Backbone amide linker in solid-phase synthesis

Recent developments in the total synthesis of natural products using the Ugi multicomponent reactions as the key strategy

The total syntheses of selected natural products using different versions of the Ugi multicomponent reaction is reviewed on a case-by-case basis. The revision covers the period 2008-2023 and includes detailed descriptions of the synthetic sequences, the use of state-of-the-art chemical reagents and strategies, as well as the advantages and limitations of the transformation and some remedial solutions. Relevant data on the isolation and bioactivity of the different natural targets are also briefly provided. The examples clearly evidence the strategic importance of this transformation and its key role in the modern natural products synthetic chemistry toolbox. This methodology proved to be a valuable means for easily building molecular complexity and efficiently delivering step-economic syntheses even of intricate structures, with a promising future.

NiH-catalyzed C-N bond formation: insights and advancements in hydroamination of unsaturated hydrocarbons

The formation of C-N bonds is a fundamental aspect of organic synthesis, and hydroamination has emerged as a pivotal strategy for the synthesis of essential amine derivatives. In recent years, there has been a surge of interest in metal hydride-catalyzed hydroamination reactions of common alkenes and alkynes. This method avoids the need for stoichiometric organometallic reagents and overcomes problems associated with specific organometallic compounds that may impact functional group compatibility. Notably, recent developments have brought to the forefront olefinic hydroamination and hydroamidation reactions facilitated by nickel hydride (NiH) catalysis. The inclusion of suitable chiral ligands has paved the way for the realization of asymmetric hydroamination reactions in the realm of olefins. This review aims to provide an in-depth exploration of the latest achievements in C-N bond formation through intermolecular hydroamination catalyzed by nickel hydrides. Leveraging this innovative approach, a diverse range of alkene and alkyne substrates can be efficiently transformed into value-added compounds enriched with C-N bonds. The intricacies of C-N bond formation are succinctly elucidated, offering a concise overview of the underlying reaction mechanisms. It is our aspiration that this comprehensive review will stimulate further progress in NiH-catalytic techniques, fine-tune reaction systems, drive innovation in catalyst design, and foster a deeper understanding of the underlying mechanisms.

Solid-Phase Synthesis of Caged Luminescent Peptides via Side Chain Anchoring

The synthesis of caged luminescent peptide substrates remains challenging, especially when libraries of the substrates are required. Most currently available synthetic methods rely on a solution-phase approach, which is less suited for parallel synthesis purposes. We herein present a solid-phase peptide synthesis (SPPS) method for the synthesis of caged aminoluciferin peptides via side chain anchoring of the P1 residue. After the synthesis of a preliminary test library consisting of 40 compounds, the synthetic method was validated and optimized for up to >100 g of resin. Subsequently, two separate larger peptide libraries were synthesized either having a P1 = lysine or arginine residue containing in total 719 novel peptide substrates. The use of a more stable caged nitrile precursor instead of caged aminoluciferin rendered our parallel synthetic approach completely suitable for SPPS and serine protease profiling was demonstrated using late-stage aminoluciferin generation.

Photo-Induced Carbonylation of Aryl Bromides for the Synthesis of Aryl Esters and Amides Under Transition Metal-Free Conditions

In this work, we developed a photo-induced carbonylation of aryl bromides under transition metal-free conditions. The reaction shows good activity with alcohol and amine nucleophiles. Various esters and amides were formed from aryl halides and alcohols and amines under mild conditions in moderate to good yields.

Palladium-Catalyzed Perfluoroalkylative Carbonylation of 2-Allylaryl Trifluoromethanesulfonates: Base-Controlled Selective Access to β-Perfluoroalkyl Amides

A palladium-catalyzed perfluoroalkylative carbonylation of 2-allylaryl trifluoromethanesulfonates has been developed. A range of 2-allyl trifluoromethanesulfonates, perfluoroalkyl halides, and amines were applied in this tandem procedure to provide the corresponding β-perfluoroalkyl amides with good functional group tolerance and high chemoselectivity. The final products were controlled by the base applied.

