Peptide Bond-Forming Reaction via Amino Acid Silyl Esters: New Catalytic Reactivity of an Aminosilane

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.

Peptide Bond Formation Between Unprotected Amino Acids: Convergent Synthesis of Oligopeptides

In the history of peptide chemical synthesis, amino acid-protecting groups have become basic, essential, and common moieties. As the use of protecting groups for amino acids effectively suppresses unwanted side reactions and enables the desired reaction to proceed selectively, their use continues regardless of the complexities of the protection processes and the waste generated by the deprotection residues. We developed peptide bond formation between unprotected amino acids to form silacyclic dipeptides. This is the first report of the proceeding cross-condensation between an unprotected amino acid and another unprotected amino acid. The selectivity, reaction yields, and purity of the products were satisfactory. In addition, we demonstrated further elongation of these compounds and achieved convergent synthesis with peptide-peptide elongation without the use of coupling reagents. Thus, these methods showed the potential to unlock a new, more efficient synthetic path toward polypeptides.

Recent advances in asymmetric synthesis of chiral amides and peptides: racemization-free coupling reagents

Over past decades, chiral amides and peptides have emerged as powerful and versatile compounds due to their various biological activities and interesting molecular architectures. Although some chiral condensation reagents have been applied successfully for their synthesis, the introduction of racemization-free methods of amino acid activation have shown lots of advantages in terms of their low cost and low toxicity. In this review, advancements in amide and peptide synthesis using racemization-free coupling reagents over the last 10 years are summarized. Various racemization-free coupling reagents have been applied in the synthesis of enantioselective amides and peptides, including ynamides, allenones, HSi[OCH(CF3)2]3, Ta(OMe)5, Nb(OEt)5, Ta(OEt)5, TCFH-NMI, water-removable ynamides, DBAA, DATB, o-NosylOXY, TCBOXY, Boc-Oxyma, NDTP, 9-silafluorenyl dichlorides, the Mukaiyama reagent, EDC and T3P. The racemization-free reagents described in this review provide an alternative greener option for the asymmetric synthesis of chiral amides and peptides. We hope that this review will inspire further studies and developments in this field.

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.

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.

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.

Synthesis of Novel Arginine Building Blocks with Increased Lipophilicity Compatible with Solid-Phase Peptide Synthesis

Arginine, due to the guanidine moiety, increases peptides' hydrophilicity and enables interactions with charged molecules, but at the same time, its presence in a peptide chain might reduce its permeability through biological membranes. This might be resolved by temporary coverage of the peptide charge by lipophilic, enzyme-sensitive alkoxycarbonyl groups. Unfortunately, such a modification of a guanidine moiety has not been reported to date and turned out to be challenging. Here, we present a new, optimized strategy to obtain arginine building blocks with increased lipophilicity that were successfully utilized in the solid-phase peptide synthesis of novel arginine vasopressin prodrugs.

Super silyl-based stable protecting groups for both the C- and N-terminals of peptides: applied as effective hydrophobic tags in liquid-phase peptide synthesis

Tag-assisted liquid-phase peptide synthesis (LPPS) is one of the important processes in peptide synthesis in pharmaceutical discovery. Simple silyl groups have positive effects when incorporated in the tags due to their hydrophobic properties. Super silyl groups contain several simple silyl groups and play an important role in modern aldol reactions. In view of the unique structural architecture and hydrophobic properties of the super silyl groups, herein, two new types of stable super silyl-based groups (tris(trihexylsilyl)silyl group and propargyl super silyl group) were developed as hydrophobic tags to increase the solubility in organic solvents and the reactivity of peptides during LPPS. The tris(trihexylsilyl)silyl group can be installed at the C-terminal of the peptides in ester form and N-terminal in carbamate form for peptide synthesis and it is compatible with hydrogenation conditions (Cbz chemistry) and Fmoc-deprotection conditions (Fmoc chemistry). The propargyl super silyl group is acid-resistant, which is compatible with Boc chemistry. Both tags are complementary to each other. The preparation of these tags requires less steps than previously reported tags. Nelipepimut-S was synthesized successfully with different strategies using these two types of super silyl tags.

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).

Diastereo- and Enantioselective Reductive Mannich-type Reaction of α,β-Unsaturated Carboxylic Acids to Ketimines: A Direct Entry to Unprotected β2,3,3 -Amino Acids

Stereoselective construction of unprotected β-amino acids is a significant challenge owing to the lack of methods for the catalytic generation of highly enantioenriched carboxylic acid enolates. In this study, a novel copper-catalyzed diastereo- and enantioselective reductive Mannich-type reaction of α,β-unsaturated carboxylic acids was developed, which provides a direct and scalable synthetic method for enantioenriched β^2,3,3 -amino acids with vicinal stereogenic centers. The protocol features in situ generation of transiently protected carboxylic acids by a hydrosilane and their diastereo- and enantioselective reductive coupling with ketimines. The synthetic utility of this process was demonstrated by a gram-scale reaction and the transformation of β-amino acids.

