Table 2: Effect of different solvents towards the catalyst-free ammonium acetate-mediated synthesis of the Hantzsch dihydropyridine.

SolventReaction outcome a

HexaneCrude dihydropyridine (58% yield, contaminated with a lot of unidentified byproducts)
AcetoneMixture of unidentified byproducts without any
formation of dihydropyridine
DichloromethaneMixture of unidentified byproducts without any formation of dihydropyridine
ChloroformMixture of unidentified byproducts with trace amount of dihydropyridine
Ethyl acetateCrude dihydropyridine (54% yield, contaminated with a lot of unidentified byproducts)
DMFExclusive formation of dihydropyridine (44% yield, remainder starting material)
DMSOExclusive formation of dihydropyridine (38% yield, remainder starting material)
EthanolCrude dihydropyridine (60% yield, contaminated with a lot of unidentified byproducts)
1-PropanolCrude dihydropyridine (39% yield, contaminated with a lot of unidentified byproducts)
2-PropanolCrude dihydropyridine (36% yield, contaminated with a lot of unidentified byproducts)
WaterExclusive formation of dihydropyridine (89% yield of crystallized product) without any formation of byproduct

a All 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.