On the two-component microwave-mediated reaction of isonitriles with carboxylic acids: regarding alleged formimidate carboxylate mixed anhydrides

Ag(I)-promoted fragment coupling of peptide thioamides

Despite advances in solid phase peptide synthesis and peptide ligation, challenges remain in the assembly of polypeptides through coupling of peptide fragments. Herein we describe a new method for peptide fragment coupling employing the Ag(I)-promoted transformation of peptide thioamides. This process proceeds via an isoimide-tethered intermediate, which undergoes an O-N acyl transfer to generate the polypeptide. This method is applicable to both solution- and solid-phase coupling.

Transition-metal and oxidant-free approach for the synthesis of diverse N-heterocycles by TMSCl activation of isocyanides

A highly efficient TMSCl-mediated addition of N-nucleophiles to isocyanides has been achieved. This transition-metal and oxidant-free strategy has been applied to the construction of various N-heterocyles such as quinazolinone, benzimidazole and benzothiazole derivatives by the use of distinct amino-based binucleophiles. The notable feature of this protocol includes its mild reaction condition, broad functional group tolerance and excellent yield.

A highly efficient TMSCl-mediated addition of N-nucleophiles to isocyanides has been achieved.

The mechanism and structure–activity relationship of amide bond formation by silane derivatives: a computational study

The detailed reaction mechanism and structure–activity relationship of substrates in silane reagent-mediated amide bond formation reactions are clarified.

Serine/Threonine Ligation: Origin, Mechanistic Aspects, and Applications

Synthetic proteins are expected to go beyond the boundary of recombinant DNA expression systems by being flexibly installed with site-specific natural or unnatural modification structures during synthesis. To enable protein chemical synthesis, peptide ligations provide effective strategies to assemble short peptide fragments obtained from solid-phase peptide synthesis (SPPS) into long peptides and proteins. In this regard, chemoselective peptide ligation represents a simple but powerful transformation realizing selective amide formation between the C-terminus and N-terminus of two side-chain-unprotected peptide fragments. These reactions are highly chemo- and regioselective to tolerate the side-chain functionalities present on the unprotected peptides, highly reactive to work with millmolar or submillimolar concentrations of the substrates, and operationally simple with mild conditions and accessible building blocks. This Account focuses on our work in the development of serine/threonine ligation (STL), which originates from a chemoselective reaction between an unprotected peptide with a C-terminal salicylaldehyde (SAL) ester and another unprotected peptide with an N-terminal serine or threonine residue. Mechanistically, STL involves imine capture, 5- endo-trig ring-chain tautomerization, O-to- N [1,5] acyl transfer to afford the N, O-benzylidene acetal-linked peptide, and acidolysis to regenerate the Xaa-Ser/Thr linkage (where Xaa is the amino acid) at the ligation site. The high abundance of serine and threonine residues (12.7%) in naturally occurring proteins and the good compatibility of STL with various C-terminal residues provide multiple choices for ligation sites. The requisite peptide C-terminal SAL esters can be prepared from the peptide fragments obtained from both Fmoc-SPPS and Boc-SPPS through four available methods (a safety-catch strategy based on phenolysis, direct coupling, ozonolysis, and the n + 1 strategy). In the synthesis of proteins (e.g., ACYP enzyme, MUC1 glycopeptide 40-mer to 80-mer, interleukin 25, and HMGA1a with variable post-translational modification patterns), both C-to- N and N-to- C sequential STL strategies have been developed through selection of temporal N-terminal protecting groups and proper design of the switch-on/off C-terminal SAL ester surrogate, respectively. In the synthesis of cyclic peptide natural products (e.g., daptomycin, teixobactin, cyclomontanin B, yunnanin C) and their analogues, intramolecular head-to-tail STL has been implemented on linear peptide SAL ester precursors containing four to 10 amino acid residues with good efficiency and minimized oligomerization. As a thiol-independent chemoselective ligation complementary to native chemical ligation, STL provides an alternative tool for the chemical synthesis of homogeneous proteins with site-specific and structure-defined modifications and cyclic peptide natural products, which lays foundation for chemical biology and medicinal studies of those molecules with biological importance and therapeutic potential.

Computational study of the mechanism of amide bond formation via CS2-releasing 1,3-acyl transfer

A systematic computational study on CS₂-releasing 1,3-acyl transfer was performed for the first time and provided deeper mechanistic insights.

