Decarboxylative couplings as versatile tools for late-stage peptide modifications

Discovery and development of tyrosine-click (Y-click) reaction for the site-selective labelling of proteins

With the versatile utility of bio-conjugated peptides and proteins in the fields of agriculture, food, cosmetics and pharmaceutical industry, the design of smart protocols to conjugate and modulate biomolecules becomes highly desirable. During this process, the most important consideration for biochemists is the retention of configurational integrity of the biomolecules. Moreover, this type of bioconjugation of peptide and protein becomes frivolous if the reaction is not performed with precise amino acid residues. Hence, chemo-selective, as well as site-selective reactions, that are biocompatible and possess an appropriate level of reactivity are necessary. Based on click chemistry, there are so many tyrosine (Y) conjugation strategies, such as sulfur-fluoride exchange (SuFEx), sulfur-triazole exchange (SuTEx), coupling with π-allyl palladium complexes, diazonium salts, diazodicarboxyamide-based reagents etc. Among these techniques, diazodicarboxyamide-based Y-conjugation, which is commonly known as the "tyrosine-click (Y-click) reaction", has met the expectations of synthetic and biochemists for the tyrosine-specific functionalization of biomolecules. Over the past one and a half decades, significant progress has been made in the classical organic synthesis approach, as well as its biochemical, photochemical, and electrochemical variants. Despite such progress and increasing importance, the Y-click reaction has not been reviewed to document variations in its methodology, applications, and broad utility. The present article aims to provide a summary of the approaches for the modulation of biomolecules at the hotspot of tyrosine residue by employing the Y-click reaction. The article also highlights its application for the mapping of proteins, imaging of living cells, and in the fields of analytical and medicinal chemistry.

On-Resin Photochemical Decarboxylative Arylation of Peptides

Here we describe the application of photochemical decarboxylative arylation as a late-stage functionalization reaction for peptides. The reaction uses redox-active esters of aspartic acid and glutamic acid on the solid phase to provide analogues of aromatic amino acids. By using aryl bromides as arylation reagents, a wide variety of amino acids can be accessed without having to synthesize them individually in solution. The reaction is compatible with proteinogenic amino acids and was used to perform a structure-activity relationship study of a PRMT5 binding peptide.

Electrochemical Synthesis of Unnatural Amino Acids Embedding 5- and 6-Membered Heteroaromatics

Using a commercially available potentiostat, the electrochemical synthesis of unnatural amino acids bearing heteroaromatics on the lateral chain has been accomplished. This strategy exploits the side-chain decarboxylative arylation of aspartic/glutamic acid, a reaction that becomes challenging with electron-rich coupling partners such as 5- and 6-membered heteroaromatics. These rings are underrepresented in unnatural amino acids, therefore allowing a wider exploration of the chemical space, given the abundance of the aryl bromides employable in this reaction.

Recent Advances in Photoinduced Modification of Amino Acids, Peptides, and Proteins

The chemical modification of biopolymers like peptides and proteins is a key technology to access vaccines and pharmaceuticals. Similarly, the tunable derivatization of individual amino acids is important as they are key building blocks of biomolecules, bioactive natural products, synthetic polymers, and innovative materials. The high diversity of functional groups present in amino acid-based molecules represents a significant challenge for their selective derivatization Recently, visible light-mediated transformations have emerged as a powerful strategy for achieving chemoselective biomolecule modification. This technique offers numerous advantages over other methods, including a higher selectivity, mild reaction conditions and high functional-group tolerance. This review provides an overview of the most recent methods covering the photoinduced modification for single amino acids and site-selective functionalization in peptides and proteins under mild and even biocompatible conditions. Future challenges and perspectives are discussed beyond the diverse types of photocatalytic transformations that are currently available.

