Direct esterification of amides by the dimethylsulfate-mediated activation of amide C-N bonds

Amides are important intermediates in organic chemistry and the pharmaceutical industry, but their low reactivity requires catalysts and/or severe reaction conditions for esterification. Here, a novel approach was devised to convert amides into esters without the use of transition metals. The method effectively overcomes the inherent low reactivity of amides by employing dimethylsulfate-mediated reaction to activate the C-N bonds. To confirm the proposed reaction mechanism, control experiments and density functional theory (DFT) calculations were conducted. The method demonstrates a wide array of substrates, including amides with typical H/alkyl/aryl substitutions, N,N-disubstituted amides, amides derived from alkyl, aryl, or vinyl carboxylic acids, and even amino acid substrates with stereocentres. Furthermore, we have shown the effectiveness of dimethylsulfate in removing acyl protective groups in amino derivatives. This study presents a method that offers efficiency and cost-effectiveness in broadening the esterification capabilities of amides, thereby facilitating their increased utilization as synthetic compounds in diverse transformations.

Effect of Mini-PEGs Modification on the Enzymatic Digestion of D-Amino Acid-Containing Peptides under the Action of PROK

It was previously reported that D-amino acid-containing peptides exhibited the ability to resist enzymatic hydrolysis. This study investigated the influence of mini-PEGs modification on enzymatic hydrolysis ability of D-amino acid-containing peptides. The results showed that PEGylation promoted enzymatic hydrolysis of the D-amino acid-containing peptide, especially, the cleavage rate of the D-amino acid-containing peptide 6-w with PEG₃ modification at the N-ends was up to 17 times higher in the presence of proteinase K (PROK) compared to those without PEG₃ modification. Moreover, analysis of the enzymatic cleavage sites demonstrated a similar cleavage pattern of the PEGylated D-amino acid-containing peptide to that of the unmodified peptide. The computational simulations further showed that the enhanced enzymatic hydrolysis ability can be attributed to the strong interaction between PROK and the peptide after PEG₃ modification and the resulting formation of a mature catalytic triad structure.

Selective Hydrolysis of Primary and Secondary Amides Enabled by Visible Light

Amide hydrolysis is a fundamentally important transformation in organic chemistry. Developing hydrolysis procedures under mild conditions with a broad substrate scope is desirable. Herein, by leveraging a photoresponsive auxiliary o-nitroanilide, we established a mild two-step protocol for the hydrolysis of primary and secondary amides. This protocol is driven by visible light irradiation at room temperature under neutral conditions, which tolerates numerous acid- and base-sensitive functional groups. Various drugs, natural product-, and amino acid-derived amides can be selectively hydrolyzed.

Chelating Picolinaldehyde Hydrazone Amides as Protecting Groups for Carboxylic Acids: Orthogonal Reactivities of Hydrazone Amides and Esters in Hydrolysis

We report a chelating hydrazone amide as a protecting group for carboxylic acids. Unlike most esters, 2-picolinaldehyde hydrazone amides are stable under acidic or basic hydrolytic conditions. However, hydrazone amides can be easily converted to the corresponding carboxylic acids via Ni-mediated hydrolysis. Orthogonal reactivities of the hydrazone amides and representative protecting groups were verified by control experiments and peptide synthesis, demonstrating that chelating hydrazone amides are highly useful protecting groups.

Catalytic Chemical Recycling of Post-Consumer Polyethylene

Among commercial plastics, polyolefins are the most widely produced worldwide but have limited recyclability. Here, we report a chemical recycling route for the conversion of post-consumer high-density polyethylene (HDPE) into telechelic macromonomers suitable for circular reprocessing. Unsaturation was introduced into HDPE by catalytic dehydrogenation using an Ir-POCOP catalyst without an alkene acceptor. Cross-metathesis with 2-hydroxyethyl acrylate followed by hydrogenation transformed the partially unsaturated HDPE into telechelic macromonomers. The direct repolymerization of the macromonomers gave a brittle material due to the low overall weight-average molecular weight. Aminolysis of telechelic macromonomers with a small amount of diethanolamine increased the overall functionality. The resulting macromonomers were repolymerized through transesterification to generate a polymer with comparable mechanical properties to the starting post-consumer HDPE waste. Depolymerization of the repolymerized material catalyzed by an organic base regenerated the telechelic macromonomers, thereby allowing waste polyethylene materials to enter a chemical recycling pathway.

