In the title hydrate, C16H19N3O3·1.5H2O, both water mol­ecules are disordered: one over two adjacent sites in a 0.498 (5):0.502 (5) ratio and one lying near a crystallographic twofold axis. The dihedral angle between the pyridine rings of the organic moleucle is 1.47 (6)