Electrochemistry-enabled residue-specific modification of peptides and proteins

Selective chemical reactions at precise amino acid residues of peptides and proteins have become an exploding field of research in the last few decades. With the emerging utility of bioconjugated peptides and proteins as drug leads and therapeutic agents, the design of smart protocols to modulate and conjugate biomolecules has become necessary. During this modification, the most important concern of biochemists is to keep intact the structural integrity of the biomolecules. Hence, a soft and selective biocompatible reaction environment is necessary. Electrochemistry, a mild and elegant tunable reaction platform to synthesize complex molecules while avoiding harsh and toxic chemicals, can provide such a reaction condition. However, this strategy is yet to be fully exploited in the field of selective modification of polypeptides. With this possibility, the use of electrochemistry as a reaction toolbox in peptide and protein chemistry is flourishing day by day. Unfortunately, there is no suitable review article summarizing the residue-specific modification of biomolecules. The present review provides a comprehensive summary of the latest manifested electrochemical approaches for the modulation of five redox-active amino acid residues, namely cysteine, tyrosine, tryptophan, histidine and methionine, found in peptides and proteins. The article also highlights the incredible potential of electrochemistry for the regio- as well as chemoselective bioconjugation strategy of biomolecules.

Visible-Light Mediated Deoxygenation of Carboxylic Acid for Late-Stage Peptide Modification Targeting Dehydroalanine

A visible-light mediated deoxygenative radical addition of carboxylic acids to dehydroalanines has been disclosed. The method can be used in β-acyl alanine derivative synthesis, including those chiral and deuterated variants, and late-stage peptide modification with various functional groups, both in the homogeneous phase and on the resin in SPPS. It provides a new tool kit for rapid construction of bioactive peptide analogues, which has been demonstrated by modification of the antimicrobial peptide Feleucin-K3.

Carbon-Centered Radicals in Protein Manipulation

Methods to directly post-translationally modify proteins are perhaps the most straightforward and operationally simple ways to create and study protein post-translational modifications (PTMs). However, precisely altering or constructing the C-C scaffolds pervasive throughout biology is difficult with common two-electron chemical approaches. Recently, there has been a surge of new methods that have utilized single electron/radical chemistry applied to site-specifically "edit" proteins that have started to create this potential-one that in principle could be near free-ranging. This review provides an overview of current methods that install such "edits", including those that generate function and/or PTMs, through radical C-C bond formation (as well as C-X bond formation via C• where illustrative). These exploit selectivity for either native residues, or preinstalled noncanonical protein side-chains with superior radical generating or accepting abilities. Particular focus will be on the radical generation approach (on-protein or off-protein, use of light and photocatalysts), judging the compatibility of conditions with proteins and cells, and novel chemical biology applications afforded by these methods. While there are still many technical hurdles, radical C-C bond formation on proteins is a promising and rapidly growing area in chemical biology with long-term potential for biological editing.

Amino Acid Sulfinate Salts as Alkyl Radical Precursors

A general approach to the synthesis of amino acid sulfinate salts from commercially available α-chiral hydroxylated amino acids is reported. These reagents are shown to be valuable precursors to alkyl radicals under mild photochemical oxidation conditions. The photochemically generated amino acid radicals engage readily with alkyl and aryl disulfide radical traps to afford a diverse suite of modified amino acids.

Copper(I)-Catalyzed Late-Stage Introduction of Oxime Ethers into Peptides at the Carboxylic Acid Site

We have developed a method of introducing biological oxime ether fragments into peptides by CuI-catalyzed late-stage modification and functionalization of peptides, utilizing their acid moiety and varied 2H-azirines. As a result of its mild conditions, high atom economy, moderate yield, and excellent functional-group tolerance, the method can provide access to late-stage peptide modification and functionalization at their acid sites both in the homogeneous phase and on resins in SPPS, providing a new tool kit for peptide functionalization, diversification, and fluorescent labeling.

Photocatalyzed Oxidative Decarboxylation Forming Aminovinylcysteine Containing Peptides

The formation of (2S,3S)-S-[(Z)-aminovinyl]-3-methyl-D-cysteine (AviMeCys) substructures was developed based on the photocatalyzed-oxidative decarboxylation of lanthionine-bearing peptides. The decarboxylative selenoetherification of the N-hydroxyphthalimide ester, generated in situ, proceeded under mild conditions at −40 °C in the presence of 1 mol% of eosin Y-Na2 as a photocatalyst and the Hantzsch ester. The following β-elimination of the corresponding N,Se-acetal was operated in a one-pot operation, led to AviMeCys substructures found in natural products in moderate to good yields. The sulfide-bridged motif, and also the carbamate-type protecting groups, such as Cbz, Teoc, Boc and Fmoc groups, were tolerant under the reaction conditions.