Backbone-Anchoring, Solid-Phase Synthesis Strategy To Access a Library of Peptidouridine-Containing Small Molecules

Nucleoside diphosphate sugar (NDP-sugar) substrates provide the inspiration for nucleoside analogue inhibitor scaffolds. By employing solid-phase synthesis, we provide a method to access a library of peptidouridine inhibitors with both minimal compound handling and purification steps. Specifically, this strategy is exemplified by generating uridine diphosphate sugar (UDP-sugar) mimics, which allow for compound elaboration by altering the dipeptide composition, the N-terminal linkage, and a pendant aryl group. To exemplify the versatility, 41 unique nucleoside analogues are presented.

An Atom-Economic Inverse Solid-Phase Peptide Synthesis Using Bn or BcM Esters of Amino Acids

An atom-economic N-to-C-directed solid-phase peptide synthesis is reported that uses benzyl (Bn) or (benzhydryl-carbamoyl)-methyl (BcM) esters of amino acids as the building blocks, which facilitate efficient hydrazinolysis, convenient conversion to acyl azide, and robust amidation with the next amino acid ester. This method is free of coupling reagents and free of protection on the side-chain OH, CO₂H, CONH2, etc., therefore exhibiting a significantly improved atom economy compared to those of BOC- or Fmoc-based C-to-N-directed approaches.

An Electrochemical Approach to Designer Peptide α-Amides Inspired by α-Amidating Monooxygenase Enzymes

Designer C-terminal peptide amides are accessed in an efficient and epimerization-free approach by pairing an electrochemical oxidative decarboxylation with a tandem hydrolysis/reduction pathway. Resembling Nature's dual enzymatic approach to bioactive primary α-amides, this method delivers secondary and tertiary amides bearing high-value functional motifs, including isotope labels and handles for bioconjugation. The protocol leverages the inherent reactivity of C-terminal carboxylates, is compatible with the vast majority of proteinogenic functional groups, and proceeds in the absence of epimerization, thus addressing major limitations associated with conventional coupling-based approaches. The utility of the method is exemplified through the synthesis of natural product acidiphilamide A via a key diastereoselective reduction, as well as bioactive peptides and associated analogues, including an anti-HIV lead peptide and blockbuster cancer therapeutic leuprolide.

Copper-Catalyzed Carbonylative Hydroamidation of Styrenes to Branched Amides

Amides are one of the most ubiquitous functional groups in synthetic and medicinal chemistry. Novel and rapid synthesis of amides remains in high demand. In this communication, a general and efficient procedure for branch-selective hydroamidation of vinylarenes with hydroxyamine derivatives enabled by copper catalysis has been developed for the first time. The reaction proceeds under mild conditions and tolerates a broad range of functional groups. Applying a chiral phosphine ligand, an enantioselective variant of this transformation was achieved, affording a variety of chiral α-amides with excellent enantioselectivities (up to 99 % ee) and high yields.

The silver catalyzed direct C-H functionalization of quinones with dialkyl amides

DMA and other dialkylamides were successfully used as synthons for the C-H functionalization of quinones. This novel amidoalkylation reaction works with a variety of substituted quinones and dialkyl/alkyl amides, such as DMF, NMP and NMA, and the corresponding products were obtained in moderate to good yields. The amidoalkylation of quinones is demonstrated for the first time. A suitable mechanism and the synthetic utility of these compounds are demonstrated. The molecular docking of compound 5 with an Alzheimer's disease (AD) associated AChE target site was studied.

Polymer-Bound Triphenylphosphine and 4,4'-Dinitroazobenzene as a Coupling Reagents for Chromatography-Free Esterification Reaction

AIM AND OBJECTIVE

Sustainable production of fine chemicals both in industries and pharmaceuticals heavily depends on the application of solid-phase synthesis route coupled with microwave technologies due to their environmentally benign nature. In this report, a microwave-assisted esterification reaction using polymer-bound triphenylphosphine and 4,4'-dinitroazobenzene reagent system was investigated.

MATERIALS AND METHODS

The solvents were obtained from Merck India. Polymer-bound triphenylphosphine (~3 mmol triphenylphosphine moiety/g) was acquired from Sigma-Aldrich. The progress of the reaction was observed by thin-layer chromatography. All the reactions were performed in Milestones StartSYNTH microwave. The NMR spectra were recorded on Bruker Avance III 300, 400, and 500 MHz FT NMR Spectrometers. Using azo compound and polymer-bound triphenyl phosphine as a coupling reagent, esterification of different carboxylic acids with alcohols was performed under microwave irradiation.

