Oxamic acids: useful precursors of carbamoyl radicals

Direct C-H functionalisation of azoles via Minisci reactions

Azoles have widespread applications in medicinal chemistry; for example, thiazoles, imidazoles, benzimidazoles, isoxazoles, tetrazoles and triazoles appear in the top 25 most frequently used N-heterocycles in FDA-approved drugs. Efficient routes for the late-stage C-H functionalisation of azole cores would therefore be highly desirable. The Minisci reaction, a nucleophilic radical addition reaction onto N-heterocyclic bases, is a direct C-H functionalisation reaction that has the potential to be a powerful method for C-H functionalisations of azole scaffolds. However, azoles have not been as widely studied as substrates for modern Minisci-type reactions as they are often more electron-rich and thus more challenging substrates compared to electron-poor 6-membered N-heterocycles such as quinolines, pyrazines and pyridines typically used in Minisci reactions. Nevertheless, with the prevalence of azole scaffolds in drug design, the Minisci reaction has the potential to be a transformative tool for late-stage C-H functionalisations to efficiently access decorated azole motifs. This review thus aims to give an overview of the C-H functionalisation of azoles via Minisci-type reactions, highlighting recent progress, existing limitations and potential areas for growth.

Photoredox-Catalyzed Carbamoyl Radical-Initiated Dearomative Spirocyclization To Access Spiro-Cyclohexadiene Oxindoles

The sustainable construction of spirocyclic compounds is important to the scientific community and the pharmaceutical industries. Herein, we demonstrate a carbamoyl radical-initiated intramolecular dearomative spirocyclization to access the spiro-cyclohexadiene oxindoles under visible light irradiation, which constitutes the first example of accessing the I-substituted derivatives that facilitate diversified transformations. Additionally, the scalability, late-stage modification of drugs, and significant antitumor activity of the products demonstrate the novel spirocyclic synthesis platform for expediting drug development.

Direct C-H amidation of 1,3-azoles: light-mediated, photosensitiser-free vs. thermal

We have developed one thermal and one light-mediated method for direct Minisci-type C-H amidation of 1,3-azoles, which are applicable to thiazoles, benzothiazoles, benzimidazoles, and for the first time, imidazoles. The new visible light-mediated approach can be rendered photosensitiser/photocatalyst-free and likely proceeds via an electron donor-acceptor (EDA) complex, the first direct Minisci-type amidation to do so.

Synthesis of polyurethanes through the oxidative decarboxylation of oxamic acids: a new gateway toward self-blown foams

Polyurethane (PU) thermoplastics and thermosets were prepared through the step-growth polymerization of in situ generated polyisocyanates through the decarboxylation of polyoxamic acids, in the presence of phenyliodine diacetate (PIDA), and polyols. The CO2 produced during the reaction allowed the access to self-blown polyurethane foams through an endogenous chemical blowing. The acetic acid released from ligand exchange at the iodine center was also shown to accelerate the polymerization reaction, avoiding the recourse to an additional catalyst. Changing simple parameters during the production process allowed us to access flexible PU foams with a wide range of properties.

Photochemical radical decarboxylative disulfuration of α-keto acids and oxamic acids

A visible-light-induced directed decarboxylative disulfuration of α-keto acids and oxamic acids was developed. As a result, a series of versatile mono acyl disulfide derivatives was synthesized under mild and sustainable reaction conditions. This protocol has a broad substrate scope, good functional-group tolerance, and excellent synthetic applications.

Silver-Catalyzed Carbofluorination of Olefins and α-Fluoroolefins with Carbamoyl Radicals

The reactivity of carbamoyl radicals, generated in situ from sodium oxamate salts, has been investigated in the context of radical carbofluorination reactions of olefins and α-fluoroolefins, respectively. Both transformations are catalyzed by silver salts and required the presence of potassium persulfate (K2S2O8) and SelectfluorTM as a radicophilic fluorine source. The reported methods provide a direct access to β-fluoroamides and β,β-difluoroamides.

