochemical requirements, such as high costs of the starting materials, harsh conditions and radioactive waste generation. In addition, it must initiate from the small cohort of available C-labeled building blocks. For long time, multi-step approaches have represented the sole available patterns. On the other side, the development of chemical reactions based on the reversible cleavage of C-C bonds might offer new opportunities and reshape retrosynthetic analysis in radiosynthesis. This review aims to provide a short survey on the recently emerged carbon isotope exchange technologies that provide effective opportunity for late-stage labeling. At present, such strategies have relied on the use of primary and easily accessible radiolabeled C1-building blocks, such as carbon dioxide, carbon monoxide and cyanides, while the activation principles have been based on thermal, photocatalytic, metal-catalyzed and biocatalytic processes.

Robust and General Late-Stage Methylation of Aryl Chlorides: Application to Isotopic Labeling of Drug-like Scaffolds

The preparation of isotopically labeled compounds for drug discovery and development presents a unique challenge. Both stable and radioactive isotopes must be incorporated into complex bioactive molecules as efficiently as possible, using precious, and often expensive, isotopically enriched reagents. Due to the ubiquity and importance of methyl groups in drug molecules, there is a requirement for a general, late-stage methylation that allows for the incorporation of both carbon and hydrogen isotopes. Herein, we report a highly efficient, robust palladium-catalyzed approach, optimized via high-throughput experimentation, for the methylation of aryl chlorides using potassium methyltrifluoroborate. A practically straightforward route to isotopically labeled methylating agents has also been developed, and the methodology applied to isotopologue synthesis, including the installation of isotopic labels in a range of drug-like scaffolds.

Low Pressure Carbonylation of Benzyl Carbonates and Carbamates for Applications in 13 C Isotope Labeling and Catalytic CO2 Reduction

Herein, we report a methodology to access isotopically labeled esters and amides from carbonates and carbamates employing an oxygen deletion strategy. This methodology utilizes a decarboxylative carbonylation approach for isotope labeling with near stoichiometric, ex situ generated 12 C, or 13 C carbon monoxide. This reaction is characterized by its broad scope, functional group tolerance, and high yields, which is showcased with the synthesis of structurally complex molecules. A complementary method that operates by the catalytic in situ generation of CO via the reduction of CO2 liberated during decarboxylation has also been developed as a proof-of-concept approach that CO2 -derived compounds can be converted to CO-containing frameworks. Mechanistic studies provide insight into the catalytic steps which highlight the impact of ligand choice to overcome challenges associated with low-pressure carbonylation methodologies, along with rational for the development of future methodologies.

Unlocking full and fast conversion in photocatalytic carbon dioxide reduction for applications in radio-carbonylation

Harvesting sunlight to drive carbon dioxide (CO₂) valorisation represents an ideal concept to support a sustainable and carbon-neutral economy. While the photochemical reduction of CO₂ to carbon monoxide (CO) has emerged as a hot research topic, the full CO₂-to-CO conversion remains an often-overlooked criterion that prevents a productive and direct valorisation of CO into high-value-added chemicals. Herein, we report a photocatalytic process that unlocks full and fast CO₂-to-CO conversion (<10 min) and its straightforward valorisation into human health related field of radiochemistry with carbon isotopes. Guided by reaction-model-based kinetic simulations to rationalize reaction optimisations, this manifold opens new opportunities for the direct access to ¹¹C- and ¹⁴C-labeled pharmaceuticals from their primary isotopic sources [¹¹C]CO₂ and [¹⁴C]CO₂.

Rapid Access to Carbon-Isotope-Labeled Alkyl and Aryl Carboxylates Applying Palladacarboxylates

Herein, we report a strategy for the formation of isotopically labeled carboxylic esters from boronic esters/acids using a readily accessible palladium carboxylate complex as an organometallic source of isotopically labeled functional groups. The reaction allows access to either unlabeled or full ¹³C- or ¹⁴C-isotopically labeled carboxylic esters, and the method is characterized by its operational simplicity, mild conditions, and general substrate scope. Our protocol is further extended to a carbon isotope replacement strategy, involving an initial decarbonylative borylation procedure. Such an approach allows access to isotopically labeled compounds directly from the unlabeled pharmaceutical, which can have implications for drug discovery programs.