Hydrolysis of Acetamide on Low-Index CeO2 Surfaces: Ceria as a Deamidation and General De-esterification Catalyst

Using DFT calculations and acetamide as the main example, we show that ceria is a potential catalyst for the hydrolysis of amide and similar bonds. The overall reaction is endergonic in the gas phase, yielding acetic acid and ammonia, but is slightly exergonic in the aqueous phase, which facilitates ionization of the products (CH₃COO^– and NH₄⁺). Neighboring Ce and O sites on the CeO₂(111), (110), and (100) facets are conducive to the formation of an activated metastable tetrahedral intermediate (TI) complex, followed by C–N bond scission. With van der Waals and solvation effects taken into account, the overall reaction energetics is found to be most favorable on the (111) facet as desorption of acetic acid is much more uphill energetically on (110) and (100). We further suggest that the Ce–O–Ce sites on ceria surfaces can activate X(=Y)–Z type bonds in amides, amidines, and carboxylate and phosphate esters, among many others that we term “generalized esters”. A Brønsted-Evans–Polanyi relationship is identified correlating the stability of the transition and final states of the X–Z generalized ester bond scission. A simple descriptor (ΣΔχ) based on the electronegativity of the atoms that constitute the bond (X, Y, Z) versus those of the catalytic site (O, Ce, Ce) captures the trend in the stability of the transition state of generalized ester bond scission and suggests a direction for modifying ceria for targeting specific organic substrates.

Chemoselective Transamidation of Thioamides by Transition-Metal-Free N-C(S) Transacylation

Thioamides represent highly valuable isosteric in the strictest sense "single-atom substitution" analogues of amides that have found broad applications in chemistry and biology. A long-standing challenge is the direct transamidation of thioamides, a process which would convert one thioamide bond (R-C(S)-NR¹ R² ) into another (R-C(S)-NR³ N⁴ ). Herein, we report the first general method for the direct transamidation of thioamides by highly chemoselective N-C(S) transacylation. The method relies on site-selective N-tert-butoxycarbonyl activation of 2° and 1° thioamides, resulting in ground-state-destabilization of thioamides, thus enabling to rationally manipulate nucleophilic addition to the thioamide bond. This method showcases a remarkably broad scope including late-stage functionalization (>100 examples). We further present extensive DFT studies that provide insight into the chemoselectivity and provide guidelines for the development of transamidation methods of the thioamide bond.

ZnCl2/PhI=O Mediated Selective ortho-Chlorination of Amides

Abstract:

A new ortho-chlorination system consisting of zinc(II) and hypervalent iodine(III) reagent was developed for ortho-chlorination of amides, and the desired products were obtained in moderate to good yields (38-85%). This highly facile and convenient methodology is tolerant of aromatic amide and alkyl amide with diverse substituted groups. A plausible mechanism has been illustrated, in which carbocation rearrangement and metal salt coordinate facilitated orthochlorination are involved.

A Guide to Directing Group Removal: 8-Aminoquinoline

The use of directing groups allows high levels of selectivity to be achieved in transition metal-catalyzed transformations. Efficient removal of these auxiliaries after successful functionalization, however, can be very challenging. This review provides a critical overview of strategies used for removal of Daugulis' 8-aminoquinoline (2005-2020), one of the most widely used N,N-bidentate directing groups. The limitations of these strategies are discussed and alternative approaches are suggested for challenging substrates. Our aim is to provide a comprehensive end-users' guide for chemists in academia and industry who want to harness the synthetic power of directing groups-and be able to remove them from their final products.

Zr-MOF-808 as Catalyst for Amide Esterification

In this work, zirconium-based metal-organic framework Zr-MOF-808-P has been found to be an efficient and versatile catalyst for amide esterification. Comparing with previously reported homogeneous and heterogeneous catalysts, Zr-MOF-808-P can promote the reaction for a wide range of primary, secondary and tertiary amides with n-butanol as nucleophilic agent. Different alcohols have been employed in amide esterification with quantitative yields. Moreover, the catalyst acts as a heterogeneous catalyst and could be reused for at least five consecutive cycles. The amide esterification mechanism has been studied on the Zr-MOF-808 at molecular level by in situ FTIR spectroscopic technique and kinetic study.

Hydrolysis of amides to carboxylic acids catalyzed by Nb2O5

An efficient heterogeneous Nb₂O₅ catalytic system has been developed for industrially important and challenging amide hydrolysis reaction to carboxylic acid through cleavage of resonance stabilized amidic C–N bond.