Carbonylative cyclization of biaryl enones with aldehydes and oxamic acids

An oxidative radical-promoted carbonylative cyclization strategy for the synthesis of phenanthren-9-(10H)-one frameworks from biaryl enones using aldehydes as the carbonyl radical sources is disclosed. The reaction proceeds through a sequential addition of a carbonyl radical to the olefin followed by cyclization with an aryl ring. The method is further extended to carbamoyl radicals generated from oxamic acids to access the corresponding phenanthrenones with amide functionalities.

Visible light mediated iron-catalyzed addition of oxamic acids to imines

Oxamic acids where shown to add to imines, providing a broad range of α-aminoacid amides in generally good yields. The process is efficient on pre-formed imines but may also be conducted using a 3-component strategy by simply mixing aldehydes, amines and oxamic acids in the presence of ferrocene, acting both as a photocatalyst under visible light and as a Lewis acid. The reaction proceeds through the addition onto the imine of a carbamoyl radical intermediate generated through a charge transfer from the carboxylate ligand to a Fe(iii) species (LMCT).

Decarboxylative Nickel- and Photoredox-Catalyzed Aminocarbonylation of (Hetero)Aryl Bromides

An efficient methodology for the photoredox- and nickel-catalyzed aminocarbonylation of (hetero)aryl bromides was developed. The utilization of readily available oxamic acids, the application of a broadly used organic photoredox catalyst (4CzIPN), and mild reaction conditions make this transformation an appealing alternative to classical amidation procedures. The generation of carbamoyl radicals was supported by trapping reactions with a hydrogen atom transfer catalyst in the presence of D2O, yielding the deuterated formamide. The generality of this deuteration protocol was confirmed for various oxamic acids.

Metal-free synthesis of carbamoylated dihydroquinolinones via cascade radical annulation of cinnamamides with oxamic acids

We report a metal-free procedure for the sustainable synthesis of carbamoylated dihydroquinolinones via tandem addition-cyclization of carbamoyl radicals to cinnamamides. Readily accessible, non-toxic and inexpensive oxamic acids are used as carbamoyl radical precursors. This highly straightforward method provides a mild and environmentally friendly route showing good atom economy and excellent functional group tolerance to obtain diverse medicinally important carbamoylated dihydroquinolinones in one pot. The cascade cyclization is also modular and step-economical with a wide substrate scope and the products were obtained in good to excellent yields. Additionally, the tolerance to air and water, operational simplicity, low cost and scalability enhance the practical value of the proposed synthetic strategy. Preliminary mechanistic studies reveal that cheap and environment-friendly ammonium persulfate acts as a radical initiator in the cascade process and generates carbamoyl radicals from oxamic acids. The synthetic utility of this method is further demonstrated by late stage functionalization of drug molecules with good yields.

Persulfate-Promoted Carbamoylation/Cyclization of Alkenes: Synthesis of Amide-Containing Quinazolinones

The incorporation of amide groups into biologically active molecules has been proven to be an efficient strategy for drug design and discovery. In this study, we present a simple and practical method for the synthesis of amide-containing quinazolin-4(3H)-ones under transition-metal-free conditions. This is achieved through a carbamoyl-radical-triggered cascade cyclization of N3-alkenyl-tethered quinazolinones. Notably, the carbamoyl radical is generated in situ from the oxidative decarboxylative process of oxamic acids in the presence of (NH4)2S2O8.

Visible-Light-Induced Synthesis of 1,2-Dicarboxyl Compounds from Carbon Dioxide, Carbamoyl-dihydropyridine, and Styrene

β-Amidated carboxylic acids, or succinamic acid derivatives, constitute a valuable chemical scaffold with broad applications in pharmaceuticals, agrochemicals, and polymer sciences. Herein, we report a redox-neutral multicomponent reaction for the synthesis of succinamic acid derivatives in good yields. This protocol involves styrene, CO2 and 1,4-carbamoyl-dihydropyridine as radical precursors. The method exhibits a broad substrate scope under mild reaction conditions, including late-stage functionalization. Moreover, by employing 13CO2, the method enables the synthesis of labeled 1,2-dicarboxylic compounds.

