Borinic acid catalysed peptide synthesis

The low loading of metal in metal-organic framework-derived NiNx@NC promotes amide formation through C-N coupling

The pyrolysis of Ni-substituted zeolitic imidazolate framework-8 produces NiNx@NC with an ultra-low loading of Ni (7.4 × 10-6 mol%). The Ni-N coordination, subnanometer particle size, and uniform distribution of NiNx on the NC support lead to excellent catalytic activity (TON = 2702) and selectivity for the amination of aldehydes to produce amides.

Emergent Organoboron Acid Catalysts

Organoboron acids are stable, organic-soluble Lewis acids with potential application as catalysts for a wide variety of chemical reactions. In this review, we summarize the utility of boronic and borinic acids, as well as boric acid, as catalysts for organic transformations. Typically, the catalytic processes exploit the Lewis acidity of trivalent boron, enabling the reversible formation of a covalent bond with oxygen. Our focus is on recent developments in the catalysis of dehydration, carbonyl condensation, acylation, alkylation, and cycloaddition reactions. We conclude that organoboron acids have a highly favorable prospectus as the source of new catalysts.

Axially Chiral Borinic Acid Catalysts: Design, Synthesis, and Application in Alkylative Desymmetrization of 1,2-Diols

A novel chiral borinic acid (CBA), an organocatalyst possessing a binaphthyl skeleton, was designed and synthesized. The synthesis of CBA was achieved with a 72% yield in four steps starting with optically pure 1,1'-bi-2-naphthol. The asymmetric catalytic activity was investigated in the desymmetrization of meso-1,2-diol.

Concise Synthesis of 2,5-Diketopiperazines via Catalytic Hydroxy-Directed Peptide Bond Formations

2,5-Diketopiperazines (DKPs) with hydroxymethyl functional groups are essential structures found in many bioactive molecules and functional materials. We have established a simple protocol for the concise synthesis of this type of DKPs through diboronic acid anhydride-catalyzed hydroxy-directed peptide bond formations. The sequential reactions in this report, which consist of three steps, an intermolecular catalytic condensation reaction in which water is the only byproduct, a simple deprotection of the nitrogen-protecting group, and an intramolecular cyclization, enabled the synthesis of functionalized DKPs in high to excellent yields without any intermediate purification. The utility of this protocol has been demonstrated by synthesizing natural products, phomamide and Cyclo(Deala-l-Leu).

Recent Advances in the Synthesis of Borinic Acid Derivatives

Borinic acids [R2B(OH)] and their chelate derivatives are a subclass of organoborane compounds used in cross-coupling reactions, catalysis, medicinal chemistry, polymer or optoelectronics materials. In this paper, we review the recent advances in the synthesis of diarylborinic acids and their four-coordinated analogs. The main strategies to build up borinic acids rely either on the addition of organometallic reagents to boranes (B(OR)3, BX3, aminoborane, arylboronic esters) or the reaction of triarylboranes with a ligand (diol, amino alcohol, etc.). After general practical considerations of borinic acids, an overview of the main synthetic methods, their scope and limitations is provided. We also discuss some mechanistic aspects.

Peptide-based drug discovery: Current status and recent advances

The progressive development of peptides from reaction vessels to life-saving drugs via rigorous preclinical and clinical assessments is fascinating. Peptide therapeutics have gained momentum with the evolution of techniques in peptide chemistry, such as microwave irradiation in solid- and solution-phase synthesis, ligation chemistry, recombinant synthesis, and amalgamation with synthetic tools, including metal catalysis. Diverse emerging technologies, such as DNA-encoded libraries (DELs) and display techniques, are changing the status quo in the discovery of peptide therapeutics. In this review, we analyzed US Food and Drug Administration (FDA)-approved peptide drugs and those in clinical trials, highlighting recent advances in peptide-based drug discovery.

