Ambient temperature synthesis of high enantiopurity N-protected peptidyl ketones by peptidyl thiol ester-boronic acid cross-coupling

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.

Nickel-Catalyzed Enantioselective Hydrothiocarbonylation of Cyclopropenes

Hydrothiocarbonylation of olefins using carbon monoxide and thiols is a powerful method to synthesize thioesters from simple building blocks. Owing to the intrinsic challenges of catalyst poisoning, transition-metal-catalyzed asymmetric thiocarbonylation, particularly when utilizing earth abundant metals, remains rare in the literature. Herein, we report a nickel-catalyzed enantioselective hydrothiocarbonylation of cyclopropenes for the synthesis of a diverse collection of functionalized thioesters in good to excellent yields with high stereoselectivity. This new method employs an inexpensive, air-stable nickel(II) precursor, which provides enhanced catalyst fidelity against CO poisoning compared to nickel(0) catalysts.

Synthesis of (-)-Cotylenol, a 14-3-3 Molecular Glue Component

Small molecules that modulate the 14-3-3 protein-protein interaction (PPI) network represent valuable therapeutics and tool compounds. However, access has been lost to 14-3-3 PPI molecular glues of the cotylenin class, leading to investigations into the practical chemical syntheses of congeners and analogues. Here we report a concise synthesis of (-)-cotylenol via a 10-step asymmetric entry into a diversifiable 5-8-5 core. This route features a mild Liebeskind-Srogl fragment coupling that tolerates unprecedented steric hindrance to produce a highly congested ketone, and a tandem Claisen-ene cascade that establishes the 8-membered ring. Late-stage control of stereochemistry and functionality leads to (-)-cotylenol and sets the stage for focused library synthesis.

Application of Oxazaborolidine Catalysts (CBS) on Enantioselective 1,4-Addition of Diarylphosphine Oxides to α,β-Unsaturated Thioesters

Here, we report the first catalytic enantioselective 1,4-addition of diarylphosphine oxides to α,β-unsaturated thioesters. Importantly, the most common and commercial oxazaborolidine (CBS) was employed as a catalyst for its new application without being activated by strong protonic acids or Lewis acids and led to the chiral thioesters in excellent yields and enantioselectivities. Furthermore, this method features mild reaction conditions (room temperature and air-insensitive), good substrate tolerance, and easy scalability.

Divergent Acyl and Decarbonylative Liebeskind-Srogl Cross-Coupling of Thioesters by Cu-Cofactor and Pd-NHC (NHC = N-Heterocyclic Carbene) Catalysis

Transition-metal-catalyzed cross-coupling reactions of thioesters by selective acyl C(O)-S cleavage have emerged as a powerful platform for the preparation of complex molecules. Herein, we report divergent Liebeskind-Srogl cross-coupling of thioesters by Pd-NHC (NHC = N-heterocyclic carbene) catalysis. The reaction provides straightforward access to functionalized ketones by highly selective C(acyl)-S cleavage under mild conditions. Most crucially, the conditions enable direct functionalization of a range of complex pharmaceuticals decorated with a palette of sensitive functional groups, providing attractive products for medicinal chemistry programs. Furthermore, decarbonylative Liebeskind-Srogl cross-coupling by C(acyl)-S/C(aryl)-C(O) cleavage is reported. Cu metal cofactor directs the reaction pathway to acyl or decarbonylative pathway. This reactivity is applicable to complex pharmaceuticals. The reaction represents the mildest decarbonylative Suzuki cross-coupling discovered to date. The Cu-directed divergent acyl and decarbonylative cross-coupling of thioesters opens up chemical space in complex molecule synthesis.

Direct Synthesis of α-Amino Acid Derivatives by Hydrative Amination of Alkynes

α-Amino acid derivatives are key components of the molecules of life. The synthesis of α-amino carbonyl/carboxyl compounds is a contemporary challenge in organic synthesis. Herein, we report a practical method for the preparation of α-amino acid derivatives via direct hydrative amination of activated alkynes under mild conditions, relying on sulfinamides as the nitrogen source. Computational studies suggest that the reaction is enabled by a new type of sulfonium [2,3]-sigmatropic rearrangement.

