Abstract
Herein, we report a facile synthetic methodology for the preparation of 2,3-dialkylquinolines from anilines and propionaldehydes. This cyclization involved environmentally friendly Nafion® NR50 as an acidic catalyst with microwave irradiation as the heating source. A series of substituted 2-ethyl-3-methylquinolines were prepared from various anilines and propionaldehyde derivatives through this protocol with good to excellent yields. Some new chemical structures were confirmed by X-ray single-crystal diffraction analysis and the related data were provided. The plausible reaction mechanism studies are also discussed.
Highlights
IntroductionA quinoline scaffold is a versatile synthetic building block in various natural products [1,2,3,4,5], exceptional pharmaceuticals [6,7,8], physical materials [9,10,11,12,13], and is an important intermediate for asymmetric synthesis [14,15,16,17,18]
In continuation of our investigations toward the synthesis of nitrogen-containing heterocyclic compounds [68,69,70,71], we recently provided a synthetic route for the Friedländer quinoline synthesis from 2-aminobenzophenone and acetylacetone catalyzed by Nafion® NR50 particles under microwave irradiation ((1) in Scheme 2) [69]
According to our previous work [69], we further examined the environmentally friendly solid acid Nafion® NR50, and the desired product 3a was obtained at 93% yield
Summary
A quinoline scaffold is a versatile synthetic building block in various natural products [1,2,3,4,5], exceptional pharmaceuticals [6,7,8], physical materials [9,10,11,12,13], and is an important intermediate for asymmetric synthesis [14,15,16,17,18]. Functionalized quinolines are broadly used in agrochemicals [19,20], dyes [21,22], and some biologically active molecules for antimalarial [23,24,25], anticancer [26,27,28,29,30], antiviral [31], antifungal [32], anti-bacterial [33], and anti-inflammatory functions [34,35]. Some reports indicated that some quinoline-containing compounds are potentially active SARS-CoV-2 inhibitors [38,39,40], and some quinoline-derived drugs, as shown in Scheme 1, are currently being investigated as a possible cure for COVID19 infection [41,42].
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