Evaluation of borinic acids as new, fast hydrogen peroxide-responsive triggers

Hydrogen peroxide (H2O2) is responsible for numerous damages when overproduced, and its detection is crucial for a better understanding of H2O2-mediated signaling in physiological and pathological processes. For this purpose, various "off-on" small fluorescent probes relying on a boronate trigger have been prepared, and this design has also been involved in the development of H2O2-activated prodrugs or theranostic tools. However, this design suffers from slow kinetics, preventing activation by H2O2 with a short response time. Therefore, faster H2O2-reactive groups are awaited. To address this issue, we have successfully developed and characterized a prototypic borinic-based fluorescent probe containing a coumarin scaffold. We determined its in vitro kinetic constants toward H2O2-promoted oxidation. We measured 1.9 × 104 m-1⋅s-1 as a second-order rate constant, which is 10,000-fold faster than its well-established boronic counterpart (1.8 m-1⋅s-1). This improved reactivity was also effective in a cellular context, rendering borinic acids an advantageous trigger for H2O2-mediated release of effectors such as fluorescent moieties.

Merging the Versatile Functionalities of Boronic Acid with Peptides

Peptides inherently feature the favorable properties of being easily synthesized, water-soluble, biocompatible, and typically non-toxic. Thus, boronic acid has been widely integrated with peptides with the goal of discovering peptide ligands with novel biological activities, and this effort has led to broad applications. Taking the integration between boronic acid and peptide as a starting point, we provide an overview of the latest research advances and highlight the versatile and robust functionalities of boronic acid. In this review, we summarize the diverse applications of peptide boronic acids in medicinal chemistry and chemical biology, including the identification of covalent reversible enzyme inhibitors, recognition, and detection of glycans on proteins or cancer cell surface, delivery of siRNAs, development of pH responsive devices, and recognition of RNA or bacterial surfaces. Additionally, we discuss boronic acid-mediated peptide cyclization and peptide modifications, as well as the facile chemical synthesis of peptide boronic acids, which paved the way for developing a growing number of peptide boronic acids.

Reductive Alkylation of Quinolines to N-Alkyl Tetrahydroquinolines Catalyzed by Arylboronic Acid

A boronic acid catalyzed one-pot tandem reduction of quinolines to tetrahydroquinolines followed by reductive alkylation by the aldehyde has been demonstrated. This step-economcial synthesis of N-alkyl tetrahydroquinolines has been achieved directly from readily available quinolines, aldehydes, and Hantzsch ester under mild reaction conditions. The mechanistic study demonstrates the unique behavior of organoboron catalysts as both Lewis acids and hydrogen-bond donors.

Synthesis of Weinreb amides using diboronic acid anhydride-catalyzed dehydrative amidation of carboxylic acids

The first successful example of the direct synthesis of Weinreb amides using catalytic hydroxy-directed dehydrative amidation of carboxylic acids using the diboronic acid anhydride catalyst is described. The methodology is applicable to the concise syntheses of eight α-hydroxyketone natural products, namely, sattabacin, 4-hydroxy sattabacin, kurasoins A and B, soraphinols A and B, and circumcins B and C.

Diboronic Acid Anhydride-Catalyzed Direct Peptide Bond Formation Enabled by Hydroxy-Directed Dehydrative Condensation

We report the catalytic direct peptide bond formations via dehydrative condensation of β-hydroxy-α-amino acids, affording the serine, threonine, or β-hydroxyvaline-derived peptides in high to excellent yields with high functional group tolerance, minimum epimerization, and excellent chemoselectivity. The key to the success of these atom-economical transformations is the use of diboronic acid anhydride catalyst for the hydroxy-directed reactions.

A Glimpse into Green Chemistry Practices in the Pharmaceutical Industry

In this Minireview, the importance and implementation of green chemistry practices in the pharmaceutical industry are illustrated. With notable examples, some of the most important industrial organic transformations are discussed along with their applications in the synthesis of drug molecules. A brief comparison between traditional unsustainable methods and modern green methods is made to shed light on the economic and environmental benefits of greener methods. Finally, green chemistry practices in the pharmaceutical industries of India and China are also discussed.

