Ultrasound-Assisted Synthesis of Substituted 1,4-Dihydropyridines in Aqueous Medium Using Phase Transfer Catalysts

To improve the reaction conditions, the condensation of 3,4-dimethoxybenzaldehyde, methyl acetoacetate, and ammonium carbonate was regarded a model reaction. Dimethyl-4-dihydro-(3,4-methoxyphenyl)-2,6-dimethylpyridine-3,5-dicarboxylate is the product of this synthesis (compound DP1). Different polymers (polyethylene glycol (PEG400), glycerin), phase-transfer catalysts (tetra-n-butylammonium bromide (TBAB), and calcium phosphate nanoparticles were tested for their catalytic activity in an aqueous model process. The model reaction was conducted under ultra sound. The reaction was conducted with two high yielding phase transfer catalysts [TBAB and TPAB, 10 mol%]. The catalyst TBAB produced 91% of compound DP1, which is higher than conventional method (84%). The catalyst TPAB also gave relatively higher yield (83%) compared with conventional method (79%). The results showed that the ideal conditions for the synthesis of 1,4-DHP analogues were 60 min of ultrasonic condensation with TBAB (10 mol%) in aqueous medium (eco-beneficial approach). Twenty compounds (DP1) were produced via the condensation of aryl/heteroaryl aldehydes with methyl acetoacetate, ethyl acetoacetate, and ammonium carbonate under optimal circumstances. IR, 1H NMR, and mass spectrum data were used to determine the structures of the produced compounds.