Recent Progress in Metal-Free Direct Synthesis of Imidazo[1,2-a]pyridines

This Mini-Review highlights the most effective protocols for metal-free direct synthesis of imidazo[1,2-a]pyridines, crucial target products and key intermediates, developed in the past decade. The emphases is given on the ecological impact of the methods and on the mechanistic aspects as well. The procedures efficiently applied in the preparation of important drugs and promising drug candidates are also underlined.

2-Aminopyridine - an unsung hero in drug discovery

2-Aminopyridine is a simple, low molecular weight and perfectly functionalised moiety known for the synthesis of diverse biological molecules. Many pharmaceutical companies across the globe aim to synthesise low-molecular weight molecules for use as pharmacophores against various biological targets. 2-Aminopyridine can serve as a perfect locomotive in the synthesis and pulling of such molecules towards respective pharmacological goals. The major advantage of this moiety is its simple design, which can be used to produce single products with minimum side reactions. Moreover, the exact weight of synthesised compounds is low, which enables facile identification of toxicity-causing metabolites in drug discovery programmes. This manuscript is a quick review of such pharmacophores derived from 2-aminopyridine.

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.

Substituent effects on linear and nonlinear optical properties of fluorescent (E)-2-(4-halophenyl)-7-arlstyrylimidazo[1,2-A]pyridine: spectroscopic and computational methods

Effects of alkylamino and bromo substituents on imidazo[1,2-a]pyridines containing donor-π-acceptor type groups were comprehensively investigated for their linear and nonlinear optical properties by solvatochromic and DFT (CAM-B3LYP and BHandHLYP) methods. The difference between the ground and excited dipole moments as well as their ratios obtained by solvatochromic analysis indicate that the excited state is more polar than the ground state for both the bromo and diethyl amino derivative. More than twofold enhancement in the excited state dipole moments was observed as revealed by the difference and ratio of dipole moment upon the introduction of alkylamino donor group and these suggest large intramolecular charge transfer in the dyes. Stabilization energy above 20 kJ/mol was observed for large number of electron donor–acceptor interactions in Natural Bonding Orbital (NBO) analysis. Bond length alternation (BLA) and Bond order alternation (BOA) values tend to zero suggesting a high degree of polarization in the dyes. Enhancement in mean polarizability (α0), first hyperpolarizability (β0) and second static hyperpolarizabilities ( $\bar \gamma$) were observed by the introduction of alkylamino and bromo group in place of chloro in spite of the fact that Hammett constant of chloro and bromo are the same. The dyes have fundamental and intrinsic properties within the Hamiltonian limits. The two-photon absorption cross section value (≈100 GM) is comparable with LDS-698, a commercial TPA dye. This investigation is important for understanding the electronic structure of imidazo[1,2-a]pyridine with active functional groups and extending the potential for optical applications.

Dual role of p-tosylchloride: copper-catalyzed sulfenylation and metal free methylthiolation of imidazo[1,2-a]pyridines

Copper-catalyzed regioselective C-3 sulfenylation of imidazo[1,2-a]pyridines using p-tosylchloride as a benign source of sulfenylating agents has been developed. On the other hand, p-tosylchloride mediated thiomethylation of imidazo[1,2-a]pyridines with dimethylsulfoxide as a source of thiomethylation under metal-free conditions was also described.

Regioselective synthesis of pyrrolo[1,2-a]imidazoles and imidazo[1,2-a]-pyridines

A concise synthesis of pyrrolo[1,2-a]imidazoles and imidazo[1,2-a]-pyridines by regioselective aza-ene additions and regioselective cyclic–condensation reactions of HKAs with ethyl 3-benzoylacrylate derivatives under catalyst-free conditions.

Graphene oxide (GO)-catalyzed multi-component reactions: green synthesis of library of pharmacophore 3-sulfenylimidazo[1,2-a]pyridines

Graphene oxide (GO)-catalyzed multi-component reactions: green synthesis of library of pharmacophore 3-sulfenylimidazo[1,2-a]pyridines