Efficient and Divergent Synthesis of α-Halogenated Amides and Esters by Double Electrophilic Activation of Ynamides

Rhodium(III)-Catalyzed Intramolecular Cyclization and Sequential Aromatization of Ynamides with Propargyl Esters: Access to 2,5-Dihydropyrroles and Pyrroles

Disclosed herein is a rhodium(III)-catalyzed intramolecular cyclization of ynamides with propargyl esters. A variety of highly functionalized 2,5-dihydropyrroles were obtained in moderate to good yields with high E/Z selectivities. Subsequent oxidation of the products gave valuable pyrrole derivatives. Additionally, scale-up reactions and late-stage derivatizations highlight the potential synthetic utility of this methodology.

Ynamide Coupling Reagent for the Chemical Cross-Linking of Proteins in Live Cells

Chemical cross-linking of proteins coupled with mass spectrometry analysis (CXMS) is a powerful method for the study of protein structure and protein-protein interactions (PPIs). However, the chemical probes used in the CXMS are limited to bidentate reactive warheads, and the available zero-length cross-linkers are restricted to 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide hydrochloride/N-hydroxysuccinimide (EDC/NHS) and 4-(4,6-dimethoxy-1,3,5-triazin-2-yl)-4-methylmorpholinium chloride (DMTMM). To alleviate this issue, an efficient coupling reagent, sulfonyl ynamide, was developed as a new zero-length cross-linker that can connect high-abundance carboxyl residues (D/E) with lysine (K) to form amide bonds in the absence of any catalyst. Significant improvement in the cross-linking efficiency and specificity in comparison with traditional EDC/NHS was achieved with model proteins, which includes inter- and intramolecular conjugations. The cross-linked structures were validated by X-ray crystallography. Importantly, this coupling reagent can be successfully used to capture interacting proteins in the whole proteome and can be a useful reagent for probing potential protein-protein interactions in situ.

Atom-economic and stereoselective catalytic synthesis of fully substituted enol esters/carbonates of amides in acyclic systems enabled by boron Lewis acid catalysis

Carboacyloxylation of internal alkynes is emerging as a powerful and straightforward strategy for enol ester synthesis. However, the reported examples come with limitations, including the utilization of noble metal catalysts, the control of regio- and Z/E selectivity, and an application in the synthesis of enol carbonates. Herein, a boron Lewis acid-catalyzed intermolecular carboacyloxylation of ynamides with esters to access fully substituted acyclic enol esters in high yield with generally high Z/E selectivity (up to >96 : 4) is reported. Most importantly, readily available allylic carbonates are also compatible with this difunctionalization reaction, representing an atom-economic, catalytic and stereoselective protocol for the construction of acyclic β,β-disubstituted enol carbonates of amides for the first time. The application of the carboacyloxylation products to decarboxylative allylations provided a ready access to enantioenriched α-quaternary amides. Moreover, experimental studies and theoretical calculations were performed to illustrate the reaction mechanism and rationalize the stereochemistry.

Copper-Catalyzed Tandem Cyclization/Direct C(sp2)-H Annulation of Azide-Ynamides via α-Imino Copper Carbenes: Access to Azepino[2,3-b:4,5-b']diindoles

A novel copper-catalyzed tandem cyclization/direct C(sp²)-H annulation of phenyl azide-ynamides via α-imino copper carbenes has been developed, which provides a concise and flexible approach for the construction of a range of valuable azepino[2,3-b:4,5-b']diindoles in mostly good to excellent yields with high chemoselectivities. This tandem reaction also exhibits a broad substrate scope, excellent functional group tolerance, simple operation, and mild reaction conditions.

I2 -Catalyzed Cycloisomerization of Ynamides: Chemoselective and Divergent Access to Indole Derivatives

Herein, an I₂ -catalyzed unprecedented cycloisomerization of ynamides is developed, furnishing various functionalized bis(indole) derivatives in generally good to excellent yields with wide substrate scope and excellent atom-economy. This protocol not only represents the first molecular-iodine-catalyzed tandem complex alkyne cycloisomerizations, but also constitutes the first chemoselective cycloisomerization of tryptamine-ynamides involving distinctively different C(sp³ )-C(sp³ ) bond cleavage and rearrangement. Moreover, chiral tetrahydropyridine frameworks containing two stereocenters are obtained with moderate to excellent diastereoselectivities and excellent enantioselectivities. Meanwhile, cycloisomerization and aromatization of ynamides produce pyrrolyl indoles with high efficiency enabled by I₂ . Additionally, control experiments and theoretical calculations reveal that this reaction probably undergoes a tandem 5-exo-dig cyclization/rearrangement process.

