Quinoline is considered one of the most ubiquitous heterocycles due to its engaging biological activities and synthetic utility over organic transformations. Over the past few decades, numerous reports have been documented in the synthesis of quinolines. The classical methods including, Skraup, Friedlander, Doebner‐von‐Miller, Conrad‐Limpach, Pfitzinger quinoline synthesis, and so forth, these are the well‐known methods to construct principal quinoline scaffold with several advantages and limitations. Recently, radical insertion or catalyzed reactions have emerged as a powerful and efficient tool to construct heterocycles with high atom efficiency and step economy. In this concern, this minireview mainly focused on the developments of Quinoline synthesis via radical reactions. In addition, a brief description of the preparation procedure, reactivity, and mechanisms is also included, where as possible. Respectively, the synthesis of quinolines is classified and summarized based on its reactivity, so it will help the researchers to grab the information in this exploration area, as Quinolines are promising pharmacophores.

A volatile organic solvent-free and choline chloride (ChCl)-based deep eutectic system (DES)-mediated sp 3 -CH functionalization of acetophenones 1 with benzyl alcohols 2 to the corresponding α, β-saturated ketones 3 is accounted for. The domino dehydrogenation–aldol condensation (hydrogenation borrowing concept) has been successfully attempted with palladium-tetrakis(triphenylphosphine) [Pd(PPh 3 ) 4 ] catalyst–xantphos ligand combination. Furthermore, a sequential Friedländer reaction of 2-aminobenzophenone 4 and palladium-catalyzed α-alkylation of the quinolinyl methyl ketone with benzyl alcohols 2 in ChCl-based DES have been successfully investigated. The C–C bond formation through sp 3 -CH functionalization involves a wide scope of the substrates, high atom efficiency, chemoselectivity, and environmentally friendly strategy.

An efficient, solvent-free synthesis of 2-acylthieno­[2,3-b]­quinolines is reported from 2-halo-quinolinyl ketones through Cu-TEMPO catalyzed dehydrogenation, sp2-C–H functionalization using elemental sulfur as thiol surrogate (sulfur source) and tetrabutylammonium acetate as an ionic reaction medium. The optimized reaction conditions give excellent product yields under mild reaction conditions with chemoselectivity and broad functional group tolerance. The synthetic importance of the synthesized molecules is showcased further by Friedländer annulation, reduction, and alkene functionalization reactions.

Over the past few decades, thienoquinolines have gained considerable attention due to their biological and synthetic applications. Few reports have been disclosed on Thienoquinoline synthesis. Hence, in the present review, we provided a comprehensive update on the synthesis of Thienoquinolines. A brief description of the preparation procedure, reactivity, and mechanisms are included, where as possible. Respectively, the thienoquinoline synthesis is classified and summarized based on its reactivity, so it will help the researchers to obtain knowledge over the previous literature reports to overcome their problems in designing a new process.

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