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The synthesis of a molecularly diverse library of tetrasubstituted alkenes containing a barbiturate motif is described. Base-induced condensation of <i>N</i>¹-substituted pyrimidine-2,4,6(1<i>H</i>,3<i>H</i>,5<i>H</i>)-triones with 5-(bis(methylthio)methylene)-2,2-dimethyl-1,3-dioxane-4,6-dione gave 3-substituted 5-(methylthio)-2<i>H</i>-pyrano[2,3-<i>d</i>]pyrimidine-2,4,7(1<i>H</i>,3<i>H</i>)-triones (‘pyranopyrimidinones’), regioselectively. A sequence of reactions involving ring-opening of the pyran moiety, displacement of the methylthio group with an amine, re-formation of the pyran ring, and after its final cleavage with an amine, gave tetrasubstituted alkenes (3-amino-3-(2,4,6-trioxotetrahydropyrimidin-5(2<i>H</i>)-ylidene)propanamides) with a diversity of substituents. Cleavage of the pyranopyrimidinones with an aniline was facilitated in 2,2,2-trifluoroethanol under microwave irradiation. Compounds were tested against <i>Escherichia coli</i>, <i>Staphylococcus aureus</i>, the yeast <i>Schizosaccharomyces pombe,</i> and the pathogenic fungus <i>Candida albicans</i>. No compounds exhibited activity against <i>E. coli</i>, whilst one compound was weakly active against <i>S. aureus</i>. Three compounds were strongly active against <i>S. pombe</i>, but none was active against <i>C. albicans</i>.