Organocatalytic Activation of Inert Hydrosilane for Peptide Bond Formation

We describe the development of a reliable catalytic protocol for peptide bond formation that is generally applicable to natural and unnatural α-amino acids, β-amino acids, and peptides bearing various functional groups. A 10 mol % loading of HSi[OCH(CF₃)₂]₃ as a catalyst was sufficient to guarantee a consistently high yield of the resulting peptide. This method facilitates the sustainable utilization of natural resources by using a catalyst and an auxiliary based on earth-abundant silicon.

An economical approach for peptide synthesis via regioselective C-N bond cleavage of lactams

An economical, solvent-free, and metal-free method for peptide synthesis via C-N bond cleavage using lactams has been developed. The method not only eliminates the need for condensation agents and their auxiliaries, which are essential for conventional peptide synthesis, but also exhibits high atom economy. The reaction is versatile because it can tolerate side chains bearing a range of functional groups, affording up to >99% yields of the corresponding peptides without racemisation or polymerisation. Moreover, the developed strategy enables peptide segment coupling, providing access to a hexapeptide that occurs as a repeat sequence in spider silk proteins.

Thionyl Fluoride-Mediated One-Pot Substitutions and Reductions of Carboxylic Acids

Thionyl fluoride (SOF₂) is an underutilized reagent that is yet to be extensively studied for its synthetic applications. We previously reported that it is a powerful reagent for both the rapid syntheses of acyl fluorides and for one-pot peptide couplings, but the full scope of these nucleophilic acyl substitutions had not been explored. Herein, we report one-pot thionyl fluoride-mediated syntheses of peptides and amides (35 examples, 45-99% yields) that were not explored in our previous study. The scope of thionyl fluoride-mediated nucleophilic acyl substitutions was also expanded to encompass esters (24 examples, 64-99% yields) and thioesters (11 examples, 24-96% yields). In addition, we demonstrate that the scope of thionyl fluoride-mediated one-pot reactions can be extended beyond nucleophilic acyl substitutions to mild reductions of carboxylic acids using NaBH₄ (13 examples, 33-80% yields).

Solvent-free amide bond formation using a variety of methoxysilanes as coupling agent

A solvent-free procedure for the formation of amides without exclusion of air and moisture is described. Using tetramethoxysilane 1, hexamethoxydisilane 2 and dodecamethoxy-neopentasilane 3 as coupling agent carboxylic acids and amines are reacted to form amides in good to excellent yields. The formation of these amides was confirmed by NMR spectroscopy and mass spectrometry. Remarkably, neopentasilane 3 exceeds the performance of the currently used monosilanes as coupling agent in terms of group tolerance and yield.

Methyltrimethoxysilane (MTM) as a Reagent for Direct Amidation of Carboxylic Acids

Methyltrimethoxysilane [MTM, CH₃Si(OMe)₃] has been demonstrated to be an effective, inexpensive, and safe reagent for the direct amidation of carboxylic acids with amines. Two simple workup procedures that provide the pure amide product without the need for further purification have been developed. The first employs an aqueous base-mediated annihilation of MTM. The second involves simple product crystallization from the reaction mixture providing a low process mass intensity direct amidation protocol.

Silicon-based hydrophobic tags applied in liquid-phase peptide synthesis: protected DRGN-1 and poly alanine chain synthesis

Two types of silicon-based hydrophobic tags, including a siloxy group containing tag and an arylsilyl group containing tag, were developed for applying them in tag-assisted liquid-phase peptide synthesis (Tag LPPS) to synthesize long peptides.

Cellular imaging properties of phosphorescent iridium(III) complexes substituted with ester or amide groups

We reported four iridium(III) complexes substituted with ester or amide groups as luminescent cellular imaging reagents. While three of the complexes stained the cytoplasm, the other complex showed the exceptional...

Rapid and column-free syntheses of acyl fluorides and peptides using ex situ generated thionyl fluoride

Thionyl fluoride (SOF₂) was first isolated in 1896, but there have been less than 10 subsequent reports of its use as a reagent for organic synthesis. This is partly due to a lack of facile, lab-scale methods for its generation. Herein we report a novel protocol for the ex situ generation of SOF₂ and subsequent demonstration of its ability to access both aliphatic and aromatic acyl fluorides in 55-98% isolated yields under mild conditions and short reaction times. We further demonstrate its aptitude in amino acid couplings, with a one-pot, column-free strategy that affords the corresponding dipeptides in 65-97% isolated yields with minimal to no epimerization. The broad scope allows for a wide range of protecting groups and both natural and unnatural amino acids. Finally, we demonstrated that this new method can be used in sequential liquid phase peptide synthesis (LPPS) to afford tri-, tetra-, penta-, and decapeptides in 14-88% yields without the need for column chromatography. We also demonstrated that this new method is amenable to solid phase peptide synthesis (SPPS), affording di- and pentapeptides in 80-98% yields.