Diboronic Acid Anhydrides as Effective Catalysts for the Hydroxy-Directed Dehydrative Amidation of Carboxylic Acids

The direct catalytic dehydrative amidation of β-hydroxycarboxylic acids with amines is described. A biphenyl-based diboronic acid anhydride with a B-O-B skeleton is shown to be an exceptionally effective catalyst for the reaction, providing β-hydroxycarboxylic amides in high to excellent yields with a low catalyst loading (minimum of 0.01 mol %, TON up to 7,500). This hydroxy-directed amidation shows excellent chemoselectivity and is applicable to gram-scale drug synthesis.

Evaluating the impacts of amino acids in the second and outer coordination spheres of Rh-bis(diphosphine) complexes for CO2 hydrogenation

The influence of a biologically inspired second and outer coordination sphere on Rh-bis(diphosphine) CO₂ hydrogenation catalysts was explored.

Promoting water dissociation performance by borinic acid for the strong-acid/base-free hydrogen evolution reaction

Borinic acid is reported as a new proton donor with promoted water dissociation performance for the strong-acid/base-free hydrogen evolution reaction.

Electrophilic boron carboxylate and phosphinate complexes

A series of carboxylic acid derivatives of the form [RC(O)OB(C₆F₅)]₂O, (R = Tol, Ph, C₆F₅, Me₂BrC, Me) were prepared with the concurrent reduction to the corresponding aldehyde. The mechanism is proposed to proceed via cyclic eight-membered ring species.

NMR Quantification of Hydrogen-Bond-Activating Effects for Organocatalysts including Boronic Acids

The hydrogen-bonding activation for 66 organocatalysts has been quantified using a ³¹P NMR binding experiment with triethylphosphine oxide (TEPO). Diverse structural classes, including phenols, diols, silanols, carboxylic acids, boronic acids, and phosphoric acids, were examined with a variety of steric and electronic modifications to understand how the structure and secondary effects contribute to hydrogen-bonding ability and catalysis. Hammett plots demonstrate high correlation for the Δδ ³¹P NMR shift to Hammett parameters, establishing the ability of TEPO binding to predict electronic trends. Upon correlation to catalytic activity in a Friedel-Crafts addition reaction, data demonstrate that ³¹P NMR shifts correlate to catalytic activity better than p K_a values. Boronic acids were investigated, and ³¹P NMR binding experiments predicted strong hydrogen-bonding ability, for which catalytic activity was confirmed, resulting in the greatest rate enhancement observed in the Friedel-Crafts addition of all organocatalysts studied. A detailed investigation supports that boronic acid activation proceeds through hydrogen-bonding interactions and not coordination with the Lewis acidic boron center. Using ³¹P NMR spectroscopy offers a simple and rapid tool to quantify and predict hydrogen-bonding abilities for the design and applications of new organocatalysts and supramolecular synthons.

Structural and biological overview of Boron-containing amino acids in the medicinal chemistry field

Amino acids are the basic structural units of proteins as well as the precursors of many compounds with biological activity. The addition of boron reportedly induces changes in the chemical-biological profile of amino acids.

METHODS

We compiled information on the biological effect of some compounds and discuss the structure-activity relationship of the addition of boron. The specific focus presently is on borinic derivatives of α-amino acids, the specific changes in biological activity caused by the addition of a boron-containing moiety, and the identification of some attractive compounds for testing as potential new drugs.

RESULTS

Borinic derivatives of α-amino acids have been widely synthesized and tested as potential new therapeutic tools. The B-N (1.65 A°) or B-C (1.61 A°) or B-O (1.50 A°) bond is often key for the stability at different pHs and temperatures and activity of these compounds. The chemical features of synthesized derivatives, such as the specific moieties and the logP, polarizability and position of the boron atom are clearly linked to their pharmacodynamic and pharmacokinetic profiles. Some mechanisms of action have been suggested or demonstrated, while those responsible for other effects remain unknown.