Stereoselective Synthesis of β-Alkoxy-β-amido Vinylbenziodoxoles via Iodo(III)etherification of Ynamides

A trans-iodo(III)etherification reaction of ynamides with benziodoxole triflate and alcohols is reported. Despite the sensitivity of ynamides and enamides toward Brønsted acid, the reaction could be successfully performed under carefully controlled conditions to afford β-alkoxy-β-amido vinylbenziodoxoles in moderate to good yields. The products could be subjected to a sequence of cross-coupling via C-I(III) bond cleavage and electrophilic halogenation of the resulting α-alkoxyenamides, allowing for the preparation of densely functionalized esters.

Highly Site-Selective Oxidative Cyclization of Ene-ynamides via Non-Noble-Metal Catalysis: Access to Functionalized Lactams

Herein, an unprecedented non-noble-metal-catalyzed oxidation/cyclization of ene-ynamides is developed, allowing the synthesis of diversely functionalized lactams in moderate to good yields with excellent diastereoselectivities without the observation of typical cyclopropanation products. In combination with Ellman's tert-butylsulfinimine chemistry, chiral γ-lactams containing three contiguous stereocenters are obtained with high diastereo- and enantioselectivity. Moreover, density functional theory (DFT) calculations indicate that this protocol probably undergoes a carbon cation or proton transfer process.

Solvent directed chemically divergent synthesis of β-lactams and α-amino acid derivatives with chiral isothiourea

A protocol for the chemically divergent synthesis of β-lactams and α-amino acid derivatives with isothiourea (ITU) catalysis by switching solvents was developed. The stereospecific Mannich reaction occurring between imine and C(1)-ammonium enolate generated zwitterionic intermediates, which underwent intramolecular lactamization and afforded β-lactam derivatives when DCM and CH₃CN were used as solvents. However, when EtOH was used as the solvent, the intermediates underwent an intermolecular esterification reaction, and α-amino acid derivatives were produced. Detailed mechanistic experiments were conducted to prove that these two kinds of products came from the same intermediates. Furthermore, chemically diversified transformations of β-lactam and α-amino acid derivatives were achieved.

A protocol for the solvent directed chemically divergent synthesis of β-lactam and α-amino acid derivatives with chiral isothiourea was reported.

Divergent Synthesis of α-Fluorinated Carbonyl and Carboxyl Derivatives by Double Electrophilic Activation of Amides

A straightforward and divergent entry to α-fluorinated carbonyl and carboxyl derivatives is reported. Upon activation of amides with triflic anhydride and a 2-halo-pyridine and subsequent trapping of the resulting keteniminium ions with nucleophiles followed by a second electrophilic activation with NFSI and final hydrolysis, a range of amides can be transformed to α-fluorinated ketones, esters, and amides under mild conditions. Moreover, this reaction can be performed to yield enantioenriched products with a traceless chiral auxiliary.

Straightforward Synthesis of Indenes by Gold-Catalyzed Intramolecular Hydroalkylation of Ynamides

An original and straightforward entry to polysubstituted indenes from readily available ynamides is reported. Upon reaction with a N-heterocyclic carbene-gold complex under mild conditions, activated keteniminium ions are generated whose unique electrophilicity triggers a [1,5]-hydride shift and a subsequent cyclization. The presence of an endocyclic enamide in the densely functionalized resulting indenes was shown to be especially useful and versatile, offering a range of opportunities for their further postfunctionalization.

A general, versatile and divergent synthesis of selectively deuterated amines

Deuterated organic molecules are of utmost importance in many areas of science and have been recently intensively investigated in medicinal chemistry due to their enhanced metabolic stability. The development of efficient and broadly applicable methods for the selective incorporation of deuterium atoms into organic molecules from readily available starting materials and reagents is therefore of extreme importance. Such methods however often lack generality and selectivity, notably in the nitrogen series. With nitrogen-containing molecules being indeed ubiquitous in medicinal chemistry, there is a strong need for efficient methods enabling the selective synthesis of deuterated amines. In this perspective, we report herein a general, versatile, divergent and metal-free synthesis of amines selectively deuterated at their α and/or β positions. Upon simple treatment of readily available ynamides with a mixture of triflic acid and triethylsilane, either deuterated or not, a range of amines can be smoothly obtained with high levels of deuterium incorporation by a unique sequence involving a domino keteniminium/iminium activation.

A general, versatile, divergent and metal-free synthesis of amines selectively deuterated at their α and/or β positions and relying on a simple treatment of ynamides with triflic acid and triethylsilane, either deuterated or not, is reported.

