Large-Scale Applications of Amide Coupling Reagents for the Synthesis of Pharmaceuticals

DMTMM-mediated amidation of sodium alginate in aqueous solutions: pH-dependent efficiency of conjugation

DMTMM-mediated amidation of sodium alginate is one of the methods used for the chemical modification of alginate with amines. However, there is a limited understanding of how the reaction conditions, particularly the pH value, influence the conjugation efficiency (CE) and the resulting degree of substitution (DS). In this study, we investigated the effect of the pH during the reaction, focusing on both neutral and weakly basic conditions, using water and buffer as solvents. Two model amines with high pKaH values were selected, furfurylamine (FFA, pKaH = 9.12) and 4-(2-aminoethyl)morpholine (AEM, pKaH = 9.93). Sodium alginate with a high mannuronate content (60 mol%) and molar mass of 168 kg·mol-1 was used for amidation. Our results show that both FFA and AEM effectively conjugate to sodium alginate under the selected reaction conditions. We found that pH significantly affects both CE and DS, which varied between 2 % to 40 % and 3 % to 53 %, respectively, depending on the specific reaction conditions. Optimal conditions were observed at neutral pH in water, whereas weak basic pH led to lower CE. Our findings thus offer a recommendation for optimizing the DMTMM-mediated amidation of sodium alginate, emphasizing the importance of pH values during the reaction.

Development of a Triazinyluronium-Based Dehydrative Condensing Reagent with No Heteroatomic Bonds

A triazinyluronium-based dehydrative condensing reagent, 2-(4,6-dimethoxy-1,3,5-triazin-2-yl)-1,1,3,3-tetramethyluronium hexafluorophosphate (DMT-TU), has been developed. Unlike commonly used guanidinium- and uronium-based reagents, DMT-TU does not contain high-energy N-N and N-O bonds, reducing its explosivity, as suggested by differential scanning calorimetry. Using DMT-TU in the presence of iPr2EtN at room temperature, carboxylic acids and amines were effectively converted to their corresponding amides. Additionally, peptide bond formation with DMT-TU exhibited suppressed racemization ratios.

A practical method for the synthesis of small peptides using DCC and HOBt as activators in H2O-THF while avoiding the use of protecting groups

The synthesis of peptides in solution proceeds through successive steps involving the removal of a protecting group and the formation of the peptide bond. While most methodological efforts have focused on the development of new protecting groups and coupling agents, methodologies based on minimal protecting groups have been less explored. In this research, a peptide synthesis methodology was developed using DCC and HOBt in THF-H2O, avoiding the use of protecting groups, reducing reaction times, and reusing HOBt during successive couplings. The reaction conditions allow the production of peptides that can directly serve as the starting material for the next coupling, leading to the creation of small peptide sequences, which in turn are precursors to biologically important molecules. Here we explore the example of Sansalvamide as a template for other active peptides. Unlike SPPS, our methodology constructs the sequence from the N-terminus to C-terminus. This unique approach could streamline peptide synthesis and facilitate the development of complex peptides efficiently.

Aminocarbonylation using CO surrogates

Aminocarbonylation reactions play a critical role in the synthesis of amides. Traditional aminocarbonylation processes often rely on carbon monoxide (CO) gas, a highly toxic and challenging reagent to handle. Recent advancements in CO surrogates address these challenges. This review looks at the various CO substitutes used in aminocarbonylation reactions. These include metal carbonyls, acids, formates, chloroform, and others that release CO. Use of CO surrogates not only improves safety but also broadens the substrate scope and operational simplicity of the aminocarbonylation reactions. This review provides a summary of recent progress made in aminocarbonylation reactions using different CO surrogates. We discuss key methodologies, catalytic systems, and mechanistic insights, highlighting the efficiency and versatility of CO surrogates in amide bond formation.

General chemoselective hindered amide coupling enabled by TCFH-catalytic Oxyma and transient imine protection

We report a general chemoselective strategy for amide bond formation with poorly nucleophilic amines in the presence of reactive primary alcohols or amines as the competing nucleophiles. The selectivity for less reactive amines over competing alcohols was achieved using TCFH and catalytic Oxyma as a highly reactive, inexpensive, and safe reagent combination. By temporarily masking more reactive amines as imines through the use of electron-deficient aldehydes, the hindered amines could be similarly coupled with high efficiency and selectivity.

A review of the known MTA-cooperative PRMT5 inhibitors

Protein arginine methyltransferase 5 (PRMT5), an epigenetic target with significant clinical potential, is closely associated with the occurrence and development of a range of tumours and has attracted considerable interest from the pharmaceutical industry and academic research communities. According to incomplete statistics, more than 10 PRMT5 inhibitors for cancer therapy have entered clinical trials in recent years. Among them, the second-generation PRMT5 inhibitors developed based on the synthetic lethal strategy demonstrate considerable clinical application value. This suggests that, following the precedent of poly ADP ribose polymerase (PARP), PRMT5 has the potential to become the next clinically applicable synthetic lethal target. However, due to the inherent dose-limiting toxicity of epigenetic target inhibitors, none of these PRMT5 inhibitors has been approved for marketing to date. In light of this, we have conducted a review of the design thoughts and the structure-activity relationship (SAR) of known methylthioadenosine (MTA)-cooperative PRMT5 inhibitors. Additionally, we have analysed the clinical safety of representative first- and second-generation PRMT5 inhibitors. This paper discusses the in vivo vulnerability of the aromatic amine moiety of the second-generation PRMT5 inhibitor based on its structure. It also considers the potential nitrosamine risk factors associated with the preparation process.

