Abstract <p>The condensation of 5-acetylpyrimidine-2,4,6(1<i>H</i>,3<i>H</i>,5<i>H</i>)-trione with 3-fluoro-, 4-fluoro-, and 2-methoxy-3,5-dichlorobenzaldehydes and methyl (4-formyl-2- oxo-2<i>H</i>-chromen-7-yl)carbamate in BuOH in the presence of piperidine and acetic acid for 4 h under reflux gave 5-[(<i>E</i>)-3-aryl-2-propenoyl]pyrimidine--2,4,6(1<i>H</i>,3<i>H</i>,5<i>H</i>)-triones) and methyl <i>N</i>-{2-oxo-4-[(<i>E</i>)-3-oxo-3-(2,4,6-trioxohexahydropyrimidin-5-yl)-1-propenyl]-2<i>H</i>-chromen-7-yl}carbamate. The reactions of 3-methoxy-4-hydroxy-, 3,5-dimethoxy-4-hydroxybenzaldehydes with 5-acetyl-pyrimidine-2,4,6(1<i>H</i>,3<i>H</i>,5<i>H</i>)-trione in dioxane in the presence of boron trifluoride etherate at room temperature forms the corresponding chalcones within 48 h. The corresponding imines and 5-arylidene derivatives were prepared by the condensation of barbituric acid with alkyl <i>C</i>-nitroso phenylcarbamates in methanol and hydroxy-substituted benzaldehydes in water. The condensation of 5-[(4-hydroxy-3,5-dimethoxyphenyl)methylene]pyrimidine-2,4,6(1<i>H</i>,3<i>H</i>,5<i>H</i>)-triones with aminoguanidine bicarbonate results in the formation of 7-hydrazinyl-5-(4-hydroxy-3,5-dimethoxyphenyl)pyrimido[4,5-<i>d</i>]pyrimidine-2,4(1<i>H</i>,3<i>H</i>)-dione. The cyclocondensation of pyrimidine-2,4,6(1<i>H</i>,3<i>H</i>,5<i>H</i>)-trione with methyl <i>N</i>-{3-[(<i>E</i>)-3-(4-methoxyphenyl)-2-propenoyl]phenyl}carbamate in acetic acid in the presence of P₂O₅ provided methyl {3-[5-(4-methoxyphenyl)-2,4-dioxo-1,3,4,5-tetrahydro-2<i>H</i>-pyrano[2,3-<i>d</i>]pyrimidin-7-yl]phenyl}carbamate.</p>