A practical green MCR strategy has been developed for the synthesis of D–A type hydrazinyl thiazolyl coumarins catalyzed by an environmentally compatible MMT K10 clay in water at RT.
Natural dolomitic limestone (NDL) is employed as a heterogeneous green catalyst for the synthesis of medicinally valuable benzimidazoles, dihydropyrimidinones, and highly functionalized pyridines via C–N, C–C, and C–S bond formations in a mixture of ethanol and H2O under ultrasound irradiation. The catalyst is characterized by XRD, FTIR, Raman spectroscopy, SEM, and EDAX analysis. The main advantages of this methodology include the wide substrate scope, cleaner reaction profile, short reaction times, and excellent isolated yields. The products do not require chromatographic purification, and the catalyst can be reused seven times. Therefore, the catalyst is a greener alternative for the synthesis of the above N-heterocycles compared to the existing reported catalysts.
An improved and greener protocol has been developed for the synthesis of 2,4‐disubstituted thiazoles via C–Br, C–S, and, C–N bond formations in a single step from readily available ketones, N‐bromosuccinimide (NBS), and thiourea catalyzed by citric acid in a mixture of ethanol and water (3:1) under reflux conditions. This method has the advantages of freedom from the isolation of lachrymatory α‐bromoketones, ease of carrying out, cleaner reaction profile, broad substrate scope, freedom from chromatographic purification, and suitability for large‐scale synthesis.
The present work describes the solvent and catalyst‐free synthesis of imidazo[1,2‐a]pyridines in excellent to nearly quantitative yields from 2‐aminopyridines and a wide variety of ω‐bromomethylketones using a grindstone procedure at 25°C to 30°C for 3 to 5 minutes. The absolute structure of the compound, 2‐(3‐bromophenyl)‐7‐methylimidazo[1,2‐a]pyridine is determined by X‐ray crystallography. This green strategy has several noteworthy advantages such as wide spread substrate scope, short reaction times, water work up and the products do not require any chromatographic purification. Moreover, the method does not require any specialized equipment and is highly economical, environmentally benign and easy to carry out in any laboratory. Hence, the developed method meets the concept of “benign by design” and is greener alternative to the reported procedures for the synthesis of imidazo[1,2‐a]pyridines.
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