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In each of ethyl N-{2-amino-5-formyl-6-[methyl(phenyl)amino]pyrimidin-4-yl}glycinate, C16H19N5O3, (I), N-{2-amino-5-formyl-6-[methyl(phenyl)amino]pyrimidin-4-yl}glycinamide, C14H16N6O2, (II), and ethyl 3-amino-N-{2-amino-5-formyl-6-[methyl(phenyl)amino]pyrimidin-4-yl}propionate, C17H21N5O3, (III), the pyrimidine ring is effectively planar, but in each of methyl N-{2-amino-6-[benzyl(methyl)amino]-5-formylpyrimidin-4-yl}glycinate, C16H19N5O3, (IV), ethyl 3-amino-N-{2-amino-6-[benzyl(methyl)amino]-5-formylpyrimidin-4-yl}propionate, C18H23N5O3, (V), and ethyl 3-amino-N-[2-amino-5-formyl-6-(piperidin-4-yl)pyrimidin-4-yl]propionate, C15H23N5O3, (VI), the pyrimidine ring is folded into a boat conformation. The bond lengths in each of (I)–(VI) provide evidence for significant polarization of the electronic structure. The molecules of (I) are linked by paired N—H...N hydrogen bonds to form isolated dimeric aggregates, and those of (III) are linked by a combination of N—H...N and N—H...O hydrogen bonds into a chain of edge-fused rings. In the structure of (IV), molecules are linked into sheets by means of two hydrogen bonds, both of N—H...O type, in the structure of (V) by three hydrogen bonds, two of N—H...N type and one of C—H...O type, and in the structure of (VI) by four hydrogen bonds, all of N—H...O type. Molecules of (II) are linked into a three-dimensional framework structure by a combination of three N—H...O hydrogen bonds and one C—H...O hydrogen bond.