Copper(II)-Mediated Aerobic Synthesis of Imidazo[1,2-a]pyridines via Cascade Aminomethylation/Cycloisomerization of Alkynes

Cu(II)-Mediated aerobic oxidative synthesis of 2-sulfonylated chromeno[4,3-c]pyrazol-4(2H)-ones

Herein, a concise and facial synthesis of 2-sulfonylated chromeno [4,3-c]pyrazol-4(2H)-ones and 2,5-dihydro-4H-pyrazolo[4,3-c]quinolin-4-ones via a Cu(II)-promoted aerobic oxidative cascade C–C/C–N bonds formations was described. The protocol has advantages of atom economy,...

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.

Dihydroquinazolinones via A3 -Type Reactions of N-Carbamoyliminium Ions

A variant of the A³ coupling reaction was developed utilizing in situ generated N-carbamoyliminium ions. The tandem INCIC/A³ -coupling sequence provided a facile one-pot synthesis of dihydroquinazolinone derivatives. The scope of the reaction was demonstrated in solution as well as on solid support. The reaction was further combined with peptide synthesis, S_N Ar reactions, CuAAC triazole formation or bromination, providing additional opportunities for further diversification of the dihydroquinazolinone scaffolds.

Fe(iii)-Catalyzed synthesis of steroidal imidazoheterocycles as potent antiproliferative agents

An efficient and practical method has been developed for the synthesis of steroidal imidazoheterocycles via cost-effective and environmentally benign FeCl₃-catalyzed oxidative amination. A library of steroidal imidazo[1,2-a]pyridines was directly synthesized from readily available 2-aminopyridines and steroidal ketones in aerobic conditions. The synthesized compounds were screened for activity on human microsomal cytochrome P450s CYP7, CYP17 and CYP21. Antiproliferative activity of two lead compounds 3ia and 3la was additionally evaluated against the human MCF-7 (breast cancer), SKOV3 (ovarian cancer), and 22Rv1 (prostate cancer) cell lines. Steroidal imidazo[1,2-a]pyridine 3la which is a substrate molecule for CYP17A1 with IC₅₀ = 1.7 μM (MCF-7), 3.0 (SKOV3), and 6.0 μM (22Rv1) has proved to be more active than reference drug cisplatin.

Iodine Promoted Efficient Synthesis of 2-Arylimidazo[1,2-a]pyridines in Aqueous Media: A Comparative Study between Micellar Catalysis and an "On-Water" Platform

In a new and environmentally sustainable approach, a series of 2-arylimidazo[1,2-a]pyridine derivatives were synthesized in aqueous media in the presence of iodine as a catalyst. The reaction proceeded by condensation of various aryl methyl ketones with 2-aminopyridines to afford 2-arylimidazo[1,2-a]pyridines in good overall yields. Although several of the reactions were efficiently performed "on water", the addition of a surfactant, namely, sodium dodecyl sulphate , was found effective in terms of substrate scope and yield enhancement. Both methods were successfully used for the gram-scale synthesis of a marketed drug, zolimidine. The simple experimental setup, water as "green" media, and inexpensive catalyst are some of the merits of this protocol.

Development of 1,3-thiazole analogues of imidazopyridines as potent positive allosteric modulators of GABAA receptors

Structure-activity relationship studies were conducted in the search for 1,3-thiazole isosteric analogs of imidazopyridine drugs (Zolpidem, Alpidem). Three series of novel γ-aminobutyric acid receptor (GABA_AR) ligands belonging to imidazo[2,1-b]thiazoles, imidazo[2,1-b][1,3,4]thiadiazoles, and benzo[d]imidazo[2,1-b]thiazoles were synthesized and characterized as active agents against GABA_AR benzodiazepine-binding site. In each of these series, potent compounds were discovered using a radioligand competition binding assay. The functional properties of highest-affinity compounds 28 and 37 as GABA_AR positive allosteric modulators (PAMs) were determined by electrophysiological measurements. In vivo studies on zebrafish demonstrated their potential for the further development of anxiolytics. Using the OECD "Fish, Acute Toxicity Test" active compounds were found safe and non-toxic. Structural bases for activity of benzo[d]imidazo[2,1-b]thiazoles were proposed using molecular docking studies. The isosteric replacement of the pyridine nuclei by 1,3-thiazole, 1,3,4-thiadiazole, or 1,3-benzothiazole in the ring-fused imidazole class of GABA_AR PAMs was shown to be promising for the development of novel hypnotics, anxiolytics, anticonvulsants, and sedatives drug-candidates.