RESULTS

Esterification of benzoic acid with 1-propanol under microwave irradiation gave a high yield of 92% propyl benzoate in 60 minutes only. Isolation of the ester products was relatively simple as both the byproducts polymer-bound triphenylphosphine oxide and hydrazine could be removed by simple filtration. The rates of reactions were found to be directly proportional to the pKa of the benzoic acids.

CONCLUSION

4,4'-Dinitroazobenzene was introduced as a novel coupling reagent, in conjugation with polymer-bound triphenylphosphine, for esterification reactions under microwave irradiation. The low moisture sensitivity of the reaction system, easy separation of the byproducts, and column chromatographyfree isolation of esters help our methods with application significance, particularly from the 'Sustainable Chemistry' perspective.

Base-mediated cascade amidination/N-alkylation of amines by alcohols

An efficient base mediated N-alkylation with nitriles as a water acceptor was described, providing a convenient method to construct the different substituted diamino compounds, ¹⁵N labeled amine molecules and could scaled up to 1 mol scale.

Automated Chemical Oligosaccharide Synthesis: Novel Approach to Traditional Challenges

Advances in carbohydrate chemistry have certainly made common oligosaccharides much more accessible. However, many current methods still rely heavily upon specialized knowledge of carbohydrate chemistry. The application of automated technologies to chemical and life science applications such as genomics and proteomics represents a vibrant field. These automated technologies also present opportunities for their application to organic synthesis, including that of the synthesis of oligosaccharides. However, application of automated methods to the synthesis of carbohydrates is an underdeveloped area as compared to other classes of biomolecules. The overarching goal of this review article is to present the advances that have been made at the interface of carbohydrate chemistry and automated technology.

An Optimized Preparation of 1,1-Dimethylallyl Esters and Their Application to Solid-Phase Peptide Synthesis

A one-step preparation of 1,1-dimethylallyl (DMA) esters was optimized for the C-terminal protection of a range of Fmoc-protected amino acids. This preparation is not sensitive to the scale of reaction and affords the corresponding DMA esters in 70-99% yield with high regioselectivity. Additionally, these DMA-protected amino acids were used with the backbone amide linker (BAL) of Albericio and Barany and found to resist diketopiperazine formation during the synthesis of a series of tripeptide esters. C-terminal DMA protection is compatible with the BAL linkage and allows for standard Fmoc-based methods to be used throughout the synthesis.

Recent Progress in the Chemical Synthesis of Class II and S-Glycosylated Bacteriocins

A wide variety of antimicrobial peptides produced by lactic acid bacteria (LAB) have been identified and studied in the last decades. Known as bacteriocins, these ribosomally synthesized peptides inhibit the growth of a wide range of bacterial species through numerous mechanisms and show a great variety of spectrum of activity. With their great potential as antimicrobial additives and alternatives to traditional antibiotics in food preservation and handling, animal production and in veterinary and medical medicine, the demand for bacteriocins is rapidly increasing. Bacteriocins are most often produced by fermentation but, in several cases, the low isolated yields and difficulties associated with their purification seriously limit their use on a large scale. Chemical synthesis has been proposed for their production and recent advances in peptide synthesis methodologies have allowed the preparation of several bacteriocins. Moreover, the significant cost reduction for peptide synthesis reagents and building blocks has made chemical synthesis of bacteriocins more attractive and competitive. From a protein engineering point of view, the chemical approach offers many advantages such as the possibility to rapidly perform amino acid substitution, use unnatural or modified residues, and make backbone and side chain modifications to improve potency, modify the activity spectrum or increase the stability of the targeted bacteriocin. This review summarized synthetic approaches that have been developed and used in recent years to allow the preparation of class IIa bacteriocins and S-linked glycopeptides from LAB. Synthetic strategies such as the use of pseudoprolines, backbone protecting groups, microwave irradiations, selective disulfide bridge formation and chemical ligations to prepare class II and S-glycosylsated bacteriocins are discussed.