Synthese von α-Aminosäurederivaten durch hydrative Aminierung von Alkinen

α‐Aminosäurederivate sind Kernbestandteile jeglichen Lebens. Die Synthese von α‐Aminocarbonyl/carboxyl Verbindungen ist jedoch weiterhin eine Herausforderung für die organische Synthese. In dieser Arbeit berichten wir von einer praktischen Herstellungsmethode für α‐Aminosäurederivate durch direkte hydrative Aminierung von aktivierten Alkinen mit Sulfinamiden unter milden Bedingungen. Computergestützte Untersuchungen legen nahe, dass eine [2,3]‐sigmatrope Sulfoniumumlagerung der zentrale Schritt der Reaktion ist.

Direct Access to Thiocyano-Thioesters from Cyclic Thioacetals via Photoredox Catalysis: An Introduction of Two Functional Groups in One Pot

The cyanation of organic compounds is an important synthetic transformation and mainly relies on a toxic CN source. Undeniably, thiocyanate salt has emerged as a very mild and environmentally benign CN source, yet its synthetic utility for cyanation is highly limited to very few types of organic compounds. Herein, we report the direct cyanation of cyclic thioacetals for accessing compounds with two different functional groups (thiocyano-thioesters) in one pot using sodium thiocyanate via photoredox catalysis. The protocol has been further extended for the direct cyanation of disulfides and diselenide to access aryl thiocyanates and aryl selenocyanate. A plausible mechanism has been proposed based on a series of control experiments, cyclic voltammetry and Stern-Volmer studies.

Stereoretentive cross-coupling of chiral amino acid chlorides and hydrocarbons through mechanistically controlled Ni/Ir photoredox catalysis

The direct modification of naturally occurring chiral amino acids to their amino ketone analogs is a significant synthetic challenge. Here, an efficient and robust cross-coupling reaction between chiral amino acid chlorides and unactivated C(sp³)–H hydrocarbons is achieved by a mechanistically designed Ni/Ir photoredox catalysis. This reaction, which proceeds under mild conditions, enables modular access to a wide variety of chiral amino ketones that retain the stereochemistry of the starting amino acids. In-depth mechanistic analysis reveals that the strategic generation of an N-acyllutidinium intermediate is critical for the success of this reaction. The barrierless reduction of the N-acyllutidinium intermediate facilitates the delivery of chiral amino ketones with retention of stereochemistry. This pathway avoids the formation of a detrimental nickel intermediate, which could be responsible for undesirable decarbonylation and transmetalation reactions that limit the utility of previously reported methods.

Chiral α-amino ketones are privileged motifs in bioorganic and medicinal chemistry. Here, the authors develop an efficient method to synthesize these structures via stereoretentive direct cross-coupling of amino acid chlorides with simple aliphatic substrates.

Suzuki-type cross-coupling of alkyl trifluoroborates with acid fluoride enabled by NHC/photoredox dual catalysis

The Suzuki–Miyaura cross-coupling of C(sp³)-hybridised boronic compounds still remains a challenging task, thereby hindering the broad application of alkyl boron substrates in carbon–carbon bond-forming reactions. Herein, we developed an NHC/photoredox dual catalytic cross-coupling of alkyl trifluoroborates with acid fluorides, providing an alternative solution to the classical acylative Suzuki coupling chemistry. With this protocol, various ketones could be rapidly synthesised from readily available materials under mild conditions. Preliminary mechanistic studies shed light on the unique radical reaction mechanism.

An acylative Suzuki-type cross-coupling of alkyl trifluoroborates and acid fluorides was developed by merging NHC organocatalysis with photoredox catalysis. A broad spectrum of ketones could be facilely synthesised under mild reaction conditions.

Rhodium-catalyzed regioselective addition of thioacids to terminal allenes: enantioselective access to branched allylic thioesters

Rhodium-catalyzed regio- and enantioselective hydrothiolation of terminal allenes with thioacids is reported for the atom-economic synthesis of chiral branched allylic thioesters. By using a rhodium(I) catalyst system, diversities of terminal allenes and thioacids afforded the corresponding branched thioesters in excellent regioselectivity, high yield, and good enantioselectivity. This method was also explored for Fmoc-protected aminothioacids for diastereoselective synthesis of the corresponding thioesters.

