CITU: A Peptide and Decarboxylative Coupling Reagent

Single step synthesis of β- and γ- aryl-substituted ß- and γ-amino acid derivatives by electrochemistry

Electrochemical transformations are a subject of increasing interest in early drug discovery due to its ability to assemble complex scaffolds under rather mild reaction conditions. In this context, we became interested in electrochemical decarboxylative cross-coupling (DCC) protocols of redox-active esters (RAEs) and halo(hetero)arenes. Starting with the one-step electrochemical synthesis of novel methylamino-substituted heterocycles we recognized the potential of this methodology to deliver a novel approach to β- and γ- amino acids by starting from the corresponding RAEs. Our work finally resulted in the delivery of novel and highly valuable trifunctional building blocks based on ß- and γ-amino-acid scaffolds.

Triflylpyridinium as Coupling Reagent for Rapid Amide and Ester Synthesis

An effective method has been developed to facilitate the synthesis of amides and esters at ambient temperature within 5 min, in which a stable and easily accessible triflylpyridinium reagent is used. Remarkably, this method not only has a wide range of substrate compatibility but also could realize the scalable synthesis of peptide and ester via a continuous flow process. Moreover, excellent chirality retentions are presented during activation of carboxylic acid.

Application of Redox-Active Ester Catalysis to the Synthesis of Pyranose Alkyl C-Glycosides

The direct coupling of shelf-stable, tetrachloro-N-hydroxyphthalimide ester (TCNHPI) glycosyl donors with a variety of alkylzinc reagents under redox catalysis is described. Alkyl C-glycosides are formed directly by a decarboxylative, Negishi-type process in 31-73% yields without the need for photocatalytic activation or additional reductants. Extension of this approach to the coupling of TCNHPI donors with stereodefined α-alkoxy furan-containing alkylzinc halides enabled de novo synthesis of methylene-linked exo-C-disaccharides via an Achmatowicz rearrangement.

Total Synthesis and Functional Evaluation of IORs, Sulfonolipid-based Inhibitors of Cell Differentiation in Salpingoeca rosetta

The choanoflagellate Salpingoeca rosetta is an important model system to study the evolution of multicellularity. In this study we developed a new, modular, and scalable synthesis of sulfonolipid IOR-1A (six steps, 27 % overall yield), which acts as bacterial inhibitor of rosette formation in S. rosetta. The synthesis features a decarboxylative cross-coupling reaction of a sulfonic acid-containing tartaric acid derivative with alkyl zinc reagents. Synthesis of 15 modified IOR-1A derivatives, including fluorescent and photoaffinity-based probes, allowed quantification of IOR-1A, localization studies within S. rosetta cells, and evaluation of structure-activity relations. In a proof of concept study, an inhibitory bifunctional probe was employed in proteomic profiling studies, which allowed to deduce binding partners in bacteria and S. rosetta. These results showcase the power of synthetic chemistry to decipher the biochemical basis of cell differentiation processes within S. rosetta.

Polymer End Group Control through a Decarboxylative Cobalt-Mediated Radical Polymerization: New Avenues for Synthesizing Peptide, Protein, and Nanomaterial Conjugates

Cobalt-mediated radical polymerizations (CMRPs) have been initiated by the radical decarboxylation of tetrachlorophthalimide activated esters. This allows for the controlled radical polymerization of activated monomers across a broad temperature range with a single cobalt species, with the incorporation of polymer end groups derived from simple carboxylic acid derivatives and termination with an organozinc reagent. This method has been applied to the synthesis of a polymer/graphene conjugate and a water-soluble protein/polymer conjugate, demonstrating the first examples of CMRP in graphene and protein conjugation.

Structural and Functional Analysis of Bacterial Sulfonosphingolipids and Rosette-Inducing Factor 2 (RIF-2) by Mass Spectrometry-Guided Isolation and Total Synthesis

We have analyzed the abundance of bacterial sulfonosphingolipids, including rosette‐inducing factors (RIFs), in seven bacterial prey strains by using high‐resolution tandem mass spectrometry (HRMS²) and molecular networking (MN) within the Global Natural Product Social Molecular Networking (GNPS) web platform. Six sulfonosphingolipids resembling RIFs were isolated and their structures were elucidated based on comparative MS and NMR studies. Here, we also report the first total synthesis of two RIF‐2 diastereomers and one congener in 15 and eight synthetic steps, respectively. For the total synthesis of RIF‐2 congeners, we employed a decarboxylative cross‐coupling reaction to synthesize the necessary branched α‐hydroxy fatty acids, and the Garner‐aldehyde approach to generate the capnine base carrying three stereogenic centers. Bioactivity studies in the choanoflagellate Salpingoeca rosetta revealed that the rosette inducing activity of RIFs is inhibited dose dependently by the co‐occurring sulfonosphingolipid sulfobacins D and F and that activity of RIFs is specific for isolates obtained from Algoriphagus.