Late‐Stage Functionalisation of Peptides on the Solid Phase by an Iodination‐Substitution Approach

The functionalisation of peptides at a late synthesis stage holds great potential, for example, for the synthesis of peptide pharmaceuticals, fluorescent biosensors or peptidomimetics. Here we describe an on‐resin iodination‐substitution reaction sequence on homoserine that is also suitable for peptide modification in a combinatorial format. The reaction sequence is accessible to a wide range of sulfur nucleophiles with various functional groups including boronic acids, hydroxy groups or aromatic amines. In this way, methionine‐like thioethers or thioesters and thiosulfonates are accessible. Next to sulfur nucleophiles, selenols, pyridines and carboxylic acids were successfully used as nucleophiles, whereas phenols did not react. The late‐stage iodination‐substitution approach is not only applicable to short peptides but also to the more complex 34‐amino‐acid WW domains. We applied this strategy to introduce 7‐mercapto‐4‐methylcoumarin into a switchable Zn^II responsive WW domain to design an iFRET‐based Zn^II sensor.

Solid-Phase Photochemical Peptide Homologation Cyclization

Forging new C(sp3)-C(sp3) bonds to central positions within a peptide backbone is critical for the development of new therapeutics and chemical probes. Currently, there are no methods for decarboxylating Asp and Glu side chains solid-phase photochemically or using such radicals to form peptide macrocycles. Herein, electron-donor-acceptor complexes between Hantzsch ester and on-resin peptide N-hydroxyphthalimide radical precursors are used to access these radicals, demonstrated with two-carbon homologations and homologation cyclizations of Atosiban and RGDf.

Solid-Phase Peptide Modification via Deaminative Photochemical Csp3 -Csp3 Bond Formation Using Katritzky Salts

Introduction of unnatural amino acids can significantly improve the binding affinity and stability of peptides. Commercial availability of such amino acids is limited, and their synthesis is a long and tedious process. We here describe a method that allows the functionalization of peptides directly on solid-support by converting lysine residues to Katritzky salts, and subjecting them to a photochemical Giese reaction under mild reaction conditions. The method avoids the need for amino acid synthesis and instead offers a late-stage modification route for rapid peptide diversification. While numerous modification approaches at the lysine amine have been described, this work provides the first example of deaminative functionalization of peptides at lysine. The two-step protocol is compatible with various substrates, lysine analogues, resins, and all proteinogenic amino acids. Finally, by leveraging solid-phase modification, this protocol facilitates the functionalization of longer peptides as was demonstrated using biologically relevant peptides of up to 15 amino acids.

Electrochemistry for the Chemoselective Modification of Peptides and Proteins

Although electrochemical strategies for small-molecule synthesis are flourishing, this technology has yet to be fully exploited for the mild and chemoselective modification of peptides and proteins. With the growing number of diverse peptide natural products being identified and the emergence of modified proteins as therapeutic and diagnostic agents, methods for electrochemical modification stand as alluring prospects for harnessing the reactivity of polypeptides to build molecular complexity. As a mild and inherently tunable reaction platform, electrochemistry is arguably well-suited to overcome the chemo- and regioselectivity issues which limit existing bioconjugation strategies. This Perspective will showcase recently developed electrochemical approaches to peptide and protein modification. The article also highlights the wealth of untapped opportunities for the production of homogeneously modified biomolecules, with an eye toward realizing the enormous potential of electrochemistry for chemoselective bioconjugation chemistry.

