“On-Water” Catalyst-Free Ecofriendly Synthesis of the Hantzsch Dihydropyridines

Table 2

Effect of different solvents towards the catalyst-free ammonium acetate-mediated synthesis of the Hantzsch dihydropyridine.

Solvent	Reaction outcomea
Hexane	Crude dihydropyridine (58% yield, contaminated with a lot of unidentified byproducts)
Acetone	Mixture of unidentified byproducts without any; formation of dihydropyridine
Dichloromethane	Mixture of unidentified byproducts without any formation of dihydropyridine
Chloroform	Mixture of unidentified byproducts with trace amount of dihydropyridine
Ethyl acetate	Crude dihydropyridine (54% yield, contaminated with a lot of unidentified byproducts)
DMF	Exclusive formation of dihydropyridine (44% yield, remainder starting material)
DMSO	Exclusive formation of dihydropyridine (38% yield, remainder starting material)
Ethanol	Crude dihydropyridine (60% yield, contaminated with a lot of unidentified byproducts)
1-Propanol	Crude dihydropyridine (39% yield, contaminated with a lot of unidentified byproducts)
2-Propanol	Crude dihydropyridine (36% yield, contaminated with a lot of unidentified byproducts)
Water	Exclusive formation of dihydropyridine (89% yield of crystallized product) without any formation of byproduct

aAll the reactions were done with 4-methylbenzaldehyde : EAA : ammonium acetate = 1 : 2 : 1.3 at the boiling point of the corresponding solvent or at 70^∘C (whichever is lower) for 1 hr.