The catalytic efficiency of ammonium dihydrogen phosphate
for the synthesis of 3,4-dihydropyrimidin-2(1H)-ones
was studied with various amount of NH4H2PO4 (Table 1). To
determine the role of the catalyst, a blank reaction of
4-nitrobenzaldehyde, ethyl acetoacetate, and urea was carried
out in the absence of catalyst. The reaction did not give the
desired product efficiently in the absence of catalyst, and it
gave many impurity byproducts after an extended time of 8 h
(entry 1). The presence of the catalyst, only 0.5 mol% of
NH4H2PO4, gave 47% conversion after 3 h (entry 2), which
showed that the catalyst has high catalytic activity. Table 1