Organoelectrophotocatalytic Generation of Acyl Radicals from Formamides and Aldehydes: Access to Acylated 3-CF3-2-Oxindoles

Organoelectrophotocatalytic generation of acyl radicals from formamides and aldehydes to synthesize acylated 3-CF3-2-oxindoles has been developed. This protocol features a monocatalytic system using 9,10-phenanthrenequinone (PQ) both as a catalyst and as a hydrogen atom transfer (HAT) reagent, which avoids the use of an external HAT reagent, metal reagent, and oxidant. A variety of acylated 3-CF3-2-oxindoles have been obtained in satisfactory yields from CF3-substituted N-arylacrylamides via a tandem radical cyclization.

Direct decarboxylative Giese amidations: photocatalytic vs. metal- and light-free

A direct intermolecular decarboxylative Giese amidation reaction from bench stable, non-toxic and environmentally benign oxamic acids has been developed, which allows for easy access to 1,4-difunctionalised compounds which are not otherwise readily accessible. Crucially, a more general acceptor substrate scope is now possible, which renders the Giese amidation applicable to more complex substrates such as natural products and chiral building blocks. Two different photocatalytic methods (one via oxidative and the other via reductive quenching cycles) and one metal- and light-free method were developed and the flexibility provided by different conditions proved to be crucial for enabling a more general substrate scope.

Cascade Cyclizations Triggered by Photochemically Generated Carbamoyl Radicals Derived from Alkyl Amines

We report on a visible light-mediated cascade carbamoylation/cyclization of acrylamides using dihydropyridyl carbamoyl donors derived from alkyl amines. Diversely selected acrylamides including 2-cyano-N-arylacrylamides, indolyl- and benzimidazolyl acrylamides, and 2-alkynyl-N-aryl acrylamides participate in this reaction, providing products in good yields. The highlights of this photochemical method include the application of alkyl amine-derived carbamoyl donors, peroxide-free reaction conditions, and a broad scope.

PIDA-mediated Oxidative Decarboxylation of Oxamic Acids. The Role of Radical Acidity Enhancement

The PIDA-mediated oxidative decarboxylation of oxamic acids in the presence of alcohols is shown to afford the corresponding urethanes under thermal conditions. Computational and experimental mechanistic exploration allows to rationalize the different reactivity of PIDA as compared to related cyclic BI-OAc and highlights the importance of the enhanced acidity of the proton in the carbamoyl radical intermediate.

Kinetics-Based Quantification of Peroxynitrite Using the Oxidative Decarbonylation of Isatin

Peroxynitrite and its radical decomposition products are highly reactive nitrogen and oxygen species that can influence the balance between health and disease in multiple organ systems. Despite vigorous research activity, real-time quantitative monitoring of peroxynitrite generated by donor compounds remains challenging. Here, we report a kinetics-based fluorescence method for quantitative tracking of peroxynitrite generation using the oxidative decarbonylation of isatin to form anthranilic acid as a fluorescent probe. This method relies on knowledge of the rate of the reaction of peroxynitrite with the probe, which we measure using stopped-flow fluorescence techniques. To the best of our knowledge, this is the first optical method capable of providing real-time quantitative measures of peroxynitrite concentrations generated from donor compounds, as demonstrated herein for SIN-1 and Angeli's salt.

Direct Minisci-Type C–H Amidation of Purine Bases

A method for the C–H carboxyamidation of purines has been developed that is capable of directly installing primary, secondary, and tertiary amides. Previous Minisci-type investigations on purines were limited to alkylations and arylations. Herein, we present the first method for the direct C–H amidation of a wide range of purines: xanthine, guanine, and adenine structures, including guanosine- and adenosine-type nucleosides. The Minisci-type reaction is also metal-free, cheap, operationally simple, scalable, and applicable to late-stage functionalizations of biologically important molecules.