A benzo[b]thiophene-derived inhibitor of virus particle assembly via targeting capsid protein residue Arg157

As a biological macromolecule, the coat protein (CP) of potato virus Y (PVY) mediates the virus' primary pathogenic behaviors. It has been gradually realized that certain residues on the CP are crucial for functions such as virus particle movement and assembly. However, there are few reports of potential drugs successfully targeting these key residues with unique mechanisms of action. Here, we disclose the first new phytovirucide that acts on the key site Arg157 (R157) on the PVY CP. In this investigation, we developed a series of benzo[b]thiophene-based compounds, strategically introducing sulfonamide functionalities to enhance their antiviral performance. Through bio-screening, derivative C54 (EC50 = 69.2 μg/mL for inactive activity) emerged as notably more effective against PVY than the established antiviral agent ningnanmycin (EC50 = 79.6 μg/mL). Mechanistic studies revealed that C54 is an inhibitor of viral particle assembly by specifically binding to the CP residue R157, thereby disrupting its interaction with RNA. These results underscore the promise of C54 as a potent antiviral lead and provide a fresh perspective on the strategic design of inhibitors focusing on viral assembly processes.

Synthesis of Amide-Containing Indolo[2,1-a]isoquinoline Derivatives via Palladium-Catalyzed Cascade Reactions

A convenient palladium-catalyzed cascade cyclization reaction for the construction of amide-containing indolo[2,1-a]isoquinolines from 2-aryl-N-propenyl indoles with aromatic amines and chloroform is described. A variety of amide-containing indolo[2,1-a]isoquinoline compounds have been successfully synthesized in moderate to good yields in which chloroform is employed as the source of CO.

Ligand-controlled palladium-catalyzed regiodivergent aminocarbonylation of tert-alcohols

Alcohols are widely available, abundant, and diverse in both commercial and natural resources. They possess low toxicity, making their use as reactants for carbonylation extremely promising. Herein, we present a robust ligand-controlled regioselective aminocarbonylation of tert-alcohols. Utilizing a commercially available palladium salt and ligand as the catalytic system, various amides containing an α-quaternary carbon or β-substituted amides can be selectively accessible. Notably, water is the only by-product of this reaction, which is consistent with the concept of green chemistry. This protocol offers a broad substrate scope, high regioselectivity, and excellent performance in scale-up reactions.

Naphthyl-functionalized acetamide derivatives: Promising agents for cholinesterase inhibition and antioxidant therapy in Alzheimer's disease

This study presents the synthesis and characterization of a series of 13 novel acetamides. These were subjected to Ellman's assay to determine the efficacy of the AChE and BChE inhibitors. Finally, we report their antioxidant activity as an alternative approach for the search for drugs to treat AD. These studies revealed that compounds 1a-1k and 2l-2m were obtained in moderate yield. Four amides (1h, 1j, 1k, and 2l) were selective for one of the enzymes (BChE); thus, those that inhibited BChE were more active than the positive control (galantamine) and showed better IC50 values (3.30-5.03 µM). The theoretical free binding energies calculated by MM-GBSA indicated that all inhibitors were more stable than rivastigmine, and the inhibition mechanisms involved the entire active site: peripheral anionic site, oxyanion hole, acyl-binding pockets, and catalytic site. We examined the cytotoxicity of compounds 1h, 1j, 1k, and 2l in human dermal cells and found that they did not exhibit any toxic effects under the tested conditions. Additionally, these compounds, which also inhibited BChE, displayed mixed inhibition and did not exhibit hemolytic effects on human erythrocytes. Furthermore, the ABTS and DPPH assays indicated that, although none of the compounds showed activity in the DPPH assay, the EC50 values for radical trapping by the ABTS method showed that compounds 1a, 1d, 1e, and 1g had EC50 values lower than 10 µg/mL, indicating their strong radical scavenging capacity. We also report the crystal structures of compounds 1c, 1d, 1f, and 1g, which are found in monoclinic crystal systems.

Rapid prediction of conformationally-dependent DFT-level descriptors using graph neural networks for carboxylic acids and alkyl amines

Data-driven reaction discovery and development is a growing field that relies on the use of molecular descriptors to capture key information about substrates, ligands, and targets. Broad adaptation of this strategy is hindered by the associated computational cost of descriptor calculation, especially when considering conformational flexibility. Descriptor libraries can be precomputed agnostic of application to reduce the computational burden of data-driven reaction development. However, as one often applies these models to evaluate novel hypothetical structures, it would be ideal to predict the descriptors of compounds on-the-fly. Herein, we report DFT-level descriptor libraries for conformational ensembles of 8528 carboxylic acids and 8172 alkyl amines towards this goal. Employing 2D and 3D graph neural network architectures trained on these libraries culminated in the development of predictive models for molecule-level descriptors, as well as the bond- and atom-level descriptors for the conserved reactive site (carboxylic acid or amine). The predictions were confirmed to be robust for an external validation set of medicinally-relevant carboxylic acids and alkyl amines. Additionally, a retrospective study correlating the rate of amide coupling reactions demonstrated the suitability of the predicted DFT-level descriptors for downstream applications. Ultimately, these models enable high-fidelity predictions for a vast number of potential substrates, greatly increasing accessibility to the field of data-driven reaction development.