An Easy Route to Aziridine Ketones and Carbinols

N,N-Dimethylaziridine-2-carboxamides react with organolithium reagents yielding 2-aziridinylketones. The reaction with one equivalent of organolithium compound is selective to amide carbonyl at a low (−78 °C) temperature. These ketones, in reaction with organolithium reagents, give symmetrical and unsymmetrical aziridinyl carbinols. The usage of excess phenyllithium may serve as a special N-Boc-protecting group cleavage method for acid-sensitive substrates.

Palladium-catalyzed benzylic C(sp3)-H carbonylative arylation of azaarylmethyl amines with aryl bromides

A highly selective palladium-catalyzed carbonylative arylation of weakly acidic benzylic C(sp3)-H bonds of azaarylmethylamines with aryl bromides under 1 atm of CO gas has been achieved. This work represents the first examples of use of such weakly acidic pronucleophiles in this class of transformations. In the presence of a NIXANTPHOS-based palladium catalyst, this one-pot cascade process allows a range of azaarylmethylamines containing pyridyl, quinolinyl and pyrimidyl moieties and acyclic and cyclic amines to undergo efficient reactions with aryl bromides and CO to provide α-amino aryl-azaarylmethyl ketones in moderate to high yields with a broad substrate scope and good tolerance of functional groups. This reaction proceeds via in situ reversible deprotonation of the benzylic C-H bonds to give the active carbanions, thereby avoiding prefunctionalized organometallic reagents and generation of additional waste. Importantly, the operational simplicity, scalability and diversity of the products highlight the potential applicability of this protocol.

Palladium-Catalyzed Linear Hydrothiocarbonylation of Unactivated Terminal Alkenes: Synthesis of Aliphatic Thioesters

A Pd-catalyzed hydrothiocarbonylation of unactivated terminal alkenes is presented. According to this protocol, aliphatic thioesters were synthesized with exclusive linear selectivity under mild reaction conditions. Good to excellent yields (up to 91% yield), broad substrate scope, broad functional group tolerance, and utility of the method demonstrated the advantages of this protocol.

Cu-Catalyzed Oxidative Thioesterification of Aroylhydrazides with Disulfides

An alternative thioesterification reaction via copper-catalyzed oxidative coupling of readily available aroylhydrazides with disulfides is developed, in which oxidative expulsion of N₂ overcomes the activation barrier between the carboxylic acid derivatives and the products. The reaction produces various thioesters in good to excellent yields with good functional group tolerance. In the reaction, stable and easily available aroylhydrazides are used as acyl sources and the relatively odorless disulfides are used as S sources. Mechanistic investigations demonstrate that the reaction of copper salt and oxidant (NH₄)₂S₂O₈ allows for achievement of tandem processes, including deprotonation, free-radical-mediated denitrogenation, and C-S bond formation.

Boronic Acids and Their Derivatives in Medicinal Chemistry: Synthesis and Biological Applications

Boron containing compounds have not been widely studied in Medicinal Chemistry, mainly due to the idea that this group could confer some toxicity. Nowadays, this concept has been demystified and, especially after the discovery of the drug bortezomib, the interest for these compounds, mainly boronic acids, has been growing. In this review, several activities of boronic acids, such as anticancer, antibacterial, antiviral activity, and even their application as sensors and delivery systems are addressed. The synthetic processes used to obtain these active compounds are also referred. Noteworthy, the molecular modification by the introduction of boronic acid group to bioactive molecules has shown to modify selectivity, physicochemical, and pharmacokinetic characteristics, with the improvement of the already existing activities. Besides, the preparation of compounds with this chemical group is relatively simple and well known. Taking into consideration these findings, this review reinforces the relevance of extending the studies with boronic acids in Medicinal Chemistry, in order to obtain new promising drugs shortly.