Nonclassical Routes for Amide Bond Formation

The present review offers an overview of nonclassical (e.g., with no pre- or in situ activation of a carboxylic acid partner) approaches for the construction of amide bonds. The review aims to comprehensively discuss relevant work, which was mainly done in the field in the last 20 years. Organization of the data follows a subdivision according to substrate classes: catalytic direct formation of amides from carboxylic and amines ( section 2 ); the use of carboxylic acid surrogates ( section 3 ); and the use of amine surrogates ( section 4 ). The ligation strategies (NCL, Staudinger, KAHA, KATs, etc.) that could involve both carboxylic acid and amine surrogates are treated separately in section 5 .

Synthesis of Peptides by Silver-Promoted Coupling of Carboxylates and Thioamides: Mechanistic Insight from Computational Studies

The mechanism of the recently described N→C direction peptide synthesis through silver-promoted coupling of N-protected amino acids with thioacetylated amino esters was explored by using density functional theory. Calculation of the potential energy surfaces for various pathways revealed that the reaction proceeds through silver-assisted addition of the carboxylate to the thioamide, which is followed by deprotonation and silver-mediated extrusion of sulfur as Ag2 S. The resulting isoimide is the key intermediate, which subsequently rearranges to an imide through a concerted pericyclic [1,3]-acyl shift (O-sp(2) N 1,3-acyl migration). The proposed mechanism clearly emphasises the requirement of two equivalents of Ag(I) and basic reaction conditions, which is in full agreement with the experimental findings. Alternative rearrangement pathways involving only one equivalent of Ag(I) or through O-sp(3) N 1,3-acyl migration can be excluded. The computations further revealed that peptide couplings involving thioformamides require significant conformational changes in the intermediate isoformimide, which slow down the rearrangement process.

In situ generation of nitrilium from nitrile ylide and the subsequent Mumm rearrangement: copper-catalyzed synthesis of unsymmetrical diacylglycine esters

A novel in situ generation of nitrilium from nitrile ylide and the subsequent Mumm rearrangement of carboxylic acid, nitrile, and diazo compounds gave various unsymmetrical diacylglycine esters.

Exploiting the Divalent Nature of Isonitriles: a novel Pictet-Spengler Amidination process

An isocyanide-based multicomponent reaction (IMCR) utilized for the rapid assembly of novel, biologically relevant dihydropyrrolo[1,2-a]quinoxalines-amidines is herein presented. Starting from 1-(2-aminophenyl)pyrroles, aldehydes, and isonitriles, the target heterocyclic scaffold is assembled in a one-pot, operationally friendly process. With three points of diversity and formation of three chemical bonds in one step, this strategy proves to be very general. Novel, mild methodology for the generation of amidines from secondary amine anilines and isonitriles is also introduced.

A new method for peptide synthesis in the N→C direction: amide assembly through silver-promoted reaction of thioamides

The Ag(i)-promoted coupling of peptide-acids with thioamides generates peptide-imides, which was exploited in the synthesis of peptides in the N→C direction.

Solid-phase peptide synthesis and solid-phase fragment coupling mediated by isonitriles

The synthesis of polypeptides on solid phase via mediation by isonitriles is described. The acyl donor is a thioacid, which presumably reacts with the isonitrile to generate a thio-formimidate carboxylate mixed anhydride intermediate. Applications of this chemistry to reiterative solid-phase peptide synthesis as well as solid-phase fragment coupling are described.

Revisiting oxytocin through the medium of isonitriles

The reaction of thioamino acids and N-terminal peptides, mediated by hindered isonitriles and hydroxybenzotriazole, gives rise to peptide bonds. In one pathway, oxytocin was synthesized by eight such reiterative amidations. In another stereospecific track, oxytocin was constructed by native chemical ligation, wherein the two building blocks were assembled by thioacid amine amidation. The NMR spectra of oxytocin and dihydrooxytocin suggest a high level of preorganization in the latter, perhaps favoring oxidative folding.

Enhancing the scope of the Diels-Alder reaction through isonitrile chemistry: emergence of a new class of acyl-activated dienophiles

α,β-Unsaturated imides, formylated at the nitrogen atom, comprise a new and valuable family of dienophiles for servicing Diels-Alder reactions. These systems are assembled through extension of recently discovered isonitrile chemistry to the domain of α,β-unsaturated acids. Cycloadditions are facilitated by Et(2)AlCl, presumably via chelation between the two carbonyl groups of the N-formyl amide. Applications of the isonitrile/Diels-Alder logic to the IMDA reaction, as well as methodologies to modify the N-formyl amide of the resultant cycloaddition product, are described. It is expected that this easily executed chemistry will provide a significant enhancement for application of Diels-Alder reactions to many synthetic targets.

A fascinating journey into history: exploration of the world of isonitriles en route to complex amides

We describe herein our recent explorations in the field of isonitrile chemistry. An array of broadly useful coupling methodologies has been developed for the formation of peptidyl and glycopeptidyl amide bonds. We further describe the application of these methods to the syntheses of complex systems, including the cyclic peptide cyclosporine A, constrained peptide systems, and heterocycles.