A Modular Approach to the Antifungal Sphingofungin Family: Concise Total Synthesis of Sphingofungin A and C

Sphingofungins are fungal natural products known to inhibit the biosynthesis of sphingolipids which play pivotal roles in various cell functions. Here, we report a short and flexible synthetic approach towards the sphingofungin family. Key step of the synthesis was a decarboxylative cross‐coupling reaction of chiral sulfinyl imines with a functionalized tartaric acid derivative, which yielded the core motif of sphingofungins carrying four consecutive stereocenters and a terminal double bond. Subsequent metathesis reaction allowed for the introduction of different side chains of choice resulting in a total of eight sphingofungins, including for the first time sphingofungin C (eight steps from commercially available protected tartaric acid with an overall yield of 6 %) and sphingofungin A (ten steps). All newly synthesized derivatives were tested for their antifungal, cell‐proliferative and antiparasitic activity unraveling their structure–activity relations.

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.

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.

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.

Clickable coupling of carboxylic acids and amines at room temperature mediated by SO2F2: a significant breakthrough for the construction of amides and peptide linkages

The construction of amide bonds and peptide linkages is one of the most fundamental transformations in all life processes and organic synthesis. The synthesis of structurally ubiquitous amide motifs is essential in the assembly of numerous important molecules such as peptides, proteins, alkaloids, pharmaceutical agents, polymers, ligands and agrochemicals. A method of SO2F2-mediated direct clickable coupling of carboxylic acids with amines was developed for the synthesis of a broad scope of amides in a simple, mild, highly efficient, robust and practical manner (>110 examples, >90% yields in most cases). The direct click reactions of acids and amines on a gram scale are also demonstrated using an extremely easy work-up and purification process of washing with 1 M aqueous HCl to provide the desired amides in greater than 99% purity and excellent yields.

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.

From d- to l-Monosaccharide Derivatives via Photodecarboxylation-Alkylation

Photodecarboxylation-alkylation of conformationally locked monosaccharides leads to inversion of stereochemistry at C5. This allows the synthesis of l-sugars from their readily available d-counterparts. Via this strategy, methyl l-guloside was synthesized from methyl d-mannoside in 21% yield over six steps.

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.

Palladium-Catalyzed Suzuki-Miyaura Reactions of Aspartic Acid Derived Phenyl Esters

Transition-metal-catalyzed transformations of amino acids and peptides could provide a powerful method for their site-selective modification. Here, we report non-decarbonylative Pd-catalyzed Suzuki-Miyaura reactions of phenyl ester derivatives of aspartic acid to form aryl-amino ketones. These products are potentially important in the synthesis of pharmaceuticals, and our methodology represents a new route to access molecules of this type.

Synthesis of thioureido peptidomimetics employing alkyl azides and dithiocarbamates

An unprecedented approach for the assembly of thioureido peptidomimetics is developed employing alkyl azides and dithiocarbamates. Dithiocarbamates react with alkyl azides with the liberation of N2 and elemental sulfur thereby leading to thiourea in a traceless manner. Thioureido peptidomimetics are thus furnished in good yields with no epimerization. This process is mild, free from the use of a base, scalable and step economic. The practicability of this methodology has been highlighted by the synthesis of N,N'-orthogonally protected thioureido peptidomimetics.

One-Pot Electrochemical Nickel-Catalyzed Decarboxylative Sp2-Sp3 Cross-Coupling

A one-pot electrochemical nickel-catalyzed decarboxylative sp²-sp³ cross-coupling reaction has been developed using redox-active esters prepared in situ from alkyl carboxylates and  N-hydroxyphthalimide tetramethyluronium hexafluorophosphate (PITU). This undivided cell one-pot method enables C-C bond formation using inexpensive, benchtop-stable reagents with isolated yields up to 95% with good functional group tolerance, which includes nitrile, ketone, ester, alkene and selectivity over other aromatic halogens.