Recent advances on the transition-metal-catalyzed synthesis of imidazopyridines: an updated coverage

A comprehensive account of recent advances in the synthesis of imidazopyridines, assisted through transition-metal-catalyzed multicomponent reactions, C-H activation/functionalization and coupling reactions are highlighted in this review article. The basic illustration of this review comprises of schemes with concise account of explanatory text. The schemes depict the reaction conditions along with a quick look into the mechanism involved to render a deep understanding of the catalytic role. At some instances optimizations of certain features have been illustrated through tables, i.e., selectivity of catalyst, loading of the catalyst and percentage yield with different substrates. Each of the reported examples has been rigorously analyzed for reacting substrates, reaction conditions and transition metals used as the catalyst. This review will be helpful to the chemists in understanding the challenges associated with the reported methods as well as the future possibilities, both in the choice of substrates and catalysts. This review would be quite appealing to a wider range of organic chemists in academia and industrial R&D sectors working in the field of heterocyclic syntheses. In a nutshell, this review will be a guiding torch to envisage: (i) the role of various transition metals in the domain dedicated towards method development and (ii) for the modifications needed thereof in the R&D sector.

Lewis Acid-Catalyzed Intermolecular Annulation: Three-Component Reaction toward Imidazo[1,2-a]pyridine Thiones

A Lewis acid-catalyzed three-component annulation reaction of 2-aminopyridines and ynals with elemental sulfur was established. A series of imidazo[1,2-a]pyridine thiones was obtained in moderate to excellent yields. The merits of this transformation include easily available starting materials, multiple C-heteroatom bond formation in one pot, good functional group tolerance, elemental sulfur as S source, operational simplicity, etc.

Efficient and "Green" Synthetic Route to Imidazo[1,2-a]pyridine by Cu(II)-Ascorbate-Catalyzed A3-Coupling in Aqueous Micellar Media

An efficient and environmentally sustainable method for the synthesis of imidazo[1,2-a]pyridine derivatives by domino A³-coupling reaction catalyzed by Cu(II)-ascorbate was developed in aqueous micellar media in the presence of sodium dodecyl sulfate (SDS). The catalyst, a dynamic combination of Cu(II)/Cu(I), was generated in situ in the reaction mixture by mixing CuSO₄ with sodium ascorbate and aided a facile 5-exo-dig cycloisomerization of alkynes with the condensation products of 2-aminopyridines and aldehydes to afford a variety of imidazo[1,2-a]pyridines in good overall yields. A simple experimental setup, water as the "green" medium, and inexpensive catalyst and auxiliary are some of the merits of this protocol.

Highly Regioselective, Acid-Catalyzed, Three-Component Cascade Reaction for the Synthesis of 2-aminopyridine-Decorated Imidazo[1,2- a]pyridine

A highly regioselective acid-catalyzed three-component reaction of 2-aminopyridine and 3-phenylpropiolaldehyde for the construction of imidazo[1,2- a]pyridine has been developed. This strategy provides a broad range of substrates and represents an efficient approach to give various 2-aminopyridine-decorated imidazo[1,2- a]pyridine in good yields.

Mesoporous particle-based microcontainers for intranasal delivery of imidazopyridine drugs

The aim of this study was to develop mesoporous containers for entrapment of imidazopyridines, such as sedative-hypnotic medicine zolpidem, anxiolytic agent alpidem and their derivatives. For this purpose, calcium carbonate (size 1.2 µm (PDI: 0.6), zeta potential: -10 mV), manganese carbonate (2.5 µm (PDI: 0.5), zeta potential: -12 mV) and titanium dioxide particles (3.7 µm (PDI: 0.4), zeta potential: -15 mV) were used. The compounds were encapsulated applying two techniques: adsorption on the preformed particles and co-precipitation during the synthesis of the particles. The polymer shell of the containers was formed by electrostatic adsorption of polyelectrolytes on the surface of the particles. The best encapsulation efficacy was shown for zolpidem incorporated into calcium carbonate (5.4%) and manganese carbonate (4.6%) by adsorption. Release of the compounds from the containers based on the proposed particles were characterised by the short time burst effect (<10 min) followed by desorption prolongation by formation of polymer shell. X-ray microtomography results demonstrate the prolonged retention of the containers with the mucoadhesive shell in the nasal cavity.

A convergent synthesis of vinyloxyimidazopyridine via Cu(i)-catalyzed three-component coupling

The synthesis of vinyloxyimidazopyridine with complete regio- and stereoselectivity has been achieved by the Cu(i)-catalyzed three-component coupling of 2-aminopyridine, 2-oxoaldehyde and alkyne. This protocol is operationally very simple and has much potential for the synthesis of heteroarylated vinyl ethers from basic chemicals. Steroidal imidazopyridinyl vinyl ether was obtained successfully from ethynylestradiol.