Aldehyde-catalysed carboxylate exchange in α-amino acids with isotopically labelled CO2

The isotopic labelling of small molecules is integral to drug development and for understanding biochemical processes. The preparation of carbon-labelled α-amino acids remains difficult and time consuming, with established methods involving label incorporation at an early stage of synthesis. This explains the high cost and scarcity of C-labelled products and presents a major challenge in 11C applications (11C t1/2 = 20 min). Here we report that aldehydes catalyse the isotopic carboxylate exchange of native α-amino acids with *CO2 (* = 14, 13, 11). Proteinogenic α-amino acids and many non-natural variants containing diverse functional groups undergo labelling. The reaction probably proceeds via the trapping of *CO2 by imine-carboxylate intermediates to generate iminomalonates that are prone to monodecarboxylation. Tempering catalyst electrophilicity was key to preventing irreversible aldehyde consumption. The pre-generation of the imine carboxylate intermediate allows for the rapid and late-stage 11C-radiolabelling of α-amino acids in the presence of [11C]CO2.

Electroreductive Ring-Opening Carboxylation of Cycloketone Oxime Esters with Carbon Dioxide

Electroreductive ring-opening carboxylation of cycloketone oxime esters with atmospheric carbon dioxide is reported. This reaction proceeded under simple constant current conditions in an undivided cell using glassy carbon as the cathode and magnesium as the sacrificial anode, providing substituted γ- and δ-cyanocarboxylic acids in moderate to good yields. Electrochemically generated cyanoalkyl radicals and cyanoalkyl anion are proposed as the key intermediates.

Late-Stage Carbon-14 Labeling and Isotope Exchange: Emerging Opportunities and Future Challenges

Carbon-14 (¹⁴C) is a gold standard technology routinely utilized in pharmaceutical and agrochemical industries for tracking synthetic organic molecules and providing their metabolic and safety profiles. While the state of the art has been dominated for decades by traditional multistep synthetic approaches, the recent emergence of late-stage carbon isotope labeling has provided new avenues to rapidly access carbon-14-labeled biologically relevant compounds. In particular, the development of carbon isotope exchange has represented a fundamental paradigm change, opening the way to unexplored synthetic transformations. In this Perspective, we discuss the recent developments in the field with a critical assessment of the literature. We subsequently discuss research directions and future challenges within this rapidly evolving field.

Recent advancements in visible-light-driven carboxylation with carbon dioxide

Carbon dioxide as a classic C1 source has long been investigated in organic synthetic chemistry. Diverse catalytic methods for CO2 activation were reported in the past several decades. In this...

Challenges and recent advancements in the transformation of CO2 into carboxylic acids: straightforward assembly with homogeneous 3d metals

Transformation of carbon dioxide (CO2) into valuable organic carboxylic acids is essential for maintaining sustainability. In this review, such CO2 thermo-, photo- and electrochemical transformations under 3d-transition metal catalysis are described from 2017 until 2022.

Nickel-Mediated Alkoxycarbonylation for Complete Carbon Isotope Replacement

Many commercial drugs, as well as upcoming pharmaceutically active compounds in the pipeline, display aliphatic carboxylic acids or derivatives thereof as key structural entities. Synthetic methods for rapidly accessing isotopologues of such compounds are highly relevant for undertaking critical pharmacological studies. In this paper, we disclose a direct synthetic route allowing for full carbon isotope replacement via a nickel-mediated alkoxycarbonylation. Employing a nickel^II pincer complex ([(N₂N)Ni-Cl]) in combination with carbon-13 labeled CO, alkyl iodide, sodium methoxide, photocatalyst, and blue LED light, it was possible to generate the corresponding isotopically labeled aliphatic carboxylates in good yields. Furthermore, the developed methodology was applied to the carbon isotope substitution of several pharmaceutically active compounds, whereby complete carbon-13 labeling was successfully accomplished. It was initially proposed that the carboxylation step would proceed via the in situ formation of a nickellacarboxylate, generated by CO insertion into the Ni-alkoxide bond. However, preliminary mechanistic investigations suggest an alternative pathway involving attack of an open shell species generated from the alkyl halide to a metal ligated CO to generate an acyl Ni^III species. Subsequent reductive elimination involving the alkoxide eventually leads to carboxylate formation. An excess of the alkoxide was essential for obtaining a high yield of the product. In general, the presented methodology provides a simple and convenient setup for the synthesis and carbon isotope labeling of aliphatic carboxylates, while providing new insights about the reactivity of the N₂N nickel pincer complex applied.