Total Synthesis of Scytonemide A Employing Weinreb AM Solid-Phase Resin

The human 20S proteasome inhibitor scytonemide A (1), a macrocyclic imine originally isolated from the cyanobacterium Scytonema hofmanni, was synthesized via a biomimetic solid-phase peptide synthesis (SPPS) approach employing the Weinreb AM resin. Utilizing this approach, cyclization of the protected heptapeptide via formation of the imine bond occurred spontaneously upon cleavage from the resin in the presence of a reducing agent and subsequent aqueous workup. The final deprotection step necessary to produce the natural product was accomplished under slightly basic conditions, facilitating cleavage of the silyl ether group while leaving the macrocycle intact. Purification of the synthetic scytonemide A was accomplished via normal-phase flash column chromatography, potentially facilitating larger scale preparation of the compound necessary for future mechanistic and SAR studies. The structure of the target compound was confirmed by NMR spectroscopy, which also shed light on differences in the spectroscopic data obtained for the synthetic and natural scytonemide A samples for some of the amide and alcohol signals in the 1H NMR spectrum.

Toward solid-phase peptide fragment ligation by a traceless-Ugi multicomponent reaction approach

A new methodology to couple peptide fragments on solid support using a traceless isocyanide-based multicomponent reaction is described. The approach uses a microwave-assisted on-resin Ugi four-component reaction to attach a carboxyl free peptide to a supported peptide bearing a free N-terminal amine via the formation of an N-protected amide bond at the ligation site. Afterward, the generated backbone amide protecting group can be efficiently removed by microwave-assisted acidolysis with trifluoroacetic acid to afford a fully deprotected peptide. This straightforward Ugi reaction/deprotection approach was applied to condense various fragment lengths and provided a variety of oligopeptides.

Thirteen decades of peptide synthesis: key developments in solid phase peptide synthesis and amide bond formation utilized in peptide ligation

A historical overview of peptide chemistry from T. Curtius to E. Fischer to M. Bergmann and L. Zervas is first presented. Next, the fundamentals of peptide synthesis with a focus on solid phase peptide synthesis by R. B. Merrifield are described. Immobilization strategies to attach the first amino acid to the resin, coupling strategies in stepwise peptide chain elongation, and approaches to synthesize difficult peptide sequences are also shown. A brief comparison between tert-butyloxycarbonyl (Boc)/benzyl (Bzl) strategy and 9-fluorenylmethoxycarbonyl (Fmoc)/tert-butyl (t -Bu) strategy utilized in solid phase peptide synthesis is given with an emphasis on the latter. Finally, the review focuses on the discovery and development of peptide ligation and the latest advances in this field including native amide bond formation strategies, these include the native chemical ligation, α-ketoacid-hydroxylamine ligation, and serine/threonine ligation which are the most commonly used chemoselective ligation methods that provide amide bond at the ligation site. This review provides an overview of the literature concerning the most important advances in the chemical synthesis of proteins and peptides covering the period from 1882 to 2017.

Epimerization-free access to C-terminal cysteine peptide acids, carboxamides, secondary amides, and esters via complimentary strategies

We present a convenient method for the diversification of peptides bearing cysteine at the C-terminus that proceeds to form a variety of carboxylic acid, carboxamide, 2° amide, and ester terminated peptides without any detectable epimerization of the α-stereocenter.

C-Terminal cysteine peptide acids are difficult to access without epimerization of the cysteine α-stereocenter. Diversification of the C-terminus after solid-phase peptide synthesis poses an even greater challenge because of the proclivity of the cysteine α-stereocenter to undergo deprotonation upon activation of the C-terminal carboxylic acid. We present herein two general strategies to access C-terminal cysteine peptide derivatives without detectable epimerization, diketopiperazine formation, or piperidinylalanine side products.

Peptide Macrocyclization Assisted by Traceless Turn Inducers Derived from Ugi Peptide Ligation with Cleavable and Resin-Linked Amines

A multicomponent approach enabling the installation of turn-inducing moieties that facilitate the macrocyclization of short and medium-size oligopeptides is described. The strategy comprises the Ugi ligation of peptide carboxylic acids and isocyanopeptides in the presence of aldehydes and acid or photolabile amines followed by cyclization and cleavage of the backbone N-substituents to render canonical cyclopeptides. Implementing the approach on solid phase with the use of Rink amide resins led to a new class of backbone amide linker strategy.

