Iron-catalyzed direct α-arylation of α-amino carbonyl compounds with indoles

A new tetrazine catalytic system for the synthesis of N-aryl-α-arylated glycine ester derivatives

A novel catalytic system has been developed, comprising 3,6-dichloro-1,2,4,5-tetrazine as the catalyst, zinc acetate as an additive, dichloromethane as the solvent, and air as the oxidizing agent. Under ambient conditions, this system effectively activates C-S bonds. Specifically, it facilitates the reaction between α-benzylthioglycine ester and electron-rich aromatics, achieving functionalization of the C-S bond. This method exhibits mild reaction conditions, broad substrate compatibility, and yields up to 91%. A plausible free radical mechanism is proposed to explain the observed reactivity.

Nickel-catalyzed oxidative thiolation of α-amino carbonyl compounds with thiols

An efficient oxidative thiolation of α-amino carbonyl compounds with thiols by the catalysis of an Earth-abundant nickel salt is disclosed for the first time. A variety of alkyl thiols and (hetero)aryl thiols underwent the reaction well with α-amino ketones and an α-amino ester to produce the desired α,α-aminothiocarbonyl compounds in good to excellent yields under ligand- and base-free conditions.

Visible-light induced cross-electrophile coupling of imines and anhydrides to synthesize α-amino ketones

α-Amino ketones are important motifs in synthetic and medicinal chemistry. Efficient methods to directly access these motifs from feasible precursors are, however, limited. Herein, a visible-light mediated reductive cross-electrophile coupling of readily available imines and anhydrides was developed. Under mild reaction conditions, the umpolung reactivity of diverse imines engaged with anhydrides gives a variety of α-amino ketones with good yields and a broad functional group compatibility. Primary mechanistic studies revealed that this transformation might proceed through a radical-radical cross coupling pathway dominantly.

Visible-Light-Promoted Cross Dehydrogenative/Decarboxylative Coupling Cascades of Glycine Ester Derivatives and β-Keto Acids

A visible-light-induced dehydrogenative/decarboxylative coupling reaction of arylglycine derivatives and β-keto acids is described. This photocatalyst- and additive-free protocol can be applied in the efficient synthesis of γ-keto glycine derivatives under ambient conditions. Further uses of this methodology and a plausible mechanism are also demonstrated.

Photoredox Cross-Dehydrogenative Coupling of N-Aryl Glycines Mediated by Mesoporous Graphitic Carbon Nitride: An Environmentally Friendly Approach to the Synthesis of Non-Proteinogenic α-Amino Acids (NPAAs) Decorated with Indoles

Indole-decorated glycine derivatives are prepared through an environmentally benign cross-dehydrogenative coupling between N-aryl glycine analogues and indoles (yield of ≤81%). Merging heterogeneous organocatalysis and photocatalysis, C–H functionalization has been achieved by selective C-2 oxidation of N-aryl glycines to afford the electrophilic imine followed by Friedel–Crafts alkylation with indole. The sustainability of the process has been taken into account in the reaction design through the implementation of a metal-free recyclable heterogeneous photocatalyst and a green reaction medium. Scale-up of the benchmark reaction (gram scale, yield of 69%) and recycling experiments (over seven runs without a loss of efficiency) have been performed to prove the robustness of the protocol. Finally, mechanistic studies were conducted employing electron paramagnetic resonance spectroscopy to unveil the roles of the photocatalyst and oxygen in the formation of odd-electron species.

One-Pot Three-Component Coupling Reaction of α-Amino Aryl Ketones, Indoles, and Perbromomethane Under Mild Conditions

A simple and efficient one-pot three-component cascade reaction of α-amino aryl ketones, indoles, and CBr₄ in moderate to good yields has been developed. This new strategy exhibits excellent mild reaction conditions and step-economy, easily accessible reactants, and simultaneous construction of three different new bonds (C=N, C–C, and N-Br) in a single step. It is worth noting that the protocol developed provides a simple and practical tool for the construction of diverse indole-containing heterocyclic frameworks, indicating its potential applications in medicinal and material chemistry.

Preparation and Application of α-Imino Ketones through One-Pot Tandem Reactions Based on Heyns Rearrangement

α-Imino ketone is a useful building block for the preparation of α-amino ketones and α-amino alcohols. However, its preparation has been seldomly seen. Herein, a metal-free and operationally simple strategy has been developed to generate α-imino ketones with high regioselectivity. Meanwhile, the method allowed for the preparation of various N,O-ketals with high regioselectivities and diastereoselectivities through cascade reactions in one pot.

NHC-Catalyzed Aza-Benzoin Condensation of N,N'-Dipyridin-2-yl Aminals with Aldehydes

α-Amino ketones are useful compounds because of their synthetic utility and bioactivities. After observing the ability of N,N'-dipyridin-2-yl aminals to form imines in situ, the synthesis of α-amino ketones using N,N'-dipyridin-2-yl aminals was proposed. Through the NHC-catalyzed aza-benzoin reaction between aromatic/aliphatic aldehydes and N,N'-dipyridin-2-yl aminals, α-amino ketones, including aromatic, heterocyclic, and aliphatic versions, were synthesized with yields up to 99%. A direct route toward N-Boc-protected α-amino ketones from N,N,N'-tris-Boc aminals was also discovered, yielding the desired N-Boc-protected α-amino ketones in yields up to 73%.