Nickel-Catalyzed Reductive Carboxylation and Amidation Reactions

The ubiquity and importance of carboxylic acids and amides in peptides, pharmaceuticals, agrochemicals, and synthetic materials has challenged chemists to design de novo catalytic carboxylation and amidation protocols. They represent a powerful alternative to canonical oxidation of alcohols and aldehydes, hydrolysis of nitriles, transamidation reactions, or condensation techniques for the synthesis of these functional groups. Among various scenarios, the recent years have witnessed considerable advances in Ni-catalyzed reductive carboxylation and amidation reactions utilizing carbon dioxide and isocyanate counterparts. This Account aims to highlight the progress made in this arena with a historical perspective, with particular emphasis on the methodologies that have emanated from our laboratories without losing sight of the underlying principles by which these reactions operate, with the ultimate goal of allowing the transition from comprehension to prediction in this exciting field.Unlike the utilization of conventional polar yet highly reactive organometallic reagents in carboxylation or amidation reactions, the utilization of nickel catalysts has allowed the use of carbon dioxide and isocyanates with less reactive and less-polarized counterparts for the formations of carboxylic acids and amides. These less reactive groups include organic halides and pseudohalides (i.e., alkyl bromides and chlorides, esters, alcohols, and ammonium salts), unsaturated hydrocarbons (i.e., alkynes, styrenes, unactivated alkenes, and dienes) or even C-H bonds, where forging the targeted C-C bond at previously unfunctionalized C-H linkages was possible, thus giving access to densely functionalized compounds that would be difficult to access otherwise. The C-H functionalization includes chain-walking scenarios, where subtle changes in the ligand and reaction conditions marked the selectivity of the transformations, and reactions via a [1,4]-Ni shift, where selective carboxylation in aromatic rings could be achieved. Conceptuality and practicality aside, these transformations have even offered the possibility of modulating and dictating the site-selectivity pattern, thus providing not only new vistas when controlling the selectivity of bond-forming reactions at specific sites within the side chain but also new knowledge in retrosynthetic analysis when accessing carboxylic acids and amide backbones. Importantly, these techniques have shown to be particularly suited for the preparation of isotopically labeled molecules when using ¹³CO₂ or even ¹⁴CO₂, thus becoming a useful endeavor in the drug discovery pipeline. Although mechanistic understanding at the molecular level still constitutes the "Achilles heel" of these transformations, the recent empirical discoveries and the rapid adoption of these protocols by the community augurs well for the widespread utilization of reductive carboxylation and amidation reactions in both academic and industrial laboratories.

Convergent Synthesis of (R)-Silodosin via Decarboxylative Cross-Coupling

A new approach to Silodosin capitalizing on a radical retrosynthetic strategy to dissect the molecule into two halves is reported. Using a reductive decarboxylative cross-coupling, a simple indoline can be coupled to a chiral pool-derived fragment to arrive at the target in only seven steps (LLS). This route avoids the use of resolution strategies or asymmetric hydrogenation that requires a subsequent Curtius rearrangement to install a key amino functionality.

Photoinduced Hydrocarboxylation via Thiol-Catalyzed Delivery of Formate Across Activated Alkenes

Herein we disclose a new photochemical process to prepare carboxylic acids from formate salts and alkenes. This redox-neutral hydrocarboxylation proceeds in high yields across diverse functionalized alkene substrates with excellent regioselectivity. This operationally simple procedure can be readily scaled in batch at low photocatalyst loading (0.01% photocatalyst). Furthermore, this new reaction can leverage commercially available formate carbon isotologues to enable the direct synthesis of isotopically labeled carboxylic acids. Mechanistic studies support the working model involving a thiol-catalyzed radical chain process wherein the atoms from formate are delivered across the alkene substrate via CO₂^•- as a key reactive intermediate.

1,2-Difunctionalized bicyclo[1.1.1]pentanes: Long-sought-after mimetics for ortho/meta-substituted arenes

The development of a versatile platform for the synthesis of 1,2-difunctionalized bicyclo[1.1.1]pentanes to potentially mimic ortho/meta-substituted arenes is described. The syntheses of useful building blocks bearing alcohol, amine, and carboxylic acid functional handles have been achieved from a simple common intermediate. Several ortho- and meta-substituted benzene analogs, as well as simple molecular matched pairs, have also been prepared using this platform. The results of in-depth ADME (absorption, distribution, metabolism, and excretion) investigations of these systems are presented, as well as computational studies which validate the ortho- or meta-character of these bioisosteres.