A novel formulation of zolpidem for direct nose-to-brain delivery: synthesis, encapsulation and intranasal administration to mice

OBJECTIVES

Anxiolytic drug zolpidem was incorporated into the microcontainers based on mesoporous calcium carbonate particles modified by diethylaminoethyl-dextran/hyaluronic acid shell. The release of zolpidem in saline solution and in polymer film modelling nasal mucosa was investigated. The anxiolytic effect of zolpidem upon intranasal administration of microcontainers and free medicine was determined by in vivo experiments on mice.

METHODS

The structures of all compounds during zolpidem synthesis were established using nuclear magnetic resonance spectroscopy. The loading efficacy and release kinetics of zolpidem were analysed by spectrophotometry. Surface morphology of formulation was investigated by scanning electron microscopy. To determine the effect of zolpidem-loaded containers administration by the intranasal route in vivo experiments was carried out applying the open field test.

KEY FINDINGS

Nasal administration of zolpidem in the form of the microcontainers based on mesoporous calcium carbonate particles modified by diethylaminoethyl-dextran/hyaluronic acid shell has a pronounced anxiolytic effect on the behaviour of the animals in the open field test.

CONCLUSIONS

The polyelectrolyte shell deposited together with zolpidem enhances the loading efficacy of the microcontainers. In vivo experiments on mice demonstrate increase in anxiolytic effect of zolpidem in microcontainers compared with upon intranasal administration of free medicine.

An improved and scalable synthesis of zolpidem via a CuI/BINOL-mediated tandem reaction of imine and alkyne

An improved and scalable method for the synthesis of zolpidem (

A nitrile-stabilized ammonium ylide as a masked C–CN synthon in heterocyclization with amidine–imine: 3-component assembly to fused pyrimidine scaffolds

The title ylides possessing a unique reactivity-pattern and nitrile's dual role cyclo-condense with amidine–amine via a pathway of electronically-assisted nucleophilic substitutions and Hofmann elimination.

Indium-based metal–organic frameworks as catalysts: synthesis of 2-nitro-3-arylimidazo[1,2-a]pyridines via oxidative amination under air using MIL-68(In) as an effective heterogeneous catalyst

The metal–organic framework MIL-68(In) has emerged as a productive heterogeneous catalyst for the synthesis of 2-nitro-3-arylimidazo[1,2-a]pyridines via oxidative amination between 2-aminopyridines and nitroalkenes using air as an oxidation agent.

Triaryl-Substituted Divinyl Ketones Cyclization: Nazarov Reaction versus Friedel-Crafts Electrophilic Substitution

The acid-catalyzed cyclization of a wide range of triaryl-substituted divinyl ketones has been studied. It was found that the reaction pathway strongly depends on the nature of the aryl substituent at the α-position to the carbonyl group. An electron-rich aromatic substituent promotes the reaction to proceed through the intramolecular Friedel-Crafts electrophilic substitution giving dihydronaphthalene derivatives. In contrast, the presence of an electron-deficient substituent is favorable for the Nazarov 4π-conrotatory cyclization yielding triaryl-substituted cyclopentenones. The electrophilic substitution reaction was applied to thiophene and thiazole derivatives.

Synthesis and antiproliferative activity evaluation of steroidal imidazo[1,2-a]pyridines

An elegant approach to unknown steroidal imidazo[1,2-a]pyridine hybrids is disclosed. Unique derivatives of androstene and estrane series containing imidazo[1,2-a]pyridine motifs were prepared from 17-ethynyl steroids in good yields via copper-catalyzed cascade aminomethylation/cycloisomerization with imines. The synthesized compounds were screened for cytotoxicity against human breast (MCF-7, MDA-MB-231, HBL-100, MDA-MB-453) and prostate (LNCaP-LN3, PC-3, DU 145) cancer cell lines. The majority of tested compounds showed activities at μM level in breast cancer cells. The hormone-responsive breast cancer cells MCF-7 were more sensitive to novel compounds than ERα-negative cells; in particular, compounds 6a,b exhibited promising cytotoxicity against this cell line with the IC50 values in the range of 3-4μM. Furthermore, compound 4a showed remarkable effects as a selective ERα receptor modulator.

Transition-metal-free C–C bond cleavage and formation: efficient synthesis of 2,3-diarylimidazo[1,2-α]pyridines from 2-aminopyridines and alkynoates

A highly efficient transition-metal-free cyclization reaction for the synthesis of 2,3-diarylimidazo[1,2-α]pyridines is described.

A catalyst- and solvent-free thermal multicomponent approach for the construction of diverse and polysubstituted 2-aminopyridines and their antibacterial activity

A multicomponent approach for the synthesis of polysubstituted 2-aminopyridines under catalyst- and solvent-free thermal conditions has been established.