Visible-Light-Mediated Modification and Manipulation of Biomacromolecules

Chemically modified biomacromolecules-i.e., proteins, nucleic acids, glycans, and lipids-have become crucial tools in chemical biology. They are extensively used not only to elucidate cellular processes but also in industrial applications, particularly in the context of biopharmaceuticals. In order to enable maximum scope for optimization, it is pivotal to have a diverse array of biomacromolecule modification methods at one's disposal. Chemistry has driven many significant advances in this area, and especially recently, numerous novel visible-light-induced photochemical approaches have emerged. In these reactions, light serves as an external source of energy, enabling access to highly reactive intermediates under exceedingly mild conditions and with exquisite spatiotemporal control. While UV-induced transformations on biomacromolecules date back decades, visible light has the unmistakable advantage of being considerably more biocompatible, and a spectrum of visible-light-driven methods is now available, chiefly for proteins and nucleic acids. This review will discuss modifications of native functional groups (FGs), including functionalization, labeling, and cross-linking techniques as well as the utility of oxidative degradation mediated by photochemically generated reactive oxygen species. Furthermore, transformations at non-native, bioorthogonal FGs on biomacromolecules will be addressed, including photoclick chemistry and DNA-encoded library synthesis as well as methods that allow manipulation of the activity of a biomacromolecule.

Organometallic AlaM Reagents for Umpolung Peptide Diversification

Selective modifications of peptides and proteins have emerged as a promising strategy to develop novel mechanistic probes and prepare compounds with translational potentials. Here, we report alanine carbastannatranes AlaSn as a universal synthon in various C-C and C-heteroatom bond-forming reactions. These reagents are compatible with peptide manipulation techniques and can undergo chemoselective conjugation in minutes when promoted by Pd(0). Despite their increased nucleophilicity and propensity to transfer the alkyl group, C(sp3)-C(sp2) coupling with AlaSn can be accomplished at room temperature under buffered conditions (pH 6.5-8.5). We also show that AlaSn can be easily transformed into several canonical L- and D-amino acids in arylation, acylation, and etherification reactions. Furthermore, AlaSn can partake in macrocyclizations exemplified by the synthesis of medium size cyclic peptides with various topologies. Taken together, metalated alanine AlaSn demonstrates unparalleled scope and represents a new type of umpolung reagents suitable for structure-activity relationship studies and peptide diversification.

Electrochemically Enabled C-Terminal Peptide Modifications

Modified peptides serve as promising therapeutic leads, valuable tools for chemical biology, and diverse functional materials. Synthetic strategies which enable the direct modification of native peptide sequences are particularly attractive for the rapid generation of designer peptides. This chapter details an operationally simple electrochemical approach to the modification of the peptide C-terminus, which proceeds via direct anodic oxidation of C-terminal peptide carboxylic acids. Electrochemical decarboxylation affords a key N,O-acetal intermediate, which can be engaged with various nucleophiles. Herein, step-by-step protocols for C-terminal arylation and sulfonylation are presented to highlight the utility of the method for the preparation of valuable functionalized peptides.

Oxime/Hydrazone Conjugation at Histidine: Late-Stage Functionalization Approach of Unprotected Peptides

Synthetic molecular probes have recently been in focus for their potential use in target deconvolution, target engagement studies, and imaging. With the field expanding, new strategies to develop such tools are in high demand. While traditional conjugation techniques relying on inherently nucleophilic amino acids such as cysteine (Cys) and lysine (Lys) or pre-incorporated non-natural amino acids are still heavily used, novel methodologies for the direct and site-selective modification of peptides are attracting increasing attention. Of particular interest are Late-Stage Functionalization (LSF) approaches based on radical chemistry as they afford mild and biocompatible alternatives to transition-metal catalysis. A recent synthetic method, which leverages the unique reactivity of histidine (His), has proven to be a promising new strategy for LSF and site-selective conjugation of unprotected peptides. In this chapter, detailed step-by-step protocols depicting the C2-alkylation of His-containing peptides, the unveiling of a ketone as handle for hydrazone conjugation, and its use to site-selectively introduce a fluorophore at this residue are discussed. In addition to its application toward the synthesis of molecular probes, this methodology can be employed in peptide-based drug discovery programs, offering the possibility to rapidly explore the chemical space surrounding peptide hits. Finally, this strategy is also amenable to the preparation of novel peptide-ASO/small molecule drug conjugates.

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.

Radical-Based Synthesis and Modification of Amino Acids

Amino acids (AAs) are key structural motifs with widespread applications in organic synthesis, biochemistry, and material sciences. Recently, with the development of milder and more versatile radical-based procedures, the use of strategies relying on radical chemistry for the synthesis and modification of AAs has gained increased attention, as they allow rapid access to libraries of novel unnatural AAs containing a wide range of structural motifs. In this Minireview, we provide a broad overview of the advancements made in this field during the last decade, focusing on methods for the de novo synthesis of α-, β-, and γ-AAs, as well as for the selective derivatisation of canonical and non-canonical α-AAs.