A three-component iodine-catalyzed oxidative coupling reaction: a heterodifunctionalization of 3-methylindoles

A metal-free method for the synthesis of heterodifunctional indole derivatives is developed through TBHP/KI-mediated oxidative coupling. The reaction constructs C-O and C-C bonds in succession with the help of tert-butyl peroxy radicals generated by the TBHP/KI catalytic system, enabling the direct realization of the heterodifunctionalization of indole in one pot. The product of this reaction is a novel heterodifunctional compound. This work might provide a new effective method for the synthesis of polycyclic indole compounds.

On the Mechanism of Cross-Dehydrogenative Couplings between N-aryl Glycinates and Indoles: A Computational Study

Despite the widespread use of cross-dehydrogenative couplings in modern organic synthesis, mechanistic studies are still rare in the literature and those applied to α-amino carbonyl compounds remain virtually unexplored. Herein, the mechanism of Co-catalyzed cross-dehydrogenative couplings of N-aryl glycinates with indoles is described. Density functional theory studies supported the formation of an imine-type intermediate as the more plausible transient electrophilic species. Likewise, key information regarding the role of the N-aryl group and free NH motif within the reaction outcome has been gained, which may set the stage for further developments in this field of expertise.

Visible light-induced aerobic oxidative cross-coupling reaction: preparation of α-indolyl glycine derivatives

Rose Bengal, an organic dye, has been used as a photocatalyst in the cross-coupling reaction between glycine derivatives and indoles.

Ketoxime peptide ligations: oxidative couplings of alkoxyamines to N-aryl peptides

Chemoselective ligation methods that preserve or introduce side chain diversity are critical for chemical synthesis of peptides and proteins. Starting from ketone substrates instead of aldehydes in oxime ligation reactions would allow substitution at the site of ligation; however, synthetic challenges to readily access ketone derivatives from common amino acid building blocks have precluded the widespread use of ketoxime peptide ligation reactions thus far. Moreover, ketones are typically much slower to react in condensation reactions compared to aldehydes. Here, one-pot catalyst-free oxidative couplings of α-substituted N-aryl peptides with alkoxyamines provide access to oxime linkages with diverse side chains. Electron-rich N-(p-Me2N-phenyl)-amino acids possessing substituents at the α-carbon were found to be uniquely capable of undergoing site-selective α-C-H oxidations in situ under an O2 atmosphere at neutral pH. Comparative studies with N-arylglycinyl peptides revealed that substitution at the α-carbon caused notable changes in reactivity, with greater sensitivity to solvent and buffer salt composition.

Synthesis of α-functionalized α-indol-3-yl carbonyls through direct SN reactions of indol-3-yl α-acyloins

A new and efficient synthesis of α-functionalized α-indol-3-yl ketones from easily available indolyl α-acyloins is reported. This process, catalyzed by Brønsted or Lewis acids, involves an uncommon direct nucleophilic substitution reaction of a secondary α-carbonyl-substituted hydroxyl group. The described methodology allows the introduction of a variety of nucleophiles such as (hetero)arenes, thiophenols, nitroanilines and 1,3-dicarbonyl derivatives. The synthesized α-indol-3-yl carbonyl compounds are important synthetic targets also useful for accessing functionalized tryptophols and furan-3-yl indoles.

Visible light-induced aerobic oxidative cross-coupling of glycine derivatives with indoles: a facile access to 3,3′ bisindolylmethanes

A rapid synthesis of 3,3′-bisindolylmethanes is achieved via a photocatalyzed double Friedel–Crafts alkylation reaction between glycine derivatives and indoles.

Metal-free photocatalyzed aerobic oxidative Csp3–H functionalization of glycine derivatives: one-step generation of quinoline-fused lactones

This reaction provides facile access to a series of valuable quinoline-fused lactones at room temperature under an air atmosphere.

Visible light-induced aerobic oxidative cross-coupling of glycine esters with α-angelicalactone: a facile pathway to γ-lactams

A rapid synthesis of substituted γ-lactams is achieved via metal-free photocatalyzed aerobic oxidative dehydrogenative formal [2 + 3] cyclization of glycine esters with α-angelicalactone.

CuCl/air-mediated oxidative coupling reaction of imidazoheterocycles with N-aryl glycine esters

A novel CuCl/air-mediated oxidative cross-coupling of imidazoheterocycles with glycine esters has been developed.

Copper-catalyzed oxidative cross-coupling of α-aminocarbonyl compounds with primary amines toward 2-oxo-acetamidines

A general and mild method for the construction of a carbon–nitrogen bond via copper-catalyzed oxidative cross-coupling of amines with α-aminocarbonyl compounds was achieved.