Ligand-Free Copper(I)-Mediated Cross-Coupling Reactions of Organostannanes with Sulfur Electrophiles

The synthesis of aryl thioether through the cross-coupling of C-S bond is a highly attractive area of research due to the prevalence of aryl thioether in bioactive natural products, functional materials, agrochemicals, and pharmaceutically active compounds. Herein, we report a ligand-free Cu(I) mediated electrophilic thiolation of organostannanes with sulfur electrophiles. A selective transfer of alkyl groups was achieved in reactions with alkyl carbastannatranes affording congested thioethers. This study offers a unified method to access diaryl and aryl alkyl thioethers and was demonstrated in the context of late-stage modifications..

Towards the Sarpagine-Ajmaline-Macroline Family of Indole Alkaloids: Enantioselective Synthesis of an N-Demethyl Alstolactone Diastereomer

the strategy involving the use of functionalized tetrahydro-6H-cycloocta[b]indol-6-one is reported as a key intermediate for synthesis of members of the sarpagine-ajmaline-macroline family of monoterpene indole alkaloids. The desired tricycle was synthesized through the following key steps: 1) Evans' syn-selective aldolization; 2) Liebeskind-Srogl cross-coupling using the phenylthiol ester of 3-chloropropanoic acid as a surrogate of acrylic thioester for the synthesis of 2,3-disubstituted indoles; and 3) ring-closing metathesis (RCM) for the formation of the eight-membered ring. An N-allylation followed by intramolecular 1,4-addition was planned for synthesis of the vobasine class of natural products. However, attempted cyclizations under a diverse set of conditions involving anionic, radical, and organopalladium/organonickel species failed to produce the bridged ring system. On the other hand, esterification of the pendant primary alcohol function with acetoacetic acid, followed by intramolecular Michael addition, afforded the desired tetracycle with excellent diastereoselectivity. Subsequent functional group manipulation and transannular cyclization of the amino alcohol afforded the N(1)-demethyl-3,5-diepi-alstolactone. We believe that the same synthetic route would afford the alstolactone should the amino alcohol with appropriate stereochemistry be used as the starting material.

Palladium-Catalyzed Enantioselective Thiocarbonylation of Styrenes

A highly enantioselective thiocarbonylation of styrenes with CO and thiols has been achieved by Pd catalysis, providing highly enantioenriched thioesters in good to excellent yields. Key to the successful execution of this reaction is the use of a chiral sulfoxide-(P-dialkyl)-phosphine (SOP) ligands. This thiocarbonylation proceeds smoothly under mild reaction conditions (1 atm CO and 0 °C) and displays broad substrate scope. Also demonstrated is that this transformation can be conducted using surrogates of CO, greatly increasing the safety aspects of running the reaction. The generality and utility of the method is manifested by its application to the synthetic transformations of thioester products and the direct acylation of cysteine-containing dipeptides. A primary mechanism was investigated and a plausible catalytic cycle was proposed.

Acyl Glycosides through Stereospecific Glycosyl Cross-Coupling: Rapid Access to C(sp3)-Linked Glycomimetics

Replacement of a glycosidic bond with hydrolytically stable C-C surrogates is an efficient strategy to access glycomimetics with improved physicochemical and pharmacological properties. We describe here a stereoretentive cross-coupling reaction of glycosyl stannanes with C(sp2)- and C(sp3)-thio(seleno)esters suitable for the preparation C-acyl glycosides as synthetic building blocks to obtain C(sp3)-linked and fluorinated glycomimetics. First, we identified a set of standardized conditions employing a Pd(0) precatalyst, CuCl additive, and phosphite ligand that provided access to C-acyl glycosides without deterioration of anomeric integrity and decarbonylation of the acyl donors (>40 examples). Second, we demonstrated that C(sp3)-glycomimetics could be introduced into the anomeric position via a direct conversion of C1 ketones. Specifically, the conversion of the carbonyl group into a CF2 mimetic is an appealing method to access valuable fluorinated analogues. We also illustrate that the introduction of other carbonyl-based groups into the C1 position of mono- and oligosaccharides can be accomplished using the corresponding acyl donors. This protocol is amenable to late-stage glycodiversification and programmed mutation of the C-O bond into hydrolytically stable C-C bonds. Taken together, stereoretentive anomeric acylation represents a convenient method to prepare a diverse set of glycan mimetics with minimal synthetic manipulations and with absolute control of anomeric configuration.