Runge–Kutta analysis for optimizing the Zn-catalyzed transesterification conditions of MA and MMA with diols to maximize monoesterified products

Terminal hydroxylated acrylate derivatives were prepared by transesterification using zinc catalyst. The time to reach the equilibrium state was analyzed by curve-fitting analysis based on the Runge–Kutta method for optimizing the best conditions.

Esterification of Tertiary Amides: Remarkable Additive Effects of Potassium Alkoxides for Generating Hetero Manganese-Potassium Dinuclear Active Species

A catalyst system of mononuclear manganese precursor 3 combined with potassium alkoxide served as a superior catalyst compared with our previously reported manganese homodinuclear catalyst 2 a for esterification of not only tertiary aryl amides, but also tertiary aliphatic amides. On the basis of stoichiometric reactions of 3 and potassium alkoxide salt, kinetic studies, and density functional theory (DFT) calculations, we clarified a plausible reaction mechanism in which in situ generated manganese-potassium heterodinuclear species cooperatively activates the carbonyl moiety of the amide and the OH moiety of the alcohols. We also revealed details of the reaction mechanism of our previous manganese homodinuclear system 2 a, and we found that the activation free energy (ΔG≠ ) for the manganese-potassium heterodinuclear complex catalyzed esterification of amides is lower than that for the manganese homodinuclear system, which was consistent with the experimental results. We further applied our catalyst system to deprotect the acetyl moiety of primary and secondary amines.

Predominant cleavage of proteins N-terminal to serines and threonines using scandium(III) triflate

Abstract

Proteolytic digestion prior to LC–MS analysis is a key step for the identification of proteins. Digestion of proteins is typically performed with trypsin, but certain proteins or important protein sequence regions might be missed using this endoproteinase. Only few alternative endoproteinases are available and chemical cleavage of proteins is rarely used. Recently, it has been reported that some metal complexes can act as artificial proteases. In particular, the Lewis acid scandium(III) triflate has been shown to catalyze the cleavage of peptide bonds to serine and threonine residues. Therefore, we investigated if this compound can also be used for the cleavage of proteins. For this purpose, several single proteins, the 20S immune-proteasome (17 proteins), and the Universal Proteomics Standard UPS1 (48 proteins) were analyzed by MALDI–MS and/or LC–MS. A high cleavage specificity N-terminal to serine and threonine residues was observed, but also additional peptides with deviating cleavage specificity were found. Scandium(III) triflate can be a useful tool in protein analysis as no other reagent has been reported yet which showed cleavage specificity within proteins to serines and threonines.

Graphic abstract

Electronic supplementary material

The online version of this article (10.1007/s00775-019-01733-7) contains supplementary material, which is available to authorized users.

Catalytic Cleavage of Amide C-N Bond: Scandium, Manganese, and Zinc Catalysts for Esterification of Amides

Amide C-N bonds are thermodynamically stable and their fission, such as by hydrolysis and alcoholysis, is considered a long-challenging organic reaction. In general, stoichiometric chemical transformations of amides into the corresponding esters and acids require harsh conditions, such as strong acids/bases at a high reaction temperature. Accordingly, the development of catalytic reactions that cleave not only primary and secondary amides, but also tertiary amides in mild conditions, is in high demand. Herein, we surveyed typical stoichiometric transformations of amides, and highlight our recent achievements in the catalytic esterification of amides using scandium, manganese, and zinc catalysts, together with some recent catalyst systems using late-transition metal reported by other groups.

Base-catalyzed selective esterification of alcohols with unactivated esters

A practical and efficient base-catalyzed esterification has been developed for the facile synthesis of a broad range of esters from simple alcohols with unactivated tert-butyl esters. This protocol could be conducted at mild conditions, providing esters in high to excellent yields with good functional tolerance. Mechanistic studies provided evidence of an exchange of the tert-butyl alkoxide metal with the alcohol, producing a new alkoxide to participate in the transesterification reaction.

Amide Bond Activation of Biological Molecules

Amide bonds are the most prevalent structures found in organic molecules and various biomolecules such as peptides, proteins, DNA, and RNA. The unique feature of amide bonds is their ability to form resonating structures, thus, they are highly stable and adopt particular three-dimensional structures, which, in turn, are responsible for their functions. The main focus of this review article is to report the methodologies for the activation of the unactivated amide bonds present in biomolecules, which includes the enzymatic approach, metal complexes, and non-metal based methods. This article also discusses some of the applications of amide bond activation approaches in the sequencing of proteins and the synthesis of peptide acids, esters, amides, and thioesters.