Radical Retrosynthesis

In The Logic of Chemical Synthesis, E. J. Corey stated that the key to retrosynthetic analysis was a "wise choice of appropriate simplifying transforms" ( Corey , E. J. ; Cheng , X.-M. The Logic of Chemical Synthesis ; John Wiley : New York , 1989 ). Through the lens of "ideality", chemists can identify opportunities that can lead to more practical, scalable, and sustainable synthesis. The percent ideality of a synthesis is defined as [(no. of construction rxns) + (no. of strategic redox rxns)]/(total no. of steps) × 100. A direct consequence of designing "wise" or "ideal" plans is that new transformations often need invention. For example, if functional group interconversions are to be avoided, one is faced with the prospect of directly functionalizing C-H bonds ( Gutekunst , W. R. ; Baran , P. S. Chem. Soc. Rev. 2011 , 40 , 1976 ; Brückl , T. ; et al. Acc. Chem. Res. 2012 , 45 , 826 ). If protecting groups are minimized, methods testing the limits of chemoselectivity require invention ( Baran , P. S. ; et al. Nature 2007 , 446 , 404 ; Young , I. S. ; Baran , P. S. Nat. Chem. 2009 , 1 , 193 ). Finally, if extraneous redox manipulations are to be eliminated, methods directly generating key skeletal bonds result ( Burns , N. Z. ; et al. Angew. Chem., Int. Ed. 2009 , 48 , 2854 ). Such analyses applied to total synthesis have seen an explosion of interest in recent years. Thus, it is the interplay of aspirational strategic demands with the limits of available methods that can influence and inspire ingenuity. E. J. Corey's sage advice holds true when endeavoring in complex molecule synthesis, but together with the tenets of the "ideal" synthesis, avoiding concession steps leads to the most strategically and tactically optimal route ( Hendrickson , J. B. J. Am. Chem. Soc. 1975 , 97 , 5784 ; Gaich , T. ; Baran , P. S. J. Org. Chem. 2010 , 75 , 4657 ). Polar disconnections are intuitive and underlie much of retrosynthetic logic. Undergraduates exposed to multistep synthesis are often taught to assemble organic molecules through the combination of positively and negatively charged synthons because, after all, opposites attract. Indeed, the most employed two-electron C-C bond forming reactions today are those based upon either classical cross-coupling reactions (e.g., Suzuki, Negishi, or Heck) or polar additions (aldol, Michael, or Grignard). These reactions are the mainstay of modern synthesis and have revolutionized the way molecules are constructed due to their robust and predictable nature. In contrast, radical chemistry is sparsely covered beyond the basic principles of radical chain processes (i.e., radical halogenation). The historical perception of radicals as somewhat uncontrollable species does not help the situation. As a result, synthetic chemists are not prone to make radical-based strategic bond disconnections during first-pass retrosynthetic analyses. Recent interest in the use of one-electron radical cross-coupling (RCC) methods has been fueled by the realization of their uniquely chemoselective profiles and the opportunities they uncover for dramatically simplifying synthesis. In general, such couplings can proceed by relying on the innate preferences of a substrate (innate RCC) or through interception with a mediator (usually a transition metal) to achieve programmed RCC. This Account presents a series of case studies illustrating the inherent strategic and tactical advantages of employing both types of radical-based cross-couplings in a variety of disparate settings. Thematically, it is clear that one-electron disconnections, while not considered to be intuitive, can serve to enable syntheses that are more direct and feature a minimal use of protecting group chemistry, functional group interconversions, and nonstrategic redox fluctuations.

TCFH-NMI: Direct Access to N-Acyl Imidazoliums for Challenging Amide Bond Formations

Challenging couplings of hindered carboxylic acids with non-nucleophilic amines to form amide bonds can be accomplished in high yields, and in many cases, with complete retention of the adjacent stereogenic centers using the combination of N, N, N', N'-tetramethylchloroformamidinium hexafluorophosphate (TCFH) and N-methylimidazole (NMI). This method allows for in situ generation of highly reactive acyl imidazolium ions, which have been demonstrated to be intermediates in the reaction. The reagent delivers high reactivity similar to acid chlorides with the ease of use of modern uronium reagents.