Synthesis of pyrano[3,2-c]quinoline derivatives using catalyst prepared from L-proline and p-toluenesulfonic acid

Abstract

This research prepared new generation of ionic liquid (Deep eutectic solvent, DES) from L-proline and p-toluenesulfonic acid. The ionic liquid has an asymmetric center in its structure because the original amino acid has an asymmetric center. Subsequently, the ionic liquid was studied catalytic activity in reactions of quinoline[3,2-c]pyrano derivatives synthesis. Conditions affecting the reaction and rate of the two products were also investigated. The results of the reaction conditions showed that aprotic polar solvents gave better yields and the best reaction conditions were: temperature: 50^oC, time: 2 h, catalytic amount: 35% mol . The ratios of two product isomers showed that the reaction always forms a mixture of two diastereomers and the trans products were synthesized with moderate enantiomeric excess (40% for four derivatives) compared with the cis products. In addition, the results on catalytic reuse also showed that the catalyst had good usability without significant decreasing activity. When benzaldehyde and aniline were changed with their derivates bearing donating electron group (-CH₃), the reaction yields slightly decrease. However, benzaldehyde bearing withdrawing electron group (-F) gave moderate yield.