Homologation of Carboxylic Acids Using a Radical-Polar Conjunctive Reagent

Homologations of organic molecules that add a carbon atom to the substrate are useful in drug discovery to access compounds with improved properties that otherwise present a synthetic challenge. Carboxylic acids are present in many bioactive molecules and are widely available building blocks for chemical synthesis, yet their direct homologation is unknown. This valuable transformation currently necessitates implementation of multistep processes that require the use of carboxylic acid derivatives rather than the native substrates, and commonly involves highly reactive and toxic reagents. Herein, we report the first one-step homologation directly from native carboxylic acids using a novel, bench-stable (1-phosphoryl)vinyl sulfonate reagent under mild conditions. This strategy was applied to a wide range of aliphatic carboxylic acid building blocks and biologically relevant complex molecules to access an array of ester, amide, and carboxylic acid homologues in a single step. The (1-phosphoryl)vinyl sulfonate reagent participates in complementary homologation protocols featuring either radical-chain transfer or organic photoredox catalysis and introduces a new synthon, the distonic acylium radical, for molecular diversification. We anticipate this strategy, which addresses a long-standing challenge in organic synthesis, will expedite drug discovery by enabling the rapid synthesis of diversified homologues.

Leveraging the Redox Promiscuity of Nickel To Catalyze C-N Coupling Reactions

This perspective details advances made in the field of Ni-catalyzed C-N bond formation. The use of this Earth abundant metal to decorate amines, amides, lactams, and heterocycles enables direct access to a variety of biologically active and industrially relevant compounds in a sustainable manner. Herein, different strategies that leverage the propensity of Ni to facilitate both one- and two-electron processes will be surveyed. The first part of this Perspective focuses on strategies that facilitate C-N couplings at room temperature by accessing oxidized Ni(III) intermediates. In this context, advances in photochemical, electrochemical, and chemically mediated processes will be analyzed. A special emphasis has been put on providing a comprehensive explanation of the different mechanistic avenues that have been proposed to facilitate these chemistries; either Ni(I/III) self-sustained cycles or Ni(0/II/III) photochemically mediated pathways. The second part of this Perspective details the ligand designs that also enable access to this reactivity via a two-electron Ni(0/II) mechanism. Finally, we discuss our thoughts on possible future directions of the field.

Synthesis of N-heterocyclic amides from imidazoheterocycles through convergent paired electrolysis

An efficient ring opening of imidazoheterocycles induced by a direct C-H azidation resulting in an unusual formation of N-heterocyclic amides has been successfully developed through convergent paired electrolysis. A broad scope of pyridylbenzamides could be obtained in moderate to excellent yields under exogenous-oxidant, electrolyte- and metal-free electrochemical conditions. The methodology was transferred to continuous flow conditions resulting in notable improvements particularly in terms of cost-efficiency over traditional batch versions.

Development of the Esterase PestE for Amide Bond Synthesis Under Aqueous Conditions: Enzyme Cascades for Converting Waste PET into Tamibarotene

A growing number of hydrolase enzymes show promiscuous acyltransferase activity, even under aqueous conditions. Here we report, for the first time, the ability of Pyrobaculum calidifontis VA1 esterase (PestE) to catalyse the formation of a wide range of amides in buffer, where the acyl donor forms a significant structural component in the amide product. The reactions occur under mild conditions and can achieve conversions up to 97 % in 6 h for formation of N-benzylfuranamide as the model reaction. We demonstrate PestE's potential in enzyme cascades to make amides from waste PET plastic and the conversion of the terephthalic acid product to tamibarotene, a drug with activity against acute leukemia. Rational mutagenesis led to identification of PestE variants F33L F289A and F33L. F33L F289A increased conversion of N-benzylfuranamide by 1.2-fold, and F33L gave a 4-fold increase in conversion to tamibarotene.

Base-Promoted Aminoamidation of Cinnamoyl Chlorides with Aryl Amines: Access to β-Amino Amides

Herein, a base-promoted strategy for the synthesis of β-amino acids derivatives from α,β-unsaturated acyl chlorides derivatives and aryl amines has been described. In the presence of triethylamine, a tandem Michael addition and nucleophilic substitution progress was generated. The current method features readily available raw materials, mild reaction conditions, high atom economy, and wide tolerance for the coupling partners.

Organoboron catalysis for direct amide/peptide bond formation

Amides and peptides are ubiquitous functional groups found in several natural and artificial materials, and they are essential for the advancement of life and material sciences. In particular, their relevance in clinical medicine and drug discovery has increased in recent years. Dehydrative condensation of readily available carboxylic acids with amines is the most "direct" method for amide synthesis; however, this methodology generally requires a stoichiometric amount of condensation agent (coupling reagent). Catalytic direct dehydrative amidation has become an "ideal" methodology for synthesizing amides from the perspective of green chemistry, with water as the only byproduct in principle, high atom efficiency, environmentally friendly, energy saving, and safety. Conversely, organoboron compounds, such as boronic acids, which are widely used in various industries as coupling reagents for Suzuki-Miyaura cross-coupling reactions or pharmaceutical structures, are environmentally friendly molecules that have low toxicity and are easy to handle. Based on the chemical properties of organoboron compounds, they have potential Lewis acidity and the ability to form reversible covalent bonds with dehydration, making them attractive as catalysts. This review explores studies on the development of direct dehydrative amide/peptide bond formation reactions from carboxylic acids using organoboron catalysis, classifying them based on chemical bonding and catalysis over approximately 25 years, from the early developmental days to 2023.