Synthesis of 13 C-labelled ω-hydroxy carboxylic acids of the general formula HO2 13 C-(CH2 )n -CH2 OH or HO2 C-(CH2 )n -13 CH2 OH (n = 12, 16, 20, 28)

¹³ C-labelled ω-hydroxy-carboxylic acids HO₂ ¹³ C-(CH₂ )_n -CH₂ OH or HO₂ C-(CH₂ )_n -¹³ CH₂ OH (n = 12, 16, 20, 28) with ¹³ C labels selectively introduced either at the carboxy group or at the primary alcohol function at the end of the hydrocarbon chain have been synthesized. Different synthetic strategies had to be applied depending on the position of the label, the chain length of the respective synthetic target and due to economic considerations. ¹³ C labels in general were introduced by nucleophilic substitution of a suitable leaving group with labelled potassium cyanide and subsequent hydrolysis of the nitriles to produce the corresponding labelled carboxy functions, which may also be reduced to give the labelled primary alcohol group. All new compounds are characterized by GC/MS, IR and NMR methods as well as by elemental analysis.

Direct Carbon Isotope Exchange of Pharmaceuticals via Reversible Decyanation

The incorporation of carbon-14 allows tracking of organic molecules and provides vital knowledge on their fate. This information is critical in pharmaceutical development, crop science, and human food safety evaluation. Herein, a transition-metal-catalyzed procedure enabling carbon isotope exchange on aromatic nitriles is described. By utilizing the radiolabeled precursor Zn([¹⁴C]CN)₂, this protocol allows the insertion of the desired carbon tag without the need for structural modifications, in a single step. By reducing synthetic costs and limiting the generation of radioactive waste, this procedure will facilitate the labeling of nitrile containing drugs and accelerate ¹⁴C-based ADME studies supporting drug development.

Late-Stage Carbon Isotope Exchange of Aryl Nitriles through Ni-Catalyzed C-CN Bond Activation

A facile one-pot strategy for ¹³CN and ¹⁴CN exchange with aryl, heteroaryl, and alkenyl nitriles using a Ni phosphine catalyst and BPh₃ is described. This late-stage carbon isotope exchange (CIE) strategy employs labeled Zn(CN)₂ to facilitate enrichment using the nonlabeled parent compound as the starting material, eliminating de novo synthesis for precursor development. A broad substrate scope encompassing multiple pharmaceuticals is disclosed, including the preparation of [¹⁴C] belzutifan to illustrate the exceptional functional group tolerance and utility of this labeling approach. Preliminary experimental and computational studies suggest the Lewis acid BPh₃ is not critical for the oxidative addition step and instead plays a role in facilitating CN exchange on Ni. This CIE method dramatically reduces the synthetic steps and radioactive waste involved in preparation of ¹⁴C labeled tracers for clinical development.

Fast Carbon Isotope Exchange of Carboxylic Acids Enabled by Organic Photoredox Catalysis

Carbazole/cyanobenzene photocatalysts promote the direct isotopic carboxylate exchange of C(sp³) acids with labeled CO₂. Substrates that are not compatible with transition-metal-catalyzed degradation-reconstruction approaches or prone to thermally induced reversible decarboxylation undergo isotopic incorporation at room temperature in short reaction times. The radiolabeling of drug molecules and precursors with [¹¹C]CO₂ is demonstrated.

Radical-Based Synthesis and Modification of Amino Acids

Amino acids (AAs) are key structural motifs with widespread applications in organic synthesis, biochemistry, and material sciences. Recently, with the development of milder and more versatile radical-based procedures, the use of strategies relying on radical chemistry for the synthesis and modification of AAs has gained increased attention, as they allow rapid access to libraries of novel unnatural AAs containing a wide range of structural motifs. In this Minireview, we provide a broad overview of the advancements made in this field during the last decade, focusing on methods for the de novo synthesis of α-, β-, and γ-AAs, as well as for the selective derivatisation of canonical and non-canonical α-AAs.