One-pot synthesis of thioesters with sodium thiosulfate as a sulfur surrogate under transition metal-free conditions

In this paper, we report an efficient synthetic method for thioester formation from sodium thiosulfate pentahydrate, organic halides, and aryl anhydrides. In the one-pot two-step reactions developed in this study, sodium thiosulfate was used as the sulfur surrogate for acylation with anhydrides, followed by substitution with organic halides through the in situ generation of thioaroylate. Furthermore, two important organic compounds could be successfully synthesized using our developed method. The advantages of the one-pot two-step reactions are operational simplicity, structurally diverse products with 42%-90% yields, use of relatively low toxic and odourless reagents, and easy applicability to large-scale operation.

Nickel-Catalyzed Suzuki-Miyaura Coupling of Aliphatic Amides

We report the Ni-catalyzed Suzuki-Miyaura coupling of aliphatic amide derivatives. Prior studies have shown that aliphatic amide derivatives can undergo Ni-catalyzed carbon-heteroatom bond formation but that Ni-mediated C-C bond formation using aliphatic amide derivatives has remained difficult. The coupling disclosed herein is tolerant of considerable variation with respect to both the amide-based substrate and the boronate coupling partner and proceeds in the presence of heterocycles and epimerizable stereocenters. Moreover, a gram-scale Suzuki-Miyaura coupling/Fischer indolization sequence demonstrates the ease with which unique polyheterocyclic scaffolds can be constructed, particularly by taking advantage of the enolizable ketone functionality present in the cross-coupled product. The methodology provides an efficient means to form C-C bonds from aliphatic amide derivatives using nonprecious-metal catalysis and offers a general platform for the heteroarylation of aliphatic acyl electrophiles.

Base-promoted ring opening of 3-chlorooxindoles for the construction of 2-aminoarylthioates and their transformation to quinazolin-4(3H)-ones

Cesium carbonate-promoted synthesis of diverse 2-aminoarylthioates via ring opening of 3-chlorooxindoles with thiols, and their synthetic applications is demonstrated.

Biocompatible metal-assisted C–C cross-coupling combined with biocatalytic chiral reductions in a concurrent tandem cascade

We present a concurrent chemo/biocatalytic one pot reaction cascade by combining a metal (Pd/Cu) assisted Liebeskind–Srogl coupling with an enantioselective enzymatic reduction for the production of chiral amines and alcohols.

Enantioselective cooperative proton-transfer catalysis using chiral ammonium phosphates

Chiral phosphorate anions are shown to be highly enantioselective templates for proton-transfer catalysis.

Palladium-catalyzed C–S bond activation and functionalization of 3-sulfenylindoles and related electron-rich heteroarenes

A novel palladium-catalyzed approach for constructing functionalized heteroarenes via C–S bond cleavage and C–C bond formation has been demonstrated.

Chemoselective C(α)–C(β) bond cleavage of saturated aryl ketones with amines leading to α-ketoamides: a copper-catalyzed aerobic oxidation process with air

An unprecedented chemoselective C(α)–C(β) bond cleavage of simple saturated aryl ketones with amines leading to α-ketoamidesvia a copper-catalyzed aerobic oxidation process with air has been disclosed.

Enantioselective Synthesis of (+)-Peganumine A

A gram-scale enantioselective total synthesis of (+)-peganumine A was accomplished in 7 steps from commercially available 6-methoxytryptamine. Key steps included (a) a Liebeskind-Srogl cross-coupling; (b) a one-pot construction of the tetracyclic skeleton from an ω-isocyano-γ-oxo-aldehyde via a sequence of an unprecedented C-C bond forming lactamization and a transannular condensation; (c) a one-pot organocatalytic process merging two achiral building blocks into an octacyclic structure via a sequence of enantioselective Pictet-Spengler reaction followed by a transannular cyclization. This last reaction created two spirocycles and a 2,7-diazabicyclo[2.2.1]heptan-3-one unit with excellent control of both the absolute and relative stereochemistry of the two newly created quaternary stereocenters.

CuI assisted desulfurative Sonogashira reaction of mercapto N-heterocyclic derivatives with alkynes

A palladium catalyzed Sonogashira reaction of mercapto N-heterocyclic derivatives with terminal alkynes by using CuI as the desulfurative reagent was described in this report.