Microwave enhanced Suzuki coupling: a diversity-oriented approach to the synthesis of highly functionalised 3-substituted-2-aryl/heteroaryl imidazo[4,5-b]pyridines

Synthesis and Antiproliferative Activity of 2,6-Disubstituted Imidazo[4,5-b]pyridines Prepared by Suzuki Cross Coupling

A series of novel 2,6-diphenyl substituted imidazo[4,5-b]pyridines was designed and synthesized using optimized Suzuki cross coupling to evaluate their biological activity in vitro. The conditions of the Suzuki coupling were evaluated and optimized using a model reaction. To study the influence of the substituents on the biological activity, we prepared N-unsubstituted and N-methyl substituted imidazo[4,5-b]pyridines with different substituents at the para position on the phenyl ring placed at position 6 on the heterocyclic scaffold. Antiproliferative activity was determined on diverse human cancer cell lines, and the selectivity of compounds with promising antiproliferative activity was determined on normal peripheral blood mononuclear cells (PBMC). Pronounced antiproliferative activity was observed for p-hydroxy substituted derivatives 13 and 19, both displaying strong activity against most of the tested cell lines (IC50 1.45-4.25 μM). The unsubstituted N-methyl derivative 19 proved to be the most active derivative. There was a dose-dependent accumulation of G2/M arrested cells in several cancer cell lines after exposure to compound 19, implying a cell cycle-phase-specific mechanism of action. Additionally, the novel series of derivatives was evaluated for antiviral activity against a broad panel of viruses, yet the majority of tested compounds did not show antiviral activity.

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.

V, Mukku N, Chanda K, Maiti B. Rapid Construction of an Imidazo[4,5-b]pyridine Skeleton from 2-Chloro-3-nitropyridine via Tandem Reaction in H2O-IPA Medium

A highly efficient, clean, and simple procedure for the synthesis of a privilege imidazo[4,5-b]pyridine scaffold from 2-chloro-3-nitropyridine in combination with environmentally benign H₂O-IPA as a green solvent is presented. The scope of the novel method has been demonstrated through the tandem sequence of S_NAr reaction with substituted primary amines followed by the in situ nitro group reduction and subsequent heteroannulation with substituted aromatic aldehydes to obtain functionalized imidazo[4,5-b]pyridines with only one chromatographic purification step. The synthesis pathway appears to be green, simple, and superior compared with other already reported procedures, with the high abundance of reagents and great ability in expanding the structural diversity.

Crystal structure of 1-ferrocenylsulfonyl-1H-imidazo[4,5-b]pyridine, C16H13FeN3O2S

C16H13FeN3O2S, triclinic, P1̅, a = 8.1311(9) Å, b = 11.6852(13) Å, c = 17.044(2) Å, α = 102.754(6)°, β = 92.548(7)°, γ = 105.563(6)°, V = 1512.3(3) Å3, Z = 4, R gt(F) = 0.0473, wR ref(F 2) = 0.1307, T = 296(2) K.

Pharmacological Potential and Synthetic Approaches of Imidazo[4,5-b]pyridine and Imidazo[4,5-c]pyridine Derivatives

The structural resemblance between the fused imidazopyridine heterocyclic ring system and purines has prompted biological investigations to assess their potential therapeutic significance. They are known to play a crucial role in numerous disease conditions. The discovery of their first bioactivity as GABA_A receptor positive allosteric modulators divulged their medicinal potential. Proton pump inhibitors, aromatase inhibitors, and NSAIDs were also found in this chemical group. Imidazopyridines have the ability to influence many cellular pathways necessary for the proper functioning of cancerous cells, pathogens, components of the immune system, enzymes involved in carbohydrate metabolism, etc. The collective results of biochemical and biophysical properties foregrounded their medicinal significance in central nervous system, digestive system, cancer, inflammation, etc. In recent years, new preparative methods for the synthesis of imidazopyridines using various catalysts have been described. The present manuscript to the best of our knowledge is the complete compilation on the synthesis and medicinal aspects of imidazo[4,5-b]pyridines and imidazo[4,5-c]pyridines reported from the year 2000 to date, including structure–activity relationships.

Mo(CO)6 as a Solid CO Source in the Synthesis of Aryl/Heteroaryl Weinreb Amides under Microwave-Enhanced Condition

The facile transformation of aryl/heteroaryl nonaflates into corresponding amides via Pd-catalyzed aminocarbonylation using Mo(CO)6 as a solid CO source under microwave-enhanced condition is reported. The method was found to be tolerant with respect to a diverse range of electronically biased aryl/heteroaryl nonaflates, and exceptional yields were obtained. The optimized protocol was further extended to a diverse range of amines.

Crystal structure of (E)-2-styryl-1-ferrocenylsulfonyl-1H-imidazo[4,5-b]pyridine, C24H19FeN3O2S

C24H19FeN3O2S, monoclinic, P21/n (no. 14), a = 8.6762(9) Å, b = 12.4684(12) Å, c = 19.256(2) Å, β = 97.496(6)°, V = 2065.3(4) Å3, Z = 4, R gt (F) = 0.0302, wR ref (F 2 ) = 0.0794, T = 296(2) K.