Enabling Modular Click Chemistry Library through Sequential Ligations of Carboxylic Acids and Amines

High-throughput synthesis and screening of chemical libraries play pivotal roles in drug discovery. Click chemistry has emerged as a powerful strategy for constructing highly modular chemical libraries. However, the development of new click reactions and unlocking new clickable building blocks remain exceedingly challenging. Herein, we describe a double-click strategy that enables the sequential ligations of widely available carboxylic acids and amines with fluorosulfuryl isocyanate (FSO2NCO) via a modular amidation/SuFEx (sulfur-fluoride exchange) process. This method provides facile access to chemical libraries of N-fluorosulfonyl amides (RCONHSO2F) and N-acylsulfamides (RCONHSO2NR'R'') in near-quantitative yields under simple and practical conditions. The robustness and efficiency of this double click strategy is showcased by the facile construction of chemical libraries in 96-well microtiter plates from a large number of carboxylic acids and amines. Preliminary biological activity screening reveals that some compounds exhibit high antimicrobial activities against Gram-positive bacterium S. aureus and drug-resistant MRSA (MIC up to 6.25 μg ⋅ mL-1). These results provide compelling evidence for the potential application of modular click chemistry library as an enabling technology in high-throughput medicinal chemistry.

Nickel-Catalyzed Reductive Amidation of Aryl Fluorosulfates with Isocyanates: Synthesis of Amides via C-O Bond Cleavage

With the assistance of nickel as catalyst, 2,2'-bipyridine (bpy) as ligand, and manganese as reducing metal, the reductive amidation of isocyanates with readily accessible aryl fluorosulfates could be successfully accomplished. The reactions proceeded effectively via C-O bond activation in DMF at room temperature, enabling the facile synthesis of a range of structurally diverse amides in moderate to high yields with broad functionality compatibility. In addition, the synthetic usefulness of the method was further demonstrated by applying the reaction in scale-up synthesis and the late-stage functionalization of complex molecules with biological activities.

To homeostasis and beyond! Recent advances in the medicinal chemistry of heterobifunctional derivatives

The field of induced proximity therapeutics has expanded dramatically over the past 3 years, and heterobifunctional derivatives continue to form a significant component of the activities in this field. Here, we review recent advances in the field from the perspective of the medicinal chemist, with a particular focus upon informative case studies, alongside a review of emerging topics such as Direct-To-Biology (D2B) methodology and utilities for heterobifunctional compounds beyond E3 ligase mediated degradation. We also include a critical evaluation of the latest thinking around the optimisation of physicochemical and pharmacokinetic attributes of these beyond Role of Five molecules, to deliver appropriate therapeutic exposure in vivo.

BF3·OEt2-mediated transamidation of unprotected primary amides under solvent-free conditions

A BF3·OEt2-mediated transamidation between unactivated amides and amines is reported, enabling access to diverse secondary and tertiary amides under transition-metal-free and solvent-free conditions. The operationally simple procedure provides a novel manifold for converting amide-amide bonds with excellent chemoselectivity. In particular, a series of amides including challenging thioamides enable direct transamidation to products with modest to excellent yields. Meanwhile, additional experiments were conducted to elucidate the mechanism of this transformation, and a plausible mechanism was proposed based on the results and related literature.

Simultaneous reaction- and analytical model building using dynamic flow experiments to accelerate process development

In modern pharmaceutical research, the demand for expeditious development of synthetic routes to active pharmaceutical ingredients (APIs) has led to a paradigm shift towards data-rich process development. Conventional methodologies encompass prolonged timelines for the development of both a reaction model and analytical models. The development of both methods are often separated into different departments and can require an iterative optimization process. Addressing this issue, we introduce an innovative dual modeling approach, combining the development of a Process Analytical Technology (PAT) strategy with reaction optimization. This integrated approach is exemplified in diverse amidation reactions and the synthesis of the API benznidazole. The platform, characterized by a high degree of automation and minimal operator involvement, achieves PAT calibration through a "standard addition" approach. Dynamic experiments are executed to screen a broad process space and gather data for fitting kinetic parameters. Employing an open-source software program facilitates rapid kinetic parameter fitting and additional in silico optimization within minutes. This highly automated workflow not only expedites the understanding and optimization of chemical processes, but also holds significant promise for time and resource savings within the pharmaceutical industry.

1-Iodoglycal: A Versatile Intermediate for the Synthesis of d-Glyco Amides and Esters Employing Carbonylative Cross-Coupling Reaction

In this study, we present the development of two catalytic processes: a Pd-PEPPSI-catalyzed aminocarbonylation and a Pd(OAc)2-Xantphos-catalyzed alkoxycarbonylation of d-glycals, utilizing carbonylative cross-coupling reactions. We explored successfully various types of aromatic amines, as well as alkyl amines and amino acids, to synthesize new d-glycal amides. However, we observed limitations in the reactivity of alkyl and heteroaromatic amines. The processes enabled the synthesis of 20 novel C1-branched glycoamides and 7 new d-gluco esters.

Sulfur-Mediated Decarboxylative Amidation of Cinnamic Acids via C═C Bond Cleavage

A new strategy for the synthesis of amides has been developed using sulfur-mediated decarboxylative coupling of cinnamic acids with amines via oxidative cleavage of the C═C bond.

Visible-light-induced Ritter-type amidation of α-hydroxy ketones in the selective synthesis of α,α-diamido and monoamido ketones

Visible light-induced, transition metal-free oxidative dehydroxylation and C-H amidation of α-hydroxy ketones involving Ritter-type amidation has been developed, leading to the selective synthesis of α,α-diamido- and α-monoamido ketones with tunable selectivity as well as broad substrate tolerance.