Metal Template Assisted Proximal Arrangement of a Nucleophile and an Electrophile: Site-Selective Acylation of α-Hydroxyamides in Polyols

Site-selective acylation of α-hydroxyl groups in amides has been achieved in the presence of other primary hydroxyl groups with intrinsic high reactivity. In this methodology, a relatively stable pyridine aldoxime ester was exploited as an acyl donor to suppress undesired acylation. The catalytic activation of a pyridine aldoxime ester with a Lewis acid produced a cationic complex, which preferentially attracted the Lewis basic α-hydroxyamide via a template effect, to thus facilitate o-acylation.

Aqueous Zinc Compounds as Residual Antimicrobial Agents for Textiles

Textiles, especially those worn by patients and medical professionals, serve as vectors for proliferating pathogens. Upstream manufacturing techniques and end-user practices, such as transition-metal embedment in textile fibers or alcohol-based disinfectants, can mitigate pathogen growth, but both techniques have their shortcomings. Fiber embedment requires complete replacement of all fabrics in a facility, and the effects of embedded nanoparticles on human health remain unknown. Alcohol-based, end-user disinfectants are short-lived because they quickly volatilize. In this work, common zinc salts are explored as an end-user residual antimicrobial agent. Zinc salts show cost-effective and long-lasting antimicrobial efficacy when solution-deposited on common textiles, such as nylon, polyester, and cotton. Unlike common alcohol-based disinfectants, these zinc salt-treated textiles mitigate microbial growth for more than 30 days and withstand commercial drying. Polyester fabrics treated with ZnO and ZnCl₂ were further explored because of their commercial ubiquity and likelihood for rapid commercialization. ZnCl₂-treated textiles were found to retain their antimicrobial coating through abrasive testing, whereas ZnO-treated textiles did not. Scanning electron microscopy, Fourier transform infrared spectroscopy, and differential scanning calorimetry analyses suggest that ZnCl₂ likely hydrolyzes and reacts with portions of the polyester fiber, chemically attaching to the fiber, whereas colloidal ZnO simply sediments and binds with weaker physical interactions.

Scandium(iii) triflate-promoted serine/threonine-selective peptide bond cleavage

The site-selective cleavage of peptide bonds is an important chemical modification that is useful not only for the structural determination of peptides, but also as an artificial modulator of peptide/protein function and properties. Here we report site-selective hydrolysis of peptide bonds at the Ser and Thr positions with a high conversion yield. This chemical cleavage relies on Sc(iii)-promoted N,O-acyl rearrangement and subsequent hydrolysis. The method is applicable to a broad scope of polypeptides with various functional groups, including a post-translationally modified peptide that is unsuitable for enzymatic hydrolysis. The system was further extended to site-selective cleavage of a native protein, Aβ1-42, which is closely related to the onset of Alzheimer's disease.

Successive addition of two different Grignard reagents to nitriles: access to α,α-disubstituted propargylamine derivatives

The successive addition of two different Grignard reagents to acyl cyanohydrins was performed with success by taking advantage of the low reactivity of alkynyl Grignard reagents. The experimental conditions were adjusted so that they were not reactive during the first addition step, but reactive only in the second one. The synthetic utility of the prepared compounds was validated by the preparation of chiral quaternary α-amino acids.

Direct/Reversible Amidation of Troponyl Alkylglycinates via Cationic Troponyl Lactones and Mechanistic Insights

The conversion of troponyl alkylglycinate acid/ester/amide derivatives (Trag acid/ester/amide) into cationic troponyl lactones (CTLs) in the presence of trifluoroacetic acid and their amidation with amines is described. The reversible amidation of Trag amides, that is, the cleavage and reformation of the Trag amide bond via CTLs is demonstrated. The direct amidation of Trag esters with the amino group of amino acid esters/peptide esters via CTLs is achieved. The direct amidation of the amine group of hydroxyl amino acid esters is selective over esterification. The Trag amide bond is stable under basic ester hydrolysis and Fmoc removal conditions. Hence, the troponyl alkylglycinates could be applicable as protecting groups for amine functionality of amino acids and peptides. The reaction mechanism was investigated by using a deuterium probe and studied by NMR and electrospray ionisation mass spectrometry techniques. Deuterium incorporation at α-CH₂ strongly supported the formation of CTLs via ketene intermediates.

