A Direct Cycloaminative Approach to Imidazole Derivatives via Dual C–H Functionalization

Mono and di ortho-C-H acetoxylation of 2-aryloxyquinoline-3-carbaldehydes

2-Aryloxyquinolines are well known for various biological activities. In this report, we have developed a novel protocol for introducing an acetoxy functional group on the aryl sp2 carbon of 2-aryloxyquinoline-3-carbaldehyde using a palladium catalyst for the first time. Interestingly, this C-H acetoxylation reaction is highly chemo- and site-selective. By modifying the reaction conditions, mono or di ortho-C-H acetoxylation products have been synthesized selectively with good yields and with good functional group tolerance.

Copper-Catalyzed Chemodivergent Synthesis of Oxazoles and Imidazolidones by Selective C-O/C-N Cyclization

Efficient synthesis of phenylalanine-derived oxazoles and imidazolidones can be achieved by copper-catalyzed reactions that are controlled by directing groups and proceed by selective C-O or C-N coupling. This strategy employs inexpensive commercial copper catalysts and readily available starting materials. It uses a convenient reaction procedure and provides a reliable approach to the versatile and flexible assembly of heterocyclic building blocks.

Di-, tetra-, and perhydropyrrolo[1,2-a]imidazoles: The Methods of Synthesis and Some Aspects of Application

The review summarizes and systematizes the literature data on the synthesis and some aspects of application of pyrrolo[1,2-a]imidazoles. Synthetic approaches are grouped according to the degree of saturation of the product pyrroloimidazole ring. The bibliography of the review includes 110 sources over the last 15 years.

Synthesis of Fully Substituted 5-(o-Hydroxybenzoyl)imidazoles via Iodine-Promoted Domino Reaction of Aurones with Amidines

An iodine-promoted domino reaction of aurones with amidines has been successfully explored. The reaction proceeds in a consecutive manner containing Michael addition, iodination, cyclization from intramolecular nucleophilic substitution, and dehydrogenative aromatization from spiro ring opening. Following this novel strategy, a variety of 1,2,4-trisubstituted 5-(o-hydroxybenzoyl)imidazoles were efficiently synthesized in moderate to good yields from readily available starting materials. A plausible mechanism has been proposed.

Iodine-Mediated Cyclization of Enamines to Imidazole-4-Carboxylic Derivatives with Sequential Removal of Nitrogen Atoms from TMSN3

An iodine-mediated oxidative [4+1] cyclization of enamines with TMSN₃ for the synthesis of 2,5-disubstituted imidazole-4-carboxylic derivatives has been developed. The mechanistic studies revealed that the reaction proceeds through a sequential removal of two nitrogen atoms from TMSN₃. The synthetic utility was further demonstrated with a gram-scale reaction and various derivatization transformations of the products.

Photocatalytic Visible-Light-Induced Nitrogen Insertion via Dual C(sp3)-H and C(sp2)-H Bond Functionalization: Access to Privileged Imidazole-based Scaffolds

Here we have demonstrated a visible-light-mediated metal-free organic-dye-catalyzed dehydrogenative N-insertion leading to highly substituted imidazoles and privileged dihydroisoquinoline-based imidazole derivatives via C(sp³)-H and C(sp²)-H bond functionalization. A sustainable, convenient, metal-free azidation/C-H aminative cyclization approach in the absence of stoichiometric oxidants is presented. This protocol involves a rare photoinduced iminyl radical as a key intermediate for the "N" insertion.

Alkynoates as Versatile and Powerful Chemical Tools for the Rapid Assembly of Diverse Heterocycles under Transition-Metal Catalysis: Recent Developments and Challenges

Heterocycles, heteroaromatics and spirocyclic entities are ubiquitous components of a wide plethora of synthetic drugs, biologically active natural products, marketed pharmaceuticals and agrochemical targets. Recognizing their high proportion in drugs and rich pharmacological potential, these invaluable structural motifs have garnered significant interest, thus enabling the development of efficient catalytic methodologies providing access to architecturally complex and diverse molecules with high atom-economy and low cost. These chemical processes not only allow the formation of diverse heterocycles but also utilize a range of flexible and easily accessible building units in a single operation to discover diversity-oriented synthetic approaches. Alkynoates are significantly important, diverse and powerful building blocks in organic chemistry due to their unique and inherent properties such as the electronic bias on carbon-carbon triple bonds posed by electron-withdrawing groups or the metallic coordination site provided by carbonyl groups. The present review highlights the comprehensive picture of the utility of alkynoates (2007-2019) for the synthesis of various heterocycles (> 50 types) using transition-metal catalysts (Ru, Rh, Pd, Ir, Ag, Au, Pt, Cu, Mn, Fe) in various forms. The valuable function of versatile alkynoates (bearing multifunctional groups) as simple and useful starting materials is explored, thus cyclizing with an array of coupling partners to deliver a broad range of oxygen-, nitrogen-, sulfur-containing heterocycles alongside fused-, and spiro-heterocyclic compounds. In addition, these examples will also focus the scope and reaction limitations, as well as mechanistic investigations into the synthesis of these heterocycles. The biological significance will also be discussed, citing relevant examples of drug molecules highlighting each class of heterocycles. This review summarizes the recent developments in the synthetic methods for the synthesis of various heterocycles using alkynoates as readily available starting materials under transition-metal catalysis.