Effect of Brønsted Acids on the Activation of Mixed Anhydride/Mixed Carbonic Anhydride and C-Terminal-Free N-Methylated Peptide Synthesis in a Micro-Flow Reactor

Amidations employing mixed (carbonic) anhydrides have long been favoured in peptide synthesis because of their cost-effectiveness and less waste generation. Despite their long history, no study has compared the effects of additives on the activation of mixed anhydrides and carbonic anhydrides. In this study, we investigated the amidation of mixed (carbonic) anhydride in the presence of a base and/or Brønsted acids. The use of NMI⋅HCl significantly improved the conversion of the mixed carbonic anhydride, while expediting nucleophilic attacks on the desired carbonyl group. In contrast, in the case of mixed anhydrides, neither the conversion nor the desired nucleophilic attack improved significantly. We developed a C-terminus-free N-methylated peptide synthesis method using mixed carbonic anhydrides in a micro-flow reactor. Fourteen N-alkylated peptides were synthesized in moderate to high yields (55-99 %) without severe racemization (<1 %). Additionally, a significant enhancement in the amidation between mixed carbonic anhydrides and bis-TMS-protected N-methyl amino acids with the inclusion of NMI⋅HCl was observed for the first time. In addition, we observed unexpected C-terminal epimerization of the C-terminus-free N-methyl peptides.

Aerobic Oxidative Synthesis of Formamides from Amines and Bioderived Formyl Surrogates

Herein we present a new strategy for the oxidative synthesis of formamides from various types of amines and bioderived formyl sources (DHA, GLA and GLCA) and molecular oxygen (O2) as oxidant on g-C3N4 supported Cu catalysts. Combined characterization data from EPR, XAFS, XRD and XPS revealed the formation of single CuN4 sites on supported Cuphen/C3N4 catalysts. EPR spin trapping experiments disclosed ⋅OOH radicals as reactive oxygen species and ⋅NR1R2 radicals being responsible for the initial C-C bond cleavage. Control experiments and DFT calculations showed that the successive C-C bond cleavage in DHA proceeds via a reaction mechanism co-mediated by ⋅NR1R2 and ⋅OOH radicals based on the well-equilibrated CuII and CuI cycle. Our catalyst has much higher activity (TOF) than those based on noble metals.

Biocatalytic Heteroaromatic Amide Formation in Water Enabled by a Catalytic Tetrad and Two Access Tunnels

The amide moiety belongs to the most common motives in pharmaceutical chemistry, present in many prescribed small-molecule pharmaceuticals. Methods for its manufacture are still in high demand, especially using water/buffer as a solvent and avoiding stoichiometric amounts of activation reagents. Herein, we identified from a library of lipases/esterases/acyltransferases and variants thereof a lipase originating from Sphingomonas sp. HXN-200 (SpL) able to form amides in aqueous solution starting from a broad scope of sterically demanding heteroaromatic ethyl esters as well as aliphatic amines, reaching isolated yields up to 99% on preparative scale and space time yields of up to 864 g L-1 d-1; thus, in selected cases, the amide was formed within minutes. The enzyme features an aspartate next to the canonical serine of the catalytic triad, which was essential for amide formation. Furthermore, the enzyme structure revealed two tunnels to the active site, presumably one for the ester and one for the amine, which permit the bringing together of the sterically demanding heteroaromatic esters and the amine in the active site. This work shows that biocatalytic amide formation starting from various five- and six-membered heteroaromatic ethyl esters in the buffer can serve as a platform for preparative amide synthesis.

Electrochemical Dehydration of Dicarboxylic Acids to Their Cyclic Anhydrides

An intramolecular electrochemical dehydration reaction of dicarboxylic acids to their cyclic anhydrides is presented. This electrolysis allows dicarboxylic acids as naturally abundant, inexpensive, safe, and readily available starting materials to be transformed into carboxylic anhydrides under mild reaction conditions. No conventional dehydration reagent is required. The obtained cyclic anhydrides are highly valuable reagents in organic synthesis, and in this report, we use them in-situ for acylation reactions of amines to synthesize amides. This work is part of the recent progress in electrochemical dehydration, which - in contrast to electrochemical dehydrogenative reactions for example - is an underexplored field of research. The reaction mechanism was investigated by 18O isotope labeling, revealing the formation of sulfate by electrochemical oxidation and hydrolysis of the thiocyanate-supporting electrolyte. This transformation is not a classical Kolbe electrolysis, because it is non-decarboxylative, and all carbon atoms of the carboxylic acid starting material are contained in the carboxylic anhydride. In total, 20 examples are shown with NMR yields up to 71 %.

Masters of Mediation: MN(SiMe3)2 in Functionalization of C(sp3)-H Latent Nucleophiles

Organoalkali compounds have undergone a far-reaching transformation being a coupling partner to a mediator in unusual organic conversions which finds its spot in the field of sustainable synthesis. Transition-metal catalysis has always been the priority in C(sp3)-H bond functionalization, however alternatively, in recent times this has been seriously challenged by earth-abundant alkali metals and their complexes arriving at new sustainable organometallic reagents. In this line, the importance of MN(SiMe3)2 (M=Li, Na, K & Cs) reagent revived in C(sp3)-H bond functionalization over recent years in organic synthesis is showcased in this minireview. MN(SiMe3)2 reagent with higher reactivity, enhanced stability, and bespoke cation-π interaction have shown eye-opening mediated processes such as C(sp3)-C(sp3) cross-coupling, radical-radical cross-coupling, aminobenzylation, annulation, aroylation, and other transformations to utilize readily available petrochemical feedstocks. This article also emphasizes the unusual reactivity of MN(SiMe3)2 reagent in unreactive and robust C-X (X=O, N, F, C) bond cleavage reactions that occurred alongside the C(sp3)-H bond functionalization. Overall, this review encourages the community to exploit the untapped potential of MN(SiMe3)2 reagent and also inspires them to take up this subject to even greater heights.