Dual-targeted peptide-conjugated multifunctional fluorescent probe with AIEgen for efficient nucleus-specific imaging and long-term tracing of cancer cells

Precisely targeted transportation of a long-term tracing regent to a nucleus with low toxicity is one of the most challenging concerns in revealing cancer cell behaviors. Here, we report a dual-targeted peptide-conjugated multifunctional fluorescent probe (cNGR-CPP-NLS-RGD-PyTPE, TCNTP) with aggregation-induced emission (AIE) characteristic, for efficient nucleus-specific imaging and long-term and low-toxicity tracing of cancer cells. TCNTP mainly consists of two components: one is a functionalized combinatorial peptide (TCNT) containing two targeted peptides (cNGR and RGD), a cell-penetrating peptide (CPP) and a nuclear localization signal (NLS), which can specifically bind to a cell surface and effectively enter into the nucleus; the other one is an AIE-active tetraphenylethene derivative (PyTPE, a typical AIEgen) as fluorescence imaging reagent. In the presence of aminopeptidase N (CD13) and integrin αvβ3, TCNTP can specifically bind to both of them using cNGR and RGD, respectively, lighting up its yellow fluorescence. Because it contains CPP, TCNTP can be effectively integrated into the cytoplasm, and then be delivered into the nucleus with the help of NLS. TCNTP exhibited strong fluorescence in the nucleus of CD13 and integrin αvβ3 overexpression cells due to the specific targeting ability, efficient transport capacity and AIE characteristic in a more crowded space. Furthermore, TCNTP can be applied for long-term tracing in living cells, scarcely affecting normal cells with negligible toxicity in more than ten passages.

Palladium-catalyzed highly regioselective hydroaminocarbonylation of aromatic alkenes to branched amides

Pd(t-Bu₃P)₂ has been successfully identified as an efficient catalyst for the hydroaminocarbonylation of aromatic alkenes to branched amides under relatively mild reaction conditions.

Selective Hydrolysis of 1-cyanocyclopropane-1-carboxylates: Concise Preparation of 1-carbamoylcyclopropane-1-carboxylates

An efficient and straightforward method has been developed for the preparation of 1-carbamoylcyclopropane-1-carboxylate derivatives via selective hydrolysis of 1-cyanocyclopropane-1-carboxylates by using hydroxylamine and sodium acetate system strategy. The structure of 1-carbamoylcyclopropane-1-carboxylate 2a was further confirmed by X-ray single crystal analysis.

Synthesis of 9-Substituted Triptycene Building Blocks for Solid-Phase Diversification and Nucleic Acid Junction Targeting

Triptycenes have been shown to bind nucleic acid three-way junctions, but rapid and efficient methods to diversify the triptycene core are lacking. An efficient synthesis of a 9-substituted triptycene scaffold is reported that can be used as a building block for solid-phase peptide synthesis and rapid diversification. The triptycene building block was diversified to produce a new class of tripeptide-triptycenes, and their binding abilities toward d(CAG)·(CTG) repeat junctions were investigated.

Preparation and self-assembly of amphiphilic polylysine dendrons

Polylysine dendrons with lipid tails prepared by divergent solid-phase synthesis showed self-assembling properties in aqueous solutions.

Polymer supported DMAP: an easily recyclable organocatalyst for highly atom-economical Henry reaction under solvent-free conditions

Polymer supported catalysts are regarded as a borderline class of catalysts, which retains the advantages of homogeneous catalysts while securing the ease of recovery by simple filtration and workup of heterogeneous systems.

A microwave-assisted highly practical chemoselective esterification and amidation of carboxylic acids

The ubiquitousness of esters and amide functionalities makes their coupling reaction one of the most sought-after organic transformations.

Selective Monoesterification of Symmetrical Diols Using Resin-Bound Triphenylphosphine

Coupling reactions to make esters and amides are among the most widely used organic transformations. We report efficient procedures for amide bond formation and for the monoesterification of symmetrical diols in excellent yields without any requirement for high dilution or slow addition using resin-bound triarylphosphonium iodide. Easy purification, low moisture sensitivity, and good to excellent yields of the products are the major advantages of this protocol.

Palladium-catalyzed hydroaminocarbonylation of alkenes with amines: a strategy to overcome the basicity barrier imparted by aliphatic amines

A novel and efficient palladium-catalyzed hydroaminocarbonylation of alkenes with aminals has been developed under mild reaction conditions, and allows the synthesis of a wide range of N-alkyl linear amides in good yields with high regioselectivity. On the basis of this method, a cooperative catalytic system operating by the synergistic combination of palladium, paraformaldehyde, and acid was established for promoting the hydroaminocarbonylation of alkenes with both aromatic and aliphatic amines, which do not react well under conventional palladium-catalyzed hydroaminocarbonylation.