Siloxy Esters as Traceless Activators of Carboxylic Acids: Boron-Catalyzed Chemoselective Asymmetric Aldol Reaction*

The catalytic asymmetric aldol reaction is among the most useful reactions in organic synthesis. Despite the existence of many prominent reports, however, the late-stage, chemoselective, catalytic, asymmetric aldol reaction of multifunctional substrates is still difficult to achieve. Herein, we identified that in situ pre-conversion of carboxylic acids to siloxy esters facilitated the boron-catalyzed direct aldol reaction, leading to the development of carboxylic acid-selective, catalytic, asymmetric aldol reaction applicable to multifunctional substrates. Combining experimental and computational studies rationalized the reaction mechanism and led to the proposal of Si/B enediolates as the active species. The silyl ester formation facilitated both enolization and catalyst turnover by acidifying the α-proton of substrates and attenuating poisonous Lewis bases to the boron catalyst.

Synthesis of Dipeptide, Amide, and Ester without Racemization by Oxalyl Chloride and Catalytic Triphenylphosphine Oxide

An efficient triphenylphosphine oxide-catalyzed amidation and esterification for the rapid synthesis of a series of dipeptides, amides, and esters is described. This reaction is applicable to challenging couplings of hindered carboxylic acids with weakly nucleophilic amines or alcohols, giving the products in good yields (67-90%) without racemization. This system employs the highly reactive intermediate Ph₃PCl₂ as the activator of the carboxylate in a catalytic manner and drives the reaction to completion in a short reaction time (less than 10 min).

Amide bond formation: beyond the dilemma between activation and racemisation

The development of methods for amide bond formation without recourse to typical condensation reagents has become an emerging research area and has been actively explored in the past quarter century. Inspired by the structure of vitamin B12, we have developed a metal-templated macrolactamisation that generates a new wave towards classical macrolactam synthesis. Further, distinct from the extensively used methods with condensation reagents or catalysts based on catalyst/reagent control our metal-catalysed methods based on substrate control can effectively address long-standing challenges such as racemisation in the field of peptide chemistry. In addition, the substrate-controlled strategy demonstrates the feasibility of "remote" peptide bond-forming reaction catalysed by a metal-ligand complex. Moreover, an originally designed hydrosilane/aminosilane system can avoid not only racemisation but also unnecessary waste production. This feature article documents our discovery and application of our original approaches in amide bond formation.

Peptide Bond Formation of Amino Acids by Transient Masking with Silylating Reagents

A one-pot peptide bond-forming reaction has been developed using unprotected amino acids and peptides. Two different silylating reagents, HSi[OCH(CF₃)₂]₃ and MTBSTFA, are instrumental for the successful implementation of this approach, being used for the activation and transient masking of unprotected amino acids and peptides at C-termini and N-termini, respectively. Furthermore, CsF and imidazole are used as catalysts, activating HSi[OCH(CF₃)₂]₃ and also accelerating chemoselective silylation. This method is versatile as it tolerates side chains that bear a range of functional groups, while providing up to >99% yields of corresponding peptides without any racemization or polymerization.

Synthesis of amide derivatives for electron deficient amines and functionalized carboxylic acids using EDC and DMAP and a catalytic amount of HOBt as the coupling reagents

A convenient protocol for amide bond formation for electron deficient amines and carboxylic acids is described. Amide coupling of aniline derivatives has been investigated with a number of reagents under a variety of reaction conditions. The use of 1 equivalent of EDC and 1 equivalent of DMAP, catalytic amount of HOBt and DIPEA provided the best results. This method is amenable to the synthesis of a range of functionalized amide derivatives with electron deficient and unreactive amines.

Silicon compounds as stoichiometric coupling reagents for direct amidation

Despite being one of the most frequently carried out chemical reactions in industry, there is currently no amidation protocol that is regarded as safe, high yielding, environmentally friendly and inexpensive. The direct amidation of a carboxylic acid with an amine is viewed as an inherently good solution for developing such a protocol. Since the 1960s, there has been a gradual development in the use of silicon reagents for direct amidation. This review covers the methods published to April 2021 for silicon reagent mediated direct amidation of a carboxylic acid with an amine. This review also covers the use of polymeric SiO2 to promote direct amidation.

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.