Expanding the limits of isonitrile-mediated amidations: on the remarkable stereosubtleties of macrolactam formation from synthetic seco-cyclosporins

The scope of isonitrile-mediated amide bond-forming reactions is further explored in this second-generation synthetic approach to cyclosporine (cyclosporin A). Both type I and type II amidations are utilized in this effort, allowing access to epimeric cyclosporins A and H from a single precursor by variation of the coupling reagents. This work lends deeper insight into the relative acylating ability of the formimidate carboxylate mixed anhydride (FCMA) intermediate, while shedding light on the far-reaching impact of remote stereochemical changes on the effective preorganization of seco-cyclosporins.

Rhodium-catalyzed annulation of N-benzoylsulfonamide with isocyanide through C-H activation

Isocyanide insertion: the first rhodium-catalyzed annulation of N -benzoylsulfonamide incorporating with isocyanide via C-H activation is described. The transformation is broadly compatible with N -benzoylsulfonamides bearing various electron-properties as well as isocyanides. From practical point of view, this methodology provides the most straightforward approach to a series of 3-(imino)isoindolinones.

On the synthesis of conformationally modified peptides through isonitrile chemistry: implications for dealing with polypeptide aggregation

A method for introducing a dimethyleneoxy constraint joining the N atoms of two consecutive amino acids in the context of a polypeptide has been developed. This constraint can profoundly affect the tendency of a polypeptide to suffer aggregation and desolubilization, and it can be readily removed under mild conditions.

Control of Nanospaces with Molecular Devices

Cavitands and capsules define nanoliter spaces for recognition, isolation and reactions of small molecules. These systems are usually self-assembled and factors such as solvent size, stoichiometry, and packing factors determine what goes into the spaces. Here we examine two switching devices to control what and when guests get in and out of these hosts: bipyridyl-metal chelation and azobenzene photoisomerization. The effects are reversible by treatment with conventional chelating agents and brief heating, respectively. Accordingly, it is possible to trigger reactions that take place within a cylindrical capsule by light, even though the reaction process is not photochemical by nature. Likewise the presence of metals can regulate reactions without acting as direct catalysts.

Total synthesis of cyclosporine: access to N-methylated peptides via isonitrile coupling reactions

Recent developments in the use of isonitriles to furnish secondary and tertiary amide bond formations have been applied to a novel total synthesis of the important cyclic polypeptide cyclosporine A. Specifically, the disclosed synthetic route demonstrates the utility of microwave-mediated carboxylic acid isonitrile couplings, thioacid isonitrile couplings at ambient temperature, and isonitrile-mediated couplings of carboxylic acids and thioacids with amines to form challenging amide bonds.

Recent Departures in the Synthesis of Peptides and Glycopeptides

In this account, we describe the results of a research program directed to the proposition that chemical synthesis can play a valuable role in identifying biologic level molecules worthy of pharma level development. We recount our journey towards the chemical synthesis of homogeneous erythropoietin, the challenges we encountered, and our efforts to address deficiencies in the current "state of the art" of glycopeptide synthesis. Here we describe new methods for the synthesis of glycopeptides that have emerged from the erythropoietin adventure, including the development of unique C-terminal acyl donors, novel amide bond forming methods, and new ligation and coupling strategies.

Thio FCMA intermediates as strong acyl donors: a general solution to the formation of complex amide bonds

Novel methodology for the formation of amide bonds under neutral conditions is described. Evidence is presented that the active acyl donors are thio FCMA intermediates, generated from the reactions of thioacids with isonitriles.

Coupling reactions of hindered isonitriles and hindered alkyl thioacids: Mechanistic studies

The coupling reaction between hindered thioacids and isonitriles is developed and described. The mechanism for the formation of the thiopyruvamide products is explored, and the method is applied to a selection of substrates.

Thio-mediated synthesis of derivatized N-linked glycopeptides using isonitrile chemistry

The reaction of oligosaccharide isonitriles with peptide thioacids in the presence of bulky thiophenol as activator to provide N-linked glycopeptides at room temperature is described.

Reaction of thioacids with isocyanates and isothiocyanates: a convenient amide ligation process

Thiocarboxylates, prepared conveniently by cleavage of 9-fluorenylmethyl or trimethoxybenzyl thioesters, react at room temperature with isocyanates and isothiocyanates to give amide bonds in good to excellent yield. A carboxylate salt is also shown to react with an electron-deficient isocyanate to give the corresponding amide in excellent yield at room temperature.