Synthesis of 13 C-labelled cutin and suberin monomeric dicarboxylic acids of the general formula HO213 C-(CH2 )n -13 CO2 H (n = 10, 12, 14, 16, 18, 20, 22, 24, 26, 28)

13 C-labeled dicarboxylic acids HO213 C-(CH2 )n -13 CO2 H (n = 10, 12, 14, 16, 18, 20, 22, 24, 26, 28) have been synthesized as internal standards for LC-MS and GC-MS analysis of cutin and suberin monomer degradation by soil-based microorganisms. Different synthetic strategies had to be applied depending on the chain length of the respective synthetic target and because of economic considerations. 13 C-labels were introduced by nucleophilic substitution of a suitable leaving group with labelled potassium cyanide and subsequent hydrolysis of the nitriles to produce the corresponding dicarboxylic acids. All new compounds are characterized by GC/MS, IR, and NMR methods as well as by elemental analysis.

Electrochemical Decarboxylative N-Alkylation of Heterocycles

An operationally simple method to employ nonactivated carboxylic acids as alkylating agents in the N-alkylation of heterocycles is reported through an electrochemically driven anodic decarboxylative process. A wide substrate scope across a range of heterocycles is demonstrated along with a series of applications that significantly reduce the step count required to access such medicinally relevant structures.

Transition-Metal-Free Carbon Isotope Exchange of Phenyl Acetic Acids

A transition-metal-free carbon isotope exchange procedure on phenyl acetic acids is described. Utilizing the universal precursor CO2 , this protocol allows the carbon isotope to be inserted into the carboxylic acid position, with no need of precursor synthesis. This procedure enabled the labeling of 15 pharmaceuticals and was compatible with carbon isotopes [14 C] and [13 C]. A proof of concept with [11 C] was also obtained with low molar activity valuable for distribution studies.

Direct reversible decarboxylation from stable organic acids in dimethylformamide solution

Many classical and emerging methodologies in organic chemistry rely on carbon dioxide (CO₂) extrusion to generate reactive intermediates for bond-forming events. Synthetic reactions that involve the microscopic reverse-the carboxylation of reactive intermediates-have conventionally been undertaken using very different conditions. We report that chemically stable C(sp³) carboxylates, such as arylacetic acids and malonate half-esters, undergo uncatalyzed reversible decarboxylation in dimethylformamide solution. Decarboxylation-carboxylation occurs with substrates resistant to protodecarboxylation by Brønsted acids under otherwise identical conditions. Isotopically labeled carboxylic acids can be prepared in high chemical and isotopic yield by simply supplying an atmosphere of ¹³CO₂ to carboxylate salts in polar aprotic solvents. An understanding of carboxylate reactivity in solution enables conditions for the trapping of aldehydes, ketones, and α,β-unsaturated esters.

Carbon-11 carboxylation of trialkoxysilane and trimethylsilane derivatives using [11C]CO2

A simple and rapid carbon-11 carboxylation radiosynthesis method.

A novel carboxylation radiosynthesis methodology is described starting from cyclotron-produced [¹¹C]CO₂ and fluoride-activated silane derivatives. Six carbon-11 labelled carboxylic acids were obtained from their corresponding trimethylsilyl and trialkoxysilyl precursors in a one-pot labelling methodology. The radiochemical yields ranged from 19% to 93% within 12 minutes post [¹¹C]CO₂ delivery with a trapping efficiency of 21–89%.

Copper-catalysed photoinduced decarboxylative alkynylation: a combined experimental and computational study

Redox-active esters (RAEs) as alkyl radical precursors have demonstrated great advantages for C–C bond formation. A decarboxylative cross-coupling method is described to afford substituted alkynes from various carboxylic acids using copper catalysts CuCl and Cu(acac)₂. The photoexcitation of copper acetylides with electron-rich NEt₃ as a ligand provides a general strategy to generate a range of alkyl radicals from RAEs of carboxylic acids, which can be readily coupled with a variety of aromatic alkynes. The scope of this cross-coupling reaction can be further expanded to aliphatic alkynes and alkynyl silanes using a catalytic amount of preformed copper-phenylacetylide. In addition, DFT calculations revealed the favorable reaction pathway and that the bidentate acetylacetonate ligand of the copper intermediate plays an important role in inhibiting the homo-coupling of the alkyne.

Redox-active esters (RAEs) as alkyl radical precursors have demonstrated great advantages for Cu-catalysed C–C bond formation.