Direct Minisci-Type C–H Amidation of Purine Bases

Synthesis of amide-functionalized isoquinoline derivatives by photo-induced carbamoyl radical cascade amidation/cyclization

Isoquinoline-1,3-dione derivatives are of paramount importance in pharmaceutical research due to their versatile bioactivities, including notable anti-tumor and antibacterial properties. This study developed a novel method to synthesize amide-functionalized isoquinoline derivatives by a cascade amidation/cyclization of N-(methacryloyl)benzamide with the carybamonyl radical, generated from oxamic acids with the organic photosensitizer 4CzIPN. Mechanistic investigations, supported by radical scavenger experiments and HRMS analysis, unequivocally established a radical-mediated reaction pathway, with control studies validating the proposed cyclization cascade. This protocol offers distinct advantages including mild reaction conditions, environmental benignity, and broad substrate scope for the synthesis of amide-functionalized isoquinoline-1,3-diones. Furthermore, this synthetic platform has been successfully extended to the synthesis of amide-functionalized oxindoles and succinimides bearing α-quaternary carbon centers, underscoring its broad synthetic utility.

Synthesis of Purine-1,4,7,10-Tetraazacyclododecane Conjugate and Its Complexation Modes with Copper(II)

Purine-1,4,7,10-tetraazacyclododecane (cyclen) conjugate was designed to study its Cu2+ ions complexation capability. Several synthetic approaches were tested to achieve the target compound. The optimal approach involved stepwise modifications of purine N9, C8, and C6 positions that, in nine consecutive steps, provided purine-cyclen conjugate. The synthetic sequence involved Mitsunobu-type alkylation at N9 and iodination at C8, followed by Stille, SNAr, CuAAC, and alkylation reactions. The designed purine-cyclen conjugate is able to complex Cu2+ ions in both the cyclen part and between the purine N7 and triazole N2 positions. The complexation pattern and equilibrium were studied using the NMR titration technique in MeCN-d3 and absorption spectra.

Late-stage C-H trifluoroacetylation of quinoxaline-2(1H)-ones using masked trifluoroacyl reagents

A strategy for trifluoroacetylation of quinoxaline-2(1H)-ones has been investigated. This strategy employs masked trifluoroacyl reagents to obtain trifluoroacetylated quinoxaline-2(1H)-ones under metal-, catalyst-, and light-free conditions. This approach is distinguished by its functional group compatibility and tolerance, as well as the simplicity of the experimental process, making it suitable for gram-scale synthesis.

Photo-induced amidation/Smiles rearrangement of alkenes for synthesizing quaternary-carbon-containing succinyldiamides

Succinyldiamide derivatives are important structural motifs in various natural products, pharmaceuticals, and functional materials. Herein, a novel, mild, and environmentally friendly method was developed for synthesizing functionalized succinyldiamides each containing a quaternary carbon center. This strategy was designed to involve photocatalytic decarboxylation of readily available oxalic monoamide using a non-precious metal photocatalyst, 4CzIPN, followed by a free-radical addition/Smiles rearrangement cascade reaction of N-aryl-N-(arylsulfonyl)acrylamides. This method was found to display several advantageous features including compatibility with many substrates, easily accessible starting materials, and operational simplicity.

Light-Mediated Direct Decarboxylative Giese Aroylations without a Photocatalyst

Previous light-mediated approaches to the direct decarboxylative Giese aroylation reaction have mainly relied on the use of a photocatalyst and a reductive quenching pathway. By exploiting a mechanistically distinct oxidative protocol, we have successfully developed a photocatalyst-free, light-mediated direct Giese aroylation methodology.

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.

Regioselective Homolytic C2-H Borylation of Unprotected Adenosine and Adenine Derivatives via Minisci Reaction

A Minisci-type borylation of unprotected adenosine, adenine nucleotide, and adenosine analogues was successfully achieved through photocatalysis or thermal activation. Despite the challenges posed by the presence of two potential reactive sites (C2 and C8) in the purine motif, the unique nucleophilic amine-ligated boryl radicals effortlessly achieved excellent C2 site selectivity and simultaneously avoided the formation of multifunctionalized products. This protocol proved effective for the late-stage borylation of some important biomolecules as well as a few antiviral and antitumor drug molecules, such as AMP, cAMP, Vidarabine, Cordycepin, Tenofovir, Adefovir, GS-441524, etc. Theoretical calculations shed light on the site selectivity, revealing that the free energy barriers for the C2-Minisci addition are further lowered through the chelation of additive Mg2+ to N3 and furyl oxygen. This phenomenon has been confirmed by an IGMH analysis. Preliminary antitumor evaluation, derivation of the C2-borylated adenosine to other analogues with high-value functionalities, along with the CuAAC click reactions, suggest the potential application of this methodology in drug molecular optimization studies and chemical biology.

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).

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.

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.