CONCLUSION

The increasing number of synthetic borinic derivatives of α-amino acids as well as the recently reported crystal structures are providing new insights into the stability of these compounds at different pHs and temperatures, their interactions on drug targets, and the ring formation of five-membered heterocycles. Further research is required to clarify the ways to achieve specific synthesis, the mechanisms involved in the observed biological effect, and the toxicological profile of this type of boron-containing compounds (BCCs).

Mechanistic Insight into Asymmetric Hetero-Michael Addition of α,β-Unsaturated Carboxylic Acids Catalyzed by Multifunctional Thioureas

Carboxylic acids and their corresponding carboxylate anions are generally utilized as Brønsted acids/bases and oxygen nucleophiles in organic synthesis. However, a few asymmetric reactions have used carboxylic acids as electrophiles. Although chiral thioureas bearing both arylboronic acid and tertiary amine were found to promote the aza-Michael addition of BnONH₂ to α,β-unsaturated carboxylic acids with moderate to good enantioselectivities, the reaction mechanism remains to be clarified. Detailed investigation of the reaction using spectroscopic analysis and kinetic studies identified tetrahedral borate complexes, comprising two carboxylate anions, as reaction intermediates. We realized a dramatic improvement in product enantioselectivity with the addition of 1 equiv of benzoic acid. In this aza-Michael reaction, the boronic acid not only activates the carboxylate ligand as a Lewis acid, together with the thiourea NH-protons, but also functions as a Brønsted base through a benzoyloxy anion to activate the nucleophile. Moreover, molecular sieves were found to play an important role in generating the ternary borate complexes, which were crucial for obtaining high enantioselectivity as demonstrated by DFT calculations. We also designed a new thiourea catalyst for the intramolecular oxa-Michael addition to suppress another catalytic pathway via a binary borate complex using steric hindrance between the catalyst and substrate. Finally, to demonstrate the synthetic versatility of both hetero-Michael additions, we used them to accomplish the asymmetric synthesis of key intermediates in pharmaceutically important molecules, including sitagliptin and α-tocopherol.

Diboron-Catalyzed Dehydrative Amidation of Aromatic Carboxylic Acids with Amines

Tetrakis(dimethylamido)diboron and tetrahydroxydiboron are herein reported as new catalysts for the synthesis of aryl amides by catalytic condensation of aromatic carboxylic acids with amines. The developed protocol is both simple and highly efficient over a broad range of substrates. This method thus represents an attractive approach for the use of diboron catalysts in the synthesis of amides without having to resort to stoichiometric or additional dehydrating agents.

Design, Synthesis and Evaluation of Oxazaborine Inhibitors of the NLRP3 Inflammasome

The NLRP3 inflammasome is an important regulator of the sterile inflammatory response, and its activation by host-derived sterile molecules leads to the intracellular activation of caspase-1, processing of the pro-inflammatory cytokines interleukin-1β (IL-1β)/IL-18, and pyroptotic cell death. Inappropriate activation of NLRP3 drives a chronic inflammatory response and is implicated in several non-communicable diseases, including gout, atherosclerosis, type II diabetes and Alzheimer's disease. In this study, we report the design, synthesis and biological evaluation of novel boron compounds (NBCs) as NLRP3 inflammasome inhibitors. Structure-activity relationships (SAR) show that 4-fluoro substituents on the phenyl rings retain NLRP3 inhibitory activity, whereas more steric and lipophilic substituents diminish activity. Loss of inhibitory activity is also observed if the CCl3 group on the oxazaborine ring is replaced by a CF3 group. These findings provide additional understanding of the NBC series and will aid in the development of these NLRP3 inhibitors as tool compounds or therapeutic candidates for sterile inflammatory diseases.

