Abstract

Background: An efficient and green synthesis of various diethyl-1,4-dihydro-2,6-dimethyl- 4-(3-aryl-1-phenyl-1H-pyrazol-4-yl)pyridine-3,5-dicarboxylate derivatives was achieved by the multicomponent condensation of 3-aryl-1-phenyl-1H-pyrazole-4-carboxaldehydes, ethyl acetoacetate, and NH4OAc in the presence of 1-methyl-2-pyrrolidonium hydrogen sulphate [HNMP][HSO₄] as a catalyst in ethanol. A series of 1, 4-dihydropyridine (DHP) derivatives have been synthesized by simply stirring at room temperature in a sealed tube for suitable time. This protocol has the number of benefits including simple work-up procedure, excellent yields and environmentally benign condition.

Methods: A 25 mL sealed tube was filled with a mixture of 3-aryl-1-phenyl-1H-pyrazole-4- carboxaldehydes 1 (1 mmol), ethyl acetoacetate 2 (0.130 g, 2 mmol), NH4OAc 3 (0.085 g, 1.2 mmol) and 100 mg of ionic liquid [HNMP][HSO₄] in 10 mL of ethanol. Then the sealed tube was capped and the reaction mixture was stirred at room temperature using a magnetic stirrer for appropriate time. The progress of the reaction was monitored by TLC. Once the reaction was completed, the content was poured over crushed ice, and solid DHP derivative thus obtained was separated by filtration. The product was dried and in some cases, the impure product was recrystallized from DMF-H₂O.

Results: We synthesized diethyl 1,4-dihydro-2,6-dimethyl-4-(3-aryl-1-phenyl-1H-pyrazol-4- yl)pyridine- 3,5-dicarboxylate 4a-j by one pot multi-component reaction of 3-aryl-1-phenyl-1H- pyrazole-4- carboxaldehydes 1, ethyl acetoacetate 2, and NH₄OAc 3 in the presence of 100mg of ionic liquid [HNMP][HSO₄] as illustrated in Table 2. The reactions worked smoothly with different 3-aryl-1- phenyl-1H-pyrazole-4-carboxaldehydes bearing both electron releasing and withdrawing groups to give the DHPs derivatives in excellent yields. The experimental and work-up method is very straightforward, suitable, and has the capability to tolerate a diversity of other functional groups.

Conclusion: In summary, we developed a new protocol for the synthesis of various diethyl 1,4- dihydro-2,6-dimethyl-4-(3-aryl-1-phenyl-1H-pyrazol-4-yl)pyridine-3,5-dicarboxylate derivatives using acidic ionic liquid [HNMP] [HSO₄] as a highly efficient catalyst. The DHPs 4a-j were obtained by simply stirring at room temperature for appropriate time. A significant benefit of this protocol is lacking of formation of side product since the reaction was carried out at room temperature. Also, this fascinating protocol offers several advantages such as cleaner reaction, simple work-up procedure, and use of non-corrosive catalysts, excellent yield with eco-friendly approach and a significant contribution, moving towards green chemistry.

Keywords: 3-aryl-1-phenyl-1<H-pyrazole-4-carboxaldehydes, 1, 4-dihydropyridine, multi-component reaction, [HNMP][HSO₄], catalyst, DMF-H_2O.

Graphical Abstract