Intramolecular [3 + 2] nitrone cycloaddition reaction: highly regio and diastereoselective synthesis of bicyclo[3.2.1]octane scaffolds

A regio and diastereoselective strategy has been developed for the synthesis of complex bicyclo[3.2.1]octane scaffolds from the reaction of readily accessible vinylogous carbonates with N-substituted hydroxylamine hydrochlorides via intramolecular 1,3-dipolar nitrone cycloaddition reaction. Wide varieties of bicyclic isoxazolidines were synthesized in high yields under catalyst free conditions through a highly efficient and operationally simple protocol.

Rhodium-Catalyzed Diastereoselective [3 + 2] Cycloaddition of Carbonyl Ylide: An Access to the Core Ring System of Cordigol and Lophirone H

This paper describes a new synthetic strategy for the construction of tricyclic chromeno/quinolino furan frameworks via creation of two new rings and three contiguous stereogenic centers with high diastereoselectivity through a rhodium-catalyzed intramolecular carbonyl ylide cycloaddition reaction for the first time. This protocol allows the synthesis of the core ring system of natural products such as cordigol and lophirone H.

Ruthenium-catalyzed, site-selective C-H activation: access to C5-substituted azaflavanone

A site-selective ruthenium-catalyzed keto group assisted C–H bond activation of 2-aryl tetrahydroquinoline (azaflavanone) derivatives has been achieved with a variety of alkenes for the first time. A wide range of substrates was utilized for the synthesis of a wide variety of alkenylated azaflavanones. This simple and efficient protocol provides the C5-substituted azaflavanone derivatives in high yields with a broad range of functional group tolerance. Further, the C5-alkenylated products were converted into substituted 2-aryl quinoline derivatives in good yields.

A site-selective ruthenium-catalyzed keto group assisted C–H bond activation of 2-aryl tetrahydroquinoline (azaflavanone) derivatives has been achieved with a variety of alkenes for the first time.

Visible light-mediated photocatalytic oxidative cleavage of activated alkynes via hydroamination: a direct approach to oxamates

The direct oxidative cleavage of activated alkynes via hydroamination has been described using organic photocatalyst under visible-light irradiation at room temperature. In this reaction, the single electron oxidation of an in situ formed enamine followed by radical coupling with an oxidant finally delivers the oxamate. The key features of this photocatalytic reaction are the mild reaction conditions, metal-free organic dye as a photocatalyst, and TBHP playing a dual role as “O” source and for the regeneration of the photocatalyst.

The direct oxidative cleavage of activated alkynes via hydroamination has been described using organic photocatalyst under visible-light irradiation at room temperature.

Studies via X-ray analysis on intermolecular interactions and energy frameworks based on the effects of substituents of three 4-aryl-2-methyl-1H-imidazoles of different electronic nature and their in vitro antifungal evaluation

The crystal structures of 2-methyl-4-phenyl-1H-imidazole, C₁₀H₁₀N₂, (3a), 4-(4-chlorophenyl)-2-methyl-1H-imidazole hemihydrate, C₁₀H₉ClN₂·0.5H₂O, (3b), and 4-(4-methoxyphenyl)-2-methyl-1H-imidazole, C₁₁H₁₂N₂O, (3c), have been analyzed. It was found that the electron-donating/withdrawing tendency of the substituent groups in the aryl ring influence the acid-base properties of the 2-methylimidazole nucleus, changing the strength of the intermolecular N-H...N interactions. This behaviour not only influences the crystal structure but also seems to have an important effect on the antifungal activity. Considering the substituent groups, that is, H in (3a), Cl in (3b) and OMe in (3c), the formation of strong N-H...N connections has the probability (3a) > (3b) > (3c), while compound (3c) proves to be more active than (3a) and (3b) at all concentrations against C. neoformans.

Copper-Catalyzed Oxygenation Approach to Oxazoles from Amines, Alkynes, and Molecular Oxygen

A novel and efficient oxygenation approach to trisubstituted oxazoles via a copper-catalyzed aerobic oxidative dehydrogenative annulation of amines, alkynes, and O₂ has been developed. This transformation combines dioxygen activation and oxidative C-H bond functionalization and provides a practical protocol for the preparation of oxazole derivatives, which are privileged units found in various bioactive compounds or other natural products. ¹⁸O-labeling experiments were conducted to reveal that oxygenation was involved in this chemistry.

Solvent-promoted catalyst-free regioselective N-incorporation multicomponent domino reaction: rapid assembly of π-functionalized [60]fullerene-fused dihydrocarbolines

An unprecedented solvent-promoted N-incorporation multicomponent domino chemistry was developed for the direct construction of π-functionalized [60]fullerene-fused dihydrocarbolines from simple hydrocarbons.