Synthesis of Imidazo[4,5-b]pyridines and Imidazo[4,5-b]pyrazines by Palladium Catalyzed Amidation of 2-Chloro-3-amino-heterocycles

Hypervalent iodine(iii) promoted C–H/C–H amination/annulation tandem reactions: synthesis of benzimidazoles from simple anilines and aldehydes

A novel hypervalent iodine mediated cascade transformation of anilines and aldehydes to benzimidazoles was developed.

Conversion of oxadiazolo[3,4-b]pyrazines to imidazo[4,5-b]pyrazines via a tandem reduction-cyclization sequence generates new mitochondrial uncouplers

We report new mitochondrial uncouplers derived from the conversion of [1,2,5]oxadiazolo[3,4-b]pyrazines to 1H-imidazo[4,5-b]pyrazines. The in situ Fe-mediated reduction of the oxadiazole fragment followed by cyclization gave access to imidazopyrazines in moderate to good yields. A selection of orthoesters also allowed functionalization on the 2-position of the imidazole ring. This method afforded a variety of imidazopyrazine derivatives with varying substitution on the 2, 5 and 6 positions. Our studies suggest that both a 2-trifluoromethyl group and N-methylation are crucial for mitochondrial uncoupling capacity.

Enzymatic and non-enzymatic crosslinks found in collagen and elastin and their chemical synthesis

This review summarized the enzymatic and non-enzymatic crosslinks found in collagen and elastin and their organic synthesis.

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.

Synthesis of N-Substituted 3-Amino-4-halopyridines: A Sequential Boc-Removal/Reductive Amination Mediated by Brønsted and Lewis Acids

N-Substituted 3-amino-4-halopyridines are valuable synthetic intermediates, as they readily provide access to imidazopyridines and similar heterocyclic systems. The direct synthesis of N-substituted 3-amino-4-halopyridines is problematic, as reductive aminations and base-promoted alkylations are difficult in these systems. A high yielding deprotection/alkylation protocol mediated by trifluoroacetic acid and trimethylsilyl trifluoromethanesulfonate is described, providing access to a wide scope of N-substituted 3-amino-4-halopyridines. This protocol furnishes many reaction products in high purity without chromatography. Similar reductive amination conditions were also established for deactivated anilines.

Eosin Y–Yb(OTf)3 catalyzed visible light mediated electrocyclization/indole ring opening towards the synthesis of heterobiaryl-pyrazolo[3,4-b]pyridines

Synthesis of heterobiaryl-pyrazolo[3,4-b]pyridines via Lewis acid and visible light photo-catalysis at room temperature.

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.

Applications of Palladium-Catalyzed C-N Cross-Coupling Reactions

Pd-catalyzed cross-coupling reactions that form C–N bonds have become useful methods to synthesize anilines and aniline derivatives, an important class of compounds throughout chemical research. A key factor in the widespread adoption of these methods has been the continued development of reliable and versatile catalysts that function under operationally simple, user-friendly conditions. This review provides an overview of Pd-catalyzed N-arylation reactions found in both basic and applied chemical research from 2008 to the present. Selected examples of C–N cross-coupling reactions between nine classes of nitrogen-based coupling partners and (pseudo)aryl halides are described for the synthesis of heterocycles, medicinally relevant compounds, natural products, organic materials, and catalysts.

Copper- and palladium-catalyzed amidation reactions for the synthesis of substituted imidazo[4,5-c]pyridines

Imidazo[4,5-c]pyridines were synthesized in three steps utilizing a palladium-catalyzed amidation/cyclization strategy. N-Aryl substrates were synthesized using copper-catalyzed amidation of 3-amino-N-Boc-4-chloropyridine. Complementary protocols for the selective chlorination of imidazo[4,5-c]pyridines at the C2 and C7 positions were also developed.

Cascade palladium catalysis: a predictable and selectable regiocontrolled synthesis of N-arylbenzimidazoles

By choice: The palladium-catalyzed cascade reaction of 2-chloroaryl sulfonates with arylamine and amide nucleophiles provides direct access to N-arylbenzimidazoles. This strategy selectively produces the heterocycles based on chemoselective oxidative addition. 2-Chloroaryl triflates (Tf) produce one regioisomer and the corresponding 2-chloroaryl mesylates (Ms) deliver the other in a selectable manner.

C-N bond forming cross-coupling reactions: an overview

Nitrogen containing compounds are of great importance because of their interesting and diverse biological activities. The construction of the C-N bond is of significant importance as it opens avenues for the introduction of nitrogen in organic molecules. Despite significant advancements in this field, the construction of the C-N bond is still a major challenge for organic chemists, due to the involvement of harsh reaction conditions or the use of expensive catalysts in many cases. Thus, it is a challenge to develop alternative, milder and cheaper methodologies for the construction of C-N bonds. Herein, we have selected some prime literature reports that may serve this purpose.

Synthesis and application of palladium precatalysts that accommodate extremely bulky di-tert-butylphosphino biaryl ligands

A series of palladacyclic precatalysts that incorporate electron-rich di-tert-butylphosphino biaryl ligands is reported. These precatalysts are easily prepared, and their use provides a general means of employing bulky ligands in palladium-catalyzed cross-coupling reactions. The application of these palladium sources to various C-N and C-O bond-forming processes is also described.

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.

Investigating the dearomative rearrangement of biaryl phosphine-ligated Pd(II) complexes

A series of monoligated L·Pd(II)(Ar)X complexes (L = dialkyl biaryl phosphine) have been prepared and studied in an effort to better understand an unusual dearomative rearrangement previously documented in these systems. Experimental and theoretical evidence suggest a concerted process involving the unprecedented Pd(II)-mediated insertion of an aryl group into an unactivated arene.