Regio- and diastereoselective synthesis of trans-3,4-diaryldihydrocoumarins via metal-free [4+2] annulation of ynamides with o-hydroxybenzyl alcohols

An efficient regio- and diastereoselective method for the construction of valuable trans-3,4-diaryldihydrocoumarins via metal-free [4+2] annulation of ynamides with o-hydroxybenzyl alcohols has been developed. Ynamides are first treated as 2-π partners to react with o-hydroxybenzyl alcohols via traceless sulfonamide directing groups, affording trans-3,4-diaryldihydrocoumarins in good yields with high regio- and diastereoselectivities. This metal-free methodology is also characterized by a wide substrate scope, good functional group tolerance, and efficiency on a gram scale.

Catalytic Asymmetric Hydroacyloxylation/Ring-Opening Reaction of Ynamides, Acids, and Aziridines

A highly enantioselective three-component reaction of ynamides with carboxylic acids and 2,2'-diester aziridines has been realized by using a chiral N,N'-dioxide/Ho(OTf)₃ complex as a Lewis acid catalyst. The process includes the formation of an α-acyloxyenamide intermediate through the addition of carboxylic acids to ynamides and the following enantioselective nucleophilic addition to in-situ-generated azomethine ylides induced by the chiral catalyst. A range of amino acyloxyenamides are delivered in moderate to good yields with good ee values. In addition, a possible catalytic cycle with a transition model is proposed to elucidate the reaction mechanism.

Ligand-Enabled Copper-Catalyzed N-Alkynylation of Sulfonamide with Alkynyl Benziodoxolone: Synthesis of Amino Acid-Derived Ynamide

Ynamides are versatile building blocks in organic synthesis. However, the synthesis of amino acid-derived ynamides is difficult but in high demand. Herein, we disclose the copper-catalyzed Csp-N coupling of sulfonamide, including amino acid and peptide derivatives, to give ynamides by using alkynyl benziodoxolones with broad functional group tolerance under mild reaction conditions. The electron-rich bipyridine as a ligand and ethanol as solvent were used for the success of this reaction. The usefulness of the obtained amino acid-derived ynamide as building block was showcased by further derivatization to unique amino acid derivatives. A control experiment to elucidate the mechanistic insight was also described.

Radical Reactions of Ynamides

Ynamides are electron-rich heteroatom-substituted alkynes with C-C triple bond directly tethered to the amide group. Over the past decades, ynamides have proven to be versatile reagents for organic synthesis and have received extensive attention. Compared with the well-established ionic reactions of ynamides, radical-based ynamide reactions have been exploited relatively seldom. Herein, radical reactions of ynamides, classified by radical attack at the α-position and β-position of ynamides, are reviewed by highlighting the reaction selectivity, scope, mechanism, and applicability. The aim of this review is to provide a comprehensive summarization of these advances, casting light on the further development of ynamide chemistry.

Synthesis of Sulfonamide-Based Ynamides and Ynamines in Water

Ynamides, though relatively more stable than ynamines, are still moisture-sensitive and prone to hydration especially under acidic and heating conditions. Here we report an environmentally benign, robust protocol to synthesize sulfonamide-based ynamides and arylynamines via Sonogashira coupling reactions in water, using a readily available quaternary ammonium salt as the surfactant.

Asymmetric synthesis with ynamides: unique reaction control, chemical diversity and applications

Ynamides are among the most powerful building blocks in organic synthesis and have become invaluable starting materials for the construction of multifunctional compounds and challenging architectures that would be difficult to prepare otherwise. The rapidly growing popularity originates from the unique reactivity and ease of manipulation of the polarized ynamide triple bond, the advance of practical methods for making them, and the simplicity of storage and handling. These attractive features and the demonstration of numerous synthetic applications have spurred the development of intriguing asymmetric reaction strategies during the last decade. An impressive variety of chemo-, regio- and stereoselective carbon-carbon and carbon-heteroatom bond forming reactions with ynamides have been developed and now significantly enrich the toolbox of synthetic chemists. This review provides a comprehensive overview of asymmetric ynamide chemistry since 2010 with a focus on the general scope, current limitations, stereochemical reaction control and mechanistic aspects.

Catalytic Asymmetric Three-component Hydroacyloxylation/ 1,4-Conjugate Addition of Ynamides

A highly enantioselective three-component hydroacyloxylation/1,4-conjugate addition of ortho-hydroxybenzyl alcohols, ynamides and carboxylic acids was developed under mild reaction conditions in the presence of a chiral N,N'-dioxide/Sc(OTf)₃ complex, which went through in situ generated ortho-quinone methides with α-acyloxyenamides, delivering a range of corresponding chiral α-acyloxyenamides derivatives containing gem(1,1)-diaryl skeletons in moderate to good yields with excellent ee values. The scale-up experiment and further derivation showed the practicality of this catalytic system. In addition, a possible catalytic cycle and transition state model was proposed to elucidate the origin of the stereoselectivity based on X-ray crystal structure of the α-acyloxyenamide intermediate and product.