ABSTRACT
The synthesis of dihydropyrimidinones and dihydropyrimidine thiones has attracted interest because of their biological activities. Although the first synthesis of dihydropyrimidinones was reported in 1893 by Biginelli, it is only since the 1990s that several new methods for their synthesis have been reported. The synthesis of 1,4-dihydropyridine derivatives such as polyhydroquinolines (PHQs) is of considerable interest due to their biological properties. The most common route to polyhydroquinolines involves a modified Hantzsch reaction involving the multicomponent coupling of an aldehyde with a β-ketoester, a diketone, and ammonium acetate. Some representative examples of catalysts used for this coupling include I2, metal triflates, ceric ammonium nitrate, Baker's yeast, organocatalysts, zeolites, Ph3P, GaCl3, thiourea dioxide, thiazolium ion, sulphamic acid, Al2(SO4)3, La2O3, and nanoparticles. Syntheses of PHQs have also been carried out in various ionic liquids as solvents. Their syntheses have also been reported under catalyst-free conditions, but the methods suffer from longer reaction times.