A Guide for Mono-Selective N-Methylation, N-Ethylation, and N-n-Propylation of Primary Amines, Amides, and Sulfonamides and Their Applicability in Late-Stage Modification

This review provides a comprehensive overview of mono-alkylation methodologies targeting crucial nitrogen moieties - amines, amides, and sulfonamides - found in organic building blocks and pharmaceuticals. Emphasizing the intersection of chemical precision with drug discovery, the central challenge addressed is achieving one-pot mono-selective short-chain N-alkylations (methylations, ethylations, and n-propylations), preventing undesired overalkylation. Additionally, sustainable, safe, and benign alternatives to traditional alkylating agents, including alcohols, carbon dioxide, carboxylic acids, nitriles, alkyl phosphates, quaternary ammonium salts, and alkyl carbonates, are explored. This review, categorized by the nature of the alkylating agent, aids researchers in selecting suitable methods for mono-selective N-alkylation.

C-C Bond Cleavage-Induced C- to N-Acyl Transfer for Synthesis of Amides

A new approach for the preparation of amides was developed using C-C bond cleavage that initiates C- to N-acyl transfer, employing activated ketones as acylation reagents and amine nucleophiles. The reaction was operational under the coupling reagent system that is commonly utilized for peptide bond formations. The method enables practical preparation of amides using linear and cyclic ketone substrates under mild conditions.

Metal-Free Bisamidation of N-Tosylhydrazones with Carboxylic Acids Promoted by Tetrabutylammonium Iodide and tert-Butyl Hydroperoxide

A versatile reaction between N-tosylhydrazones and carboxylic acids to access bisamides is reported. This metal-free, room-temperature reaction was catalyzed by TBAI, while TBHP served as the oxidant. Broad substrate scope and good functional group tolerance are the key features of the strategy. Subsequent intramolecular N-arylation of suitably substituted bisamides readily afforded functionalized 3-indazolones.

Use of ionic liquids in amidation reactions for proteolysis targeting chimera synthesis

Selective degradation of disease-causing proteins using proteolysis targeting chimeras (PROTACs) has gained great attention, thanks to its several advantages over traditional therapeutic modalities. Despite the advances made so far, the structural chemical complexity of PROTACs poses challenges in their synthetic approaches. PROTACs are typically prepared through a convergent approach, first synthesizing two fragments separately (target protein and E3 ligase ligands) and then coupling them to produce a fully assembled PROTAC. The amidation reaction represents the most common coupling exploited in PROTACs synthesis. Unfortunately, the overall isolated yields of such synthetic procedures are usually low due to one or more purification steps to obtain the final PROTAC with acceptable purity. In this work, we focused our attention on the optimization of the final amidation step for the synthesis of an anti-SARS-CoV-2 PROTAC by investigating different amidation coupling reagents and a range of alternative solvents, including ionic liquids (ILs). Among the ILs screened, [OMIM][ClO4] emerged as a successful replacement for the commonly used DMF within the HATU-mediated amidation reaction, thus allowing the synthesis of the target PROTAC under mild and sustainable conditions in very high isolated yields. With the optimised conditions in hand, we explored the scalability of the synthetic approach and the substrate scope of the reaction by employing different E3 ligase ligand (VHL and CRBN)-based intermediates containing linkers of different lengths and compositions or by using different target protein ligands. Interestingly, in all cases, we obtained high isolated yields and complete conversion in short reaction times.

Deoxyfluorinated amidation and esterification of carboxylic acid by pyridinesulfonyl fluoride

Amide bond synthesis is one of the most used reactions in medicinal chemistry. We report an amide bond formation reaction through deoxyfluorinated carboxylic acids under mild conditions using 2-pyridinesulfonyl fluoride. The reaction procedure has been used in a one-pot synthesis of amides and esters via in situ generation of acyl fluoride. This one-pot synthetic method provides easy access to amides and esters. Using this method, we have sequentially synthesized a tetrapeptide and calceolarioside-B glycoside derivative with good yields.

Integration of caveolin-mediated cytosolic delivery and enzyme-responsive releasing of squalenoyl nanoparticles enhance the anti-cancer efficacy of chidamide in pancreatic cancer

We explored the potential of overcoming the dense interstitial barrier in pancreatic cancer treatment by enhancing the uptake of hydrophilic chemotherapeutic drugs. In this study, we synthesized the squalenoyl-chidamide prodrug (SQ-CHI), linking lipophilic squalene (SQ) with the hydrophilic antitumor drug chidamide (CHI) through a trypsin-responsive bond. Self-assembled nanoparticles with sigma receptor-bound aminoethyl anisamide (AEAA) modification, forming AEAA-PEG-SQ-CHI NPs (A-C NPs, size 116.6 ± 0.4 nm), and reference nanoparticles without AEAA modification, forming mPEG-SQ-CHI NPs (M-C NPs, size 88.3 ± 0.3 nm), were prepared. A-C NPs exhibited significantly higher in vitro CHI release (74.7 %) in 0.5 % trypsin medium compared to release (20.2 %) in medium without trypsin. In vitro cell uptake assays revealed 3.6 and 2.3times higher permeation of A-C NPs into tumorspheres of PSN-1/HPSC or CFPAC-1/HPSC, respectively, compared to M-C NPs. Following intraperitoneal administration to subcutaneous tumor-bearing nude mice, the A-C NPs group demonstrated significant anti-pancreatic cancer efficacy, inducing cancer cell apoptosis and inhibiting proliferation in vivo. Mechanistic studies revealed that AEAA surface modification on nanoparticles promoted intracellular uptake through caveolin-mediated endocytosis. This nanoparticle system presents a novel therapeutic approach for pancreatic cancer treatment, offering a delivery strategy to enhance efficacy through improved tumor permeation, trypsin-responsive drug release, and specific cell surface receptor-mediated intracellular uptake.

