Background: Quinolines are one of the most popular N-heteroaromatic compounds incorporated into the structures of many pharmaceuticals. The classical methods for the synthesis of quinoline derivatives include Skraup, Doebner–Von Miller, Conrad–Limbach, Combes, and Pfitzinger quinoline syntheses but the best-known method for the preparation of quinolines is Friedlander quinoline synthesis. A number of methods have been reported for the synthesis of quinolines involving a variety of catalysts. However, many of these procedures are not satisfactory because of harsh reaction conditions, poor yields, prolonged time period, and use of hazardous and often expensive catalyst. Hence, mild synthetic protocol for the synthesis of quinolines is highly desirable.
Objective: Mild and efficient synthesis of quinolines in the presence of sulfonyl functionalized imidazolium ionic liquids is presented in this manuscript.
Method: Various functional quinolines were synthesized by one–pot condensation reaction of 2– aminoaryl carbonyl compounds with an activated α–CH acid such as cyclic or acyclic β–dicarbonyl or aromatic carbonyl compounds in the presence of recyclable sulfonyl functionalized imidazolium ionic liquids.
Results: Sulfonyl functionalized imidazolium ionic liquids exhibited remarkable activity towards synthesizing a broad range of quinolines and 1-(4-sulfonylbutyl)-3-methylimidazolium trifluoromethanesulfonate ionic liquid was found to be most efficient compared to other ionic liquids used for the synthesis of parallel library of quinolines.
Conclusion: A cascade protocol for the synthesis of functional quinolines is developed by the condensation of 2-amino-5-chlorobenzophenone (or 2-aminobenzophenone) with a variety of cyclic or acyclic β-diketones or aromatic ketones in the presence of SO3H-functionalized ILs. The recyclability of ionic liquids, simple work-up procedure, short reaction time and high yields are the important attributes of the present protocol.
Keywords: Amino benzophenone, carbonyl compounds, condensation reaction, ketones, quinolines, sulfonyl ionic liquids.