Small peptide diversification through photoredox-catalyzed oxidative C-terminal modification

A photoredox-catalyzed oxidative decarboxylative coupling of small peptides is reported, giving access to a variety of N,O-acetals. They were used as intermediates for the addition of phenols and indoles, leading to novel peptide scaffolds and bioconjugates. Amino acids with nucleophilic side chains, such as serine, threonine, tyrosine and tryptophan, could also be used as partners to access tri- and tetrapeptide derivatives with non-natural cross-linking.

Photocatalytic methods for amino acid modification

This tutorial review introduces photocatalysis for amino acid modification and summarises recent advances in the field.

Peptide macrocyclization by transition metal catalysis

Peptide macrocyclization has traditionally relied on lactam, lactone and disulfide bond-forming reactions that aim at introducing conformational constraints into small peptide sequences. With the advent of ruthenium-catalyzed ring-closing metathesis and copper-catalyzed alkyne-azide cycloaddition, peptide chemists embraced transition metal catalysis as a powerful macrocyclization tool with relevant applications in chemical biological and peptide drug discovery. This article provides a comprehensive overview of the reactivity and methodological diversification of metal-catalyzed peptide macrocyclization as a special class of late-stage peptide derivatization method. We report the evolution from classic palladium-catalyzed cross-coupling approaches to more modern oxidative versions based on C-H activation, heteroatom alkylation/arylation and annulation processes, in which aspects such as chemoselectivity and diversity generation at the ring-closing moiety became dominant over the last years. The transit from early cycloadditions and alkyne couplings as ring-closing steps to very recent 3d metal-catalyzed macrocyclization methods is highlighted. Similarly, the new trends in decarboxylative radical macrocyclizations and the interplay between photoredox and transition metal catalysis are included. This review charts future perspectives in the field hoping to encourage further progress and applications, while bringing attention to the countless possibilities available by diversifying not only the metal, but also the reactivity modes and tactics to bring peptide functional groups together and produce structurally diverse macrocycles.

Site-selective aqueous C-H acylation of tyrosine-containing oligopeptides with aldehydes

The development of useful synthetic tools to label amino acids within a peptide framework for the ultimate modification of proteins in a late-stage fashion is a challenging task of utmost importance within chemical biology. Herein, we report the first Pd-catalyzed C-H acylation of a collection of Tyr-containing peptides with aldehydes. This water-compatible tagging technique is distinguished by its site-specificity, scalability and full tolerance of sensitive functional groups. Remarkably, it provides straightforward access to a high number of oligopeptides with altered side-chain topology including mimetics of endomorphin-2 and neuromedin N, thus illustrating its promising perspectives toward the diversification of structurally complex peptides and chemical ligation.

Transition-Metal-Mediated Modification of Biomolecules

The site-selective modification of biomolecules has grown spectacularly in recent years. The presence of a large number of functional groups in a biomolecule makes its chemo- and regioselective modification a challenging goal. In this context, transition-metal-mediated reactions are emerging as a powerful tool owing to their unique reactivity and good functional group compatibility, allowing highly efficient and selective bioconjugation reactions that operate under mild conditions. This Minireview focuses on the current state of organometallic chemistry for bioconjugation, highlighting the potential of transition metals for the development of chemoselective and site-specific methods for functionalization of peptides, proteins and nucleic acids. The importance of the selection of ligands attached to the transition metal for conferring the desired chemoselectivity will be highlighted.

Total synthesis of biseokeaniamides A-C and late-stage electrochemically-enabled peptide analogue synthesis

The first total synthesis of cytotoxic cyanobacterial peptide natural products biseokeaniamides A-C is reported employing a robust solid-phase approach to peptide backbone construction followed by coupling of a key thiazole building block. To rapidly access natural product analogues, we have optimized an operationally simple electrochemical oxidative decarboxylation-nucleophilic addition pathway which exploits the reactivity of native C-terminal peptide carboxylates and abrogates the need for building block syntheses. Electrochemically-generated N,O-acetal intermediates are engaged with electron-rich aromatics and organometallic reagents to forge modified amino acids and peptides. The value of this late-stage modification method is highlighted by the expedient and divergent production of bioactive peptide analogues, including compounds which exhibit enhanced cytotoxicity relative to the biseokeaniamide natural products.