Process Mass Intensity (PMI): A Holistic Analysis of Current Peptide Manufacturing Processes Informs Sustainability in Peptide Synthesis

Small molecule therapeutics represent the majority of the FDA-approved drugs. Yet, many attractive targets are poorly tractable by small molecules, generating a need for new therapeutic modalities. Due to their biocompatibility profile and structural versatility, peptide-based therapeutics are a possible solution. Additionally, in the past two decades, advances in peptide design, delivery, formulation, and devices have occurred, making therapeutic peptides an attractive modality. However, peptide manufacturing is often limited to solid-phase peptide synthesis (SPPS), liquid phase peptide synthesis (LPPS), and to a lesser extent hybrid SPPS/LPPS, with SPPS emerging as a predominant platform technology for peptide synthesis. SPPS involves the use of excess solvents and reagents which negatively impact the environment, thus highlighting the need for newer technologies to reduce the environmental footprint. Herein, fourteen American Chemical Society Green Chemistry Institute Pharmaceutical Roundtable (ACS GCIPR) member companies with peptide-based therapeutics in their portfolio have compiled Process Mass Intensity (PMI) metrics to help inform the sustainability efforts in peptide synthesis. This includes PMI assessment on 40 synthetic peptide processes at various development stages in pharma, classified according to the development phase. This is the most comprehensive assessment of synthetic peptide environmental metrics to date. The synthetic peptide manufacturing process was divided into stages (synthesis, purification, isolation) to determine their respective PMI. On average, solid-phase peptide synthesis (SPPS) (PMI ≈ 13,000) does not compare favorably with other modalities such as small molecules (PMI median 168-308) and biopharmaceuticals (PMI ≈ 8300). Thus, the high PMI for peptide synthesis warrants more environmentally friendly processes in peptide manufacturing.

Cesium Carbonate Promoted Direct Amidation of Unactivated Esters with Amino Alcohol Derivatives

Cesium carbonate promoted direct amidation of unactivated esters with amino alcohols was developed without the use of transition-metal catalysts and coupling reagents. This method enabled the synthesis of several serine-containing oligopeptides and benzamide derivatives with yields up to 90%. The methodology proceeds under mild reaction conditions and exhibits no racemization for most naturally occurring amino acid substrates. The reaction demonstrates good compatibility with primary alkyl and benzyl esters and broad tolerance for a range of amino acid substrates with nonpolar and protected side chains. The hydroxy group on the amine nucleophile was found to be critical for the reaction to be successful. A likely mechanism involving cesium coordination to the substrates enabling the subsequent proximity-driven acyl transfer was proposed. The practicality of this approach was demonstrated in the preparation of a biologically active nicotinamide derivative in a reasonable yield.

Direct additive-free N-formylation and N-acylation of anilines and synthesis of urea derivatives using green, efficient, and reusable deep eutectic solvent ([ChCl][ZnCl2]2)

In the current report, we introduce a simple, mild efficient and green protocol for N-formylation and N-acetylation of anilines using formamide, formic acid, and acetic acid as inexpensive, nontoxic, and easily available starting materials just with heating along stirring in [ChCl][ZnCl2]2 as a durable, reusable deep eutectic solvent (DES), which acts as a dual catalyst and solvent system to produce a wide range of formanilides and acetanilides. Also, a variety of unsymmetrical urea derivatives were synthesized by the reaction of phenyl isocyanate with a range of amine compounds using this benign DES in high to excellent yields. [ChCl][ZnCl2]2 showed good recycling and reusability up to four runs without considerable loss of its catalytic activity.

(NH4)2S2O8 promoted tandem radical cyclization of quinazolin-4(3H)-ones with oxamic acids for the construction of fused quinazolinones under metal-free conditions

A novel cascade radical addition/cyclization reaction of non-activated olefins and oxamic acids has been proposed. Under transition metal-free conditions, 36 quinazolinone derivatives containing an amide moiety were successfully synthesized, with the highest yield being 81%. This method involves the preparation of aminoacyl fused quinazolinone derivatives under mild conditions, offering advantages such as a high yield, a broad substrate compatibility, and a high atom economy.

TiF4-catalyzed direct amidation of carboxylic acids and amino acids with amines

Unlike other metal fluorides, catalytic titanium tetrafluoride enhances the direct amidation of aromatic and aliphatic carboxylic acids and N-protected amino acids in refluxing toluene. While aromatic acids were converted to amides with 10 mol% of the catalyst within 24 h, aliphatic acids underwent a faster reaction (12 h), with lower catalyst loading (5 mol%). This protocol is equally efficient with alkyl and aryl amines providing a variety of carboxamides and peptides in 60-99% 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.

Reactive deep eutectic solvents for EDC-mediated amide synthesis

The sustainability of amide bond formation is an ever-present topic in the pharmaceutical industry, as it represents the common motif in many clinically approved drugs. Despite many procedures for accomplishing eco-friendly amide synthesis having been developed, this transformation still remains a contemporary challenge. Herein, we report a greener approach for amide synthesis by using Reactive Deep Eutectic Solvents (RDESs) acting as both the reaction medium and reactants. The procedure not only avoids the use of hazardous solvents but also provides operationally simple product recovery with high purity and efficiency, without chromatographic purification. This approach was efficiently applied to the synthesis of a key intermediate in the production of an active pharmaceutical ingredient like atenolol. The green metrics of the gram-scale procedure were compared to the conventional industrial strategy showing an advancement in the greening of amide synthesis.

Hexylsilane-mediated direct amidation of amino acids with a catalytic amount of 1,2,4-triazole

In this study, we report amino acid amidation using hexylsilane and a catalytic amount of 1,2,4-triazole. The conventional protection/deprotection method for the α-amino group of amino acids is not required. The corresponding α-amino amides were obtained in moderate to good yields with low to no racemization.