Tetramethyl Orthosilicate (TMOS) as a Reagent for Direct Amidation of Carboxylic Acids

Tetramethyl orthosilicate (TMOS) is shown to be an effective reagent for direct amidation of aliphatic and aromatic carboxylic acids with amines and anilines. The amide products are obtained in good to quantitative yields in pure form directly after workup without the need for any further purification. A silyl ester as the putative activated intermediate is observed by NMR methods. Amidations on a 1 mol scale are demonstrated with a favorable process mass intensity.

Direct amidation of unprotected amino acids using B(OCH2CF3)3

A commercially available borate ester, B(OCH2CF3)3, can be used to achieve protecting-group free direct amidation of α-amino acids with a range of amines in cyclopentyl methyl ether. The method can be applied to the synthesis of medicinally relevant compounds, and can be scaled up to obtain gram quantities of products.

Catalytic Oligopeptide Synthesis

Waste-free catalytic assembly of α-amino acids is fueled by a multiboron catalyst that features a characteristic B₃NO₂ heterocycle, providing a versatile catalytic protocol wherein functionalized natural α-amino acid units are accommodated and commonly used protecting groups are tolerated. The facile dehydrative conditions eliminate the use of engineered peptide coupling reagents, exemplifying a greener catalytic alternative for peptide coupling. The catalysis is sufficiently robust to enable pentapeptide synthesis, constructing all four amide bond linkages in a catalytic fashion.

Mechanistic insights into boron-catalysed direct amidation reactions

The generally accepted monoacyloxyboron mechanism of boron-catalysed direct amidation is brought into question in this study, and new alternatives are proposed.

The generally accepted monoacyloxyboron mechanism of boron-catalysed direct amidation is brought into question in this study, and new alternatives are proposed. We have carried out a detailed investigation of boron-catalysed amidation reactions, through study of the interaction between amines/carboxylic acids and borinic acids, boronic acids and boric acid, and have isolated and characterised by NMR/X-ray crystallography many of the likely intermediates present in catalytic amidation reactions. Rapid reaction between amines and boron compounds was observed in all cases, and it is proposed that such boron–nitrogen interactions are highly likely to take place in catalytic amidation reactions. These studies also clearly show that borinic acids are not competent catalysts for amidation, as they either form unreactive amino-carboxylate complexes, or undergo protodeboronation to give boronic acids. It therefore seems that at least three free coordination sites on the boron atom are necessary for amidation catalysis to occur. However, these observations are not consistent with the currently accepted ‘mechanism’ for boron-mediated amidation reactions involving nucleophilic attack of an amine onto a monomeric acyloxyboron intermediate, and as a result of our observations and theoretical modelling, alternative proposed mechanisms are presented for boron-mediated amidation reactions. These are likely to proceed via the formation of a dimeric B–X–B motif (X = O, NR), which is uniquely able to provide activation of the carboxylic acid, whilst orchestrating the delivery of the amine nucleophile to the carbonyl group. Quantum mechanical calculations of catalytic cycles at the B3LYP+D3/Def2-TZVPP level (solvent = CH₂Cl₂) support the proposal of several closely related potential pathways for amidation, all of which are likely to be lower in energy than the currently accepted mechanism.

The ortho-substituent on 2,4-bis(trifluoromethyl)phenylboronic acid catalyzed dehydrative condensation between carboxylic acids and amines

The ortho-substituent of boronic acid plays a key role in preventing the coordination of amines to the boron atom.

Borate esters: Simple catalysts for the sustainable synthesis of complex amides

Chemical reactions for the formation of amide bonds are among the most commonly used transformations in organic chemistry, yet they are often highly inefficient. A novel protocol for amidation using a simple borate ester catalyst is reported. The process presents significant improvements over other catalytic amidation methods in terms of efficiency and safety, with an unprecedented substrate scope including functionalized heterocycles and even unprotected amino acids. The method was used to access a wide range of functionalized amide derivatives, including pharmaceutically relevant targets, important synthetic intermediates, a catalyst, and a natural product.