Fe-catalyzed esterification of amides via C–N bond activation

An efficient Fe-catalyzed esterification of primary, secondary, and tertiary amides with various alcohols was performed. Esterification was accomplished with inexpensive, environmentally friendly FeCl₃·6H₂O, and with high functional group tolerance

tert-Butyl Nitrite-Mediated Synthesis of N-Nitrosoamides, Carboxylic Acids, Benzocoumarins, and Isocoumarins from Amides

This work reports tert-butyl nitrite (TBN) as a multitask reagent for (1) the controlled synthesis of N-nitrosoamide from N-alkyl amides, (2) hydrolysis of N-methoxyamides to carboxylic acids, (3) metal- and oxidant-free benzocoumarin synthesis from ortho-aryl-N-methoxyamides via N-H, C-N, and C-H bond activation, and (4) isocoumarin synthesis using Ru(II)/PEG as a recyclable catalytic system via ortho-C-H activation and TBN as an oxygen source. The sequential functional group interconversion of amide to acid has also been examined using IR spectroscopic analysis. Additionally, this methodology is highly advantageous due to short reaction time, gram scale synthesis, and broad substrate scope.

Mild Amide-Cleavage Reaction Mediated by Electrophilic Benzylation

An extremely mild method for amide-cleavage by using the triazine-based benzylating reagent 4-(4,6-diphenoxy-1,3,5-triazin-2-yl)-4-benzylmorpholinium trifluoromethanesulfonate (DPT-BM), which spontaneously releases benzyl cation species when being dissolved at room temperature, has been developed. O-Benzylation of the amide with DPT-BM and the subsequent hydrolysis of the resulting intermediate benzyl imidate salt afford the corresponding amine and benzyl ester, which can be converted by hydrogenolysis into a carboxylic acid under neutral conditions. O-Benzylation proceeds depending on both steric and electronic factors around the amide group. Thus, some amides have been selectively cleaved over other amides. Furthermore, intramolecular chemoselective cleavage of an amide group in the presence of an ester group was achieved. Such selective hydrolytic reactions cannot be performed with Meerwein reagents as well as under acidic or basic hydrolytic conditions.

K2CO3-promoted formation of aryl esters from primary aryl amides by the acyl-acyl exchange process

A new acyl-acyl exchange reaction has been developed for the formation of aryl esters from primary aryl amides. The reaction could occur under mild reaction conditions with catalytic quantities of K2CO3, and could afford moderate to good yields of the desired products.

Site-selective chemical cleavage of peptide bonds

Site-selective cleavage of extremely unreactive peptide bonds is a very important chemical modification that provides invaluable information regarding protein sequence, and it acts as a modulator of protein structure and function for therapeutic applications. For controlled and selective cleavage, a daunting task, chemical reagents must selectively recognize or bind to one or more amino acid residues in the peptide chain and selectively cleave a peptide bond. Building on this principle, we have developed an approach that utilizes a chemical reagent to selectively modify the serine residue in a peptide chain and leads to the cleavage of a peptide backbone at the N-terminus of the serine residue. After cleavage, modified residues can be converted back to the original fragments. This method exhibits broad substrate scope and selectively cleaves various bioactive peptides with post-translational modifications (e.g. N-acetylation and -methylation) and mutations (d- and β-amino acids), which are a known cause of age related diseases.

Glutamic Acid Selective Chemical Cleavage of Peptide Bonds

Site-specific hydrolysis of peptide bonds at glutamic acid under neutral aqueous conditions is reported. The method relies on the activation of the backbone amide chain at glutamic acid by the formation of a pyroglutamyl (pGlu) imide moiety. This activation increases the susceptibility of a peptide bond toward hydrolysis. The method is highly specific and demonstrates broad substrate scope including cleavage of various bioactive peptides with unnatural amino acid residues, which are unsuitable substrates for enzymatic hydrolysis.

Green alternative solvents for the copper-catalysed arylation of phenols and amides

The use of alkyl acetates as green organic solvents for the Cu-catalysed arylation of phenols and amides is reported.

Cleavage of unactivated amide bonds by ammonium salt-accelerated hydrazinolysis

Hydrazinolysis of unactivated amide bonds is significantly accelerated by the addition of ammonium salts. The reactions proceed at 50-70 °C to give amines with broad substrate scope that outperforms existing amide bond cleavage reactions. Application to peptide and amino sugar derivatives is also demonstrated.

Site-Selective Peptide/Protein Cleavage

Site-selective peptide/protein degradation through chemical cleavage methods is an important modification of biologically relevant macromolecules which complements enzymatic hydrolysis. In this review, recent progress in chemical, site-selective peptide bond cleavage is overviewed, with an emphasis on postulated mechanisms and their implications on reactivity, selectivity, and substrate scope.