Decarboxylative Couplings for Late-Stage Peptide Modifications

The application of designer peptides in medicinal chemistry, chemical biology, and materials science has generated new interest in synthetic methods for the structural modification of amino acids. Strategies which facilitate the direct diversification of proteinogenic functional groups within unprotected peptide substrates are particularly attractive as they leverage modern solution- and solid-phase protocols-tools which are now both robust and routine-for the synthesis of native peptides. Accordingly, a recent approach to the decarboxylative functionalization of peptidic carboxylic acids, including aspartic/glutamic acid residues and α-carboxylic acids, has proven to be a promising new strategy for peptide modification. This synthetic method merges conventional strategies for the activation of carboxylic acids with transition metal-catalyzed cross-coupling chemistry to forge new C-C bonds for the late-stage introduction of valuable synthetic handles. This chapter details a step-by-step protocol for the activation and nickel-catalyzed decarboxylative alkylation of a simple peptide substrate to highlight the broad utility of this strategy for the synthesis of designer peptides.

Visible-Light-Induced Deaminative Thioesterification of Amino Acid Derived Katritzky Salts via Electron Donor-Acceptor Complex Formation

A visible-light-mediated deaminative thioesterification of amino acid derived Katritzky salts with thiobenzoic acid has been developed, which provides a novel synthetic method for the synthesis of α-mercapto acid derivatives under mild conditions. This photoredox catalyst-free generation of alkyl radicals via C-N bond cleavage is enabled by the formation of an electron-donor-acceptor (EDA) complex between the Katritzky salt and thiobenzoic acid anion, which represents a new entry for EDA complex chemistry.

Pd-catalyzed site-selective C(sp2)-H radical acylation of phenylalanine containing peptides with aldehydes

The site-selective functionalization of C-H bonds within a peptide framework remains a challenging task of prime synthetic importance. Herein, the first Pd-catalyzed δ-C(sp2)-H acylation of Phe containing peptides with aldehydes is described. This oxidative coupling is distinguished by its site-specificity, tolerance of sensitive functional groups, scalability, and enantiospecificity and exhibits entire chemoselectivity for Phe motifs over other amino acid units. The compatibility of this dehydrogenative acylation platform with a number of oligopeptides of high structural complexity illustrates its ample opportunities for the late-stage peptide modification and bioconjugation.

Silver-Catalyzed Decarboxylative Alkylfluorination of Alkenes

A decarboxylation of alkyl carboxylic acids for alkylfluorination of alkene was developed, with the catalysis of silver(I) and Selectfluor as both the oxidant and fluorine source. This reaction is highly chemoselective, producing the decarboxylative alkylfluorination products rather than the competitive fluorination of aliphatic carboxylic acids. This practical transformation proceeds efficiently in aqueous media at room temperature and exhibits a large range of functional-group tolerance in various primary and secondary aliphatic carboxylates and alkenes.

Ketones from Nickel-Catalyzed Decarboxylative, Non-Symmetric Cross-Electrophile Coupling of Carboxylic Acid Esters

Synthesis of the C-C bonds of ketones relies upon one high-availability reagent (carboxylic acids) and one low-availability reagent (organometallic reagents or alkyl iodides). We demonstrate here a ketone synthesis that couples two different carboxylic acid esters, N-hydroxyphthalimide esters and S-2-pyridyl thioesters, to form aryl alkyl and dialkyl ketones in high yields. The keys to this approach are the use of a nickel catalyst with an electron-poor bipyridine or terpyridine ligand, a THF/DMA mixed solvent system, and ZnCl2 to enhance the reactivity of the NHP ester. The resulting reaction can be used to form ketones that have previously been difficult to access, such as hindered tertiary/tertiary ketones with strained rings and ketones with α-heteroatoms. The conditions can be employed in the coupling of complex fragments, including a 20-mer peptide fragment analog of Exendin(9-39) on solid support.