Crystal structure of 3-ferrocenylsulfonyl-2-(4-methoxyphenyl)-3H-imidazo[4,5-b]pyridine, C23H19FeN3O3S

C23H19FeN3O3S, monoclinic, P21/n (no. 14), a = 13.1372(6) Å, b = 9.6866(5) Å, c = 15.9700(8) Å, β = 92.499(2)°, V = 2030.33(17) Å3, Z = 4, R gt (F) = 0.0294, wR ref (F 2 ) = 0.0807, T = 296(2)K

Palladium-Catalyzed Suzuki Cross-Coupling of 2-Halo-Deazapurines with Potassium Organotrifluoroborate Salts in the Regioselective Synthesis of Imidazo[4,5-b]pyridine Analogues

In this paper, we report the use of potassium organotrifluoroborate salts as nucleophilic organoboron reagents in the Suzuki cross-coupling reactions of 2-halo deazapurines. Regio-isomeric C-2-substituted imidazo[4,5-b]pyridine analogues were synthesized by employing this protocol in good to excellent yields. Whereas aryl and heteroaryl trifluoroborates reacted readily to give the coupled products in high yields, alkyltrifluoroborates were found to be less reactive. The utilization of tetrabutylammonium acetate was found to play a substantial role in enhancing the reaction rates of the cross-coupling process. Also, a comparative study was performed between boronic acids and potassium organotrifluoroborate salts.

Palladium-catalyzed carbonylation of aryl halides: an efficient, heterogeneous and phosphine-free catalytic system for aminocarbonylation and alkoxycarbonylation employing Mo(CO)6 as a solid carbon monoxide source

Preparation and characterization of palladium nanoparticles immobilized on magnetic methionine-functionalized chitosan as a highly efficient, air stable, and readily reusable heterogeneous catalyst in carbonylation reactions.

Regioselective oxidative Pd-catalysed coupling of alkylboronic acids with pyridin-2-yl-substituted heterocycles

A total of 19 alkylated heterocycles (thiophenes, benzo-thiophenes, pyrroles, furans) were prepared (36–99% yield) from the respective pyridin-2-yl-substituted precursors employing alkylboronic acids as the alkylating reagents in an oxidative (Ag₂CO₃ and 2,6-dimethyl-1,4-benzoquinone as oxidants) Pd-catalysed coupling reaction.

Cu(OAc)2–Et3N mediated oxidative coupling of α-azido ketones with pyridinium ylides: utilizing in situ generated imines for regioselective synthesis of imidazo[1,2-a]pyridines

Imidazo[1,2-a]pyridines are synthesized via Cu(OAc)₂–Et₃N mediated one-pot coupling of pyridines, α-bromo ketones and α-azido ketones.

Palladium-catalyzed direct ortho-sulfonylation of azobenzenes with arylsulfonyl chlorides via C–H activation

ortho-Sulfonylated azobenzenes were obtained in moderate to high yields by direct ortho-sulfonylation of azobenzenes' C–H bond with arylsulfonyl chlorides.

Suzuki–Miyaura reaction by heterogeneously supported Pd in water: recent studies

This review summarizes the progress made essentially in the last fifteen years in the Suzuki–Miyaura coupling reaction by heterogeneous palladium catalysis in water as the sole solvent.

Polymer supported Pd catalyzed thioester synthesis via carbonylation of aryl halides under phosphine free conditions

A new polymer anchored Pd(ii) complex has been synthesized and characterized. The catalytic performance of this complex has been tested for thioester synthesis via carbonylation of aryl halides and thiols under phosphine free conditions. This catalyst showed excellent catalytic activity and recyclability.

Polymer supported Pd catalyzed carbonylation of aryl bromides for the synthesis of aryl esters and amides

A new polymer anchored Pd(ii) Schiff base catalyst has been synthesized for the evaluation of alkoxycarbonylation and aminocarbonylation reactions of various substituted aryl bromides with alcohols and amines.

A rapid and modified approach for C-7 amination and amidation of 4-methyl-7-nonafluorobutylsulfonyloxy coumarins under microwave irradiation

A rapid approach for the synthesis of an array of 4-methyl-7-substituted coumarins has been developed.

Regioselective C2-arylation of imidazo[4,5-b]pyridines

We show that N3-MEM-protected imidazo[4,5-b]pyridines undergo efficient C2-functionalisation via direct C-H arylation. Twenty-two substituted imidazo[4,5-b]pyridines are prepared and iterative, selective elaboration of functionalised imidazo[4,5-b]pyridines gives 2,7- and 2,6-disubstituted derivatives in good yields from common intermediates. Mechanistic observations are consistent with a concerted-metallation-deprotonation mechanism facilitated by coordination of copper(I)iodide to the imidazo[4,5-b]pyridine.

Microwave-Assisted Synthesis of Isopropylβ-(3,4-Dihydroxyphenyl)-α-hydroxypropanoate

Using microwave irradiation heating, isopropylβ-(3,4-dihydroxyphenyl)-α-hydroxypropanoate was synthesised from 3,4-dihydroxybenzaldehyde and acetylglycine through the formation of 2-methyl-4-(3,4-acetoxybenzylene)oxazol-5-ones,α-acetylamino-β-(3,4-diacetoxyphenyl)acrylic acid, andβ-(3,4-dihydroxyphenyl)pyruvic acid followed by Clemmensen reduction and esterification. The reaction conditions in terms of operating parameters were optimised by using an orthogonal design of experiment (ODOE) approach, including reaction temperature, reaction time, and microwave power level. Compared with conventional heating, the reaction time was significantly reduced for all reactions and the product yields were increased (except for the third-step reaction) under microwave heating conditions. The most remarkable microwave enhancement was found in the step of isopropylβ-(3,4-dihydroxyphenyl)-α-hydroxypropanoate production where the reaction time was reduced from 10 hrs (conventional heating) to 25 mins (microwave heating) whilst the yield was increased from 75.6% to 87.1%, respectively.