Copper-catalyzed trichloromethylative carbonylation of ethylene

Difunctionalization of alkenes is an efficient strategy for the synthesis of complex compounds from readily available starting materials. Herein, we developed a copper-catalyzed visible-light-mediated trichloromethylative carbonylation of ethylene by employing commercially available CCl4 and CO as trichloromethyl and carbonyl sources, respectively. With this protocol, various nucleophiles including amines, phenols, and alcohols can be rapidly transformed into β-trichloromethyl carboxylic acid derivatives with good functional-group tolerance. Bis-vinylated γ-trichloromethyl amides can also be obtained by adjusting the pressure of carbon monoxide and ethylene. In addition, this photocatalytic system can be successfully applied in the late-stage functionalization of bioactive molecules and pharmaceutical derivatives as well.

Broad Spectrum Enantioselective Amide Bond Synthetase from Streptoalloteichus hindustanus

The synthesis of amide bonds is one of the most frequently performed reactions in pharmaceutical synthesis, but the requirement for stoichiometric quantities of coupling agents and activated substrates in established methods has prompted interest in biocatalytic alternatives. Amide Bond Synthetases (ABSs) actively catalyze both the ATP-dependent adenylation of carboxylic acid substrates and their subsequent amidation using an amine nucleophile, both within the active site of the enzyme, enabling the use of only a small excess of the amine partner. We have assessed the ability of an ABS from Streptoalloteichus hindustanus (ShABS) to couple a range of carboxylic acid substrates and amines to form amine products. ShABS displayed superior activity to a previously studied ABS, McbA, and a remarkable complementary substrate specificity that included the enantioselective formation of a library of amides from racemic acid and amine coupling partners. The X-ray crystallographic structure of ShABS has permitted mutational mapping of the carboxylic acid and amine binding sites, revealing key roles for L207 and F246 in determining the enantioselectivity of the enzyme with respect to chiral acid and amine substrates. ShABS was applied to the synthesis of pharmaceutical amides, including ilepcimide, lazabemide, trimethobenzamide, and cinepazide, the last with 99% conversion and 95% isolated yield. These findings provide a blueprint for enabling a contemporary pharmaceutical synthesis of one of the most significant classes of small molecule drugs using biocatalysis.

Aminocarbonylation Reaction Using a Pd-Sn Heterobimetallic Catalyst: Three-Component Coupling for Direct Access of the Amide Functionality

A heterobimetallic "Pd-Sn" catalyst, namely, PdCl(PPh3)2SnCl3, efficiently catalyzes the aminocarbonylation reaction of aryl iodides with amines under the atmospheric pressure of CO in the absence of a base and additive. Primary, secondary, and alkyl amines all afforded the corresponding amides in good to excellent yields with high selectivity. A broad range of functional groups were tolerated. The method was further extended to the synthesis of biologically active isoindoline-1,3-diones in the presence of triethylamine. A mechanism is proposed for the reaction.

One-Pot Synthesis of Cereblon Proteolysis Targeting Chimeras via Photoinduced C(sp2)-C(sp3) Cross Coupling and Amide Formation for Proteolysis Targeting Chimera Library Synthesis

In this study, a one-pot synthesis via photoinduced C(sp2)-C(sp3) coupling followed by amide formation to access proteolysis targeting chimeras (PROTACs) was developed. The described protocol was studied on cereblon (CRBN)-based E3-ligase binders and (+)-JQ-1, a bromodomain inhibitor, to generate BET (bromodomain and extra terminal domain) targeting protein degraders. The generated PROTACs were profiled in vitro and tested for their degradation ability with several potent candidates identified. Upfront, the individual reactions of the one-pot transformation were carefully optimized for CRBN binder functionalization and multiple heterobifunctional linker moieties were designed and synthesized. Separate scopes detailing the C(sp2)-C(sp3) coupling and one-pot PROTAC synthesis are described in this report as well as a library miniaturization study showing the high-throughput compatibility. Overall, the developed protocol provides rapid access to PROTACs in a single process, thereby allowing efficient generation of CRBN-based PROTAC libraries.

Epimerisation in Peptide Synthesis

Epimerisation is basically a chemical conversion that includes the transformation of an epimer into another epimer or its chiral partner. Epimerisation of amino acid is a side reaction that sometimes happens during peptide synthesis. It became the most avoided reaction because the process affects the overall conformation of the molecule, eventually even altering the bioactivity of the peptide. Epimerised products have a high similarity of physical characteristics, thus making it difficult for them to be purified. In regards to amino acids, epimerisation is very important in keeping the chirality of the assembled amino acids unchanged during the peptide synthesis and obtaining the desirable product without any problematic purification. In this review, we report several factors that induce epimerisation during peptide synthesis, including how to characterise and affect the bioactivities. To avoid undesirable epimerisation, we also describe several methods of suppressing the process.

Atom-Economical Synthesis of N-Arylamides Utilizing Isopropenyl Esters with Heterogeneous Acid Catalysts

Existing methods for the catalytic synthesis of N-arylamides are limited by a narrow substrate scope, high catalyst costs, and complicated purification processes of products. To overcome these limitations, this study developed an ecofriendly method for the synthesis of N-arylamides using isopropenyl esters. Isopropenyl esters activated using heterogeneous acid catalysts reacted smoothly even with less reactive arylamines to afford N-arylamides in high yields. This method exhibits a wide substrate scope and is applicable for the synthesis of various N-arylamides (33 examples, 46-99% yield). The developed method enabled the obtainment of high-purity products with a facile workup procedure and showed excellent process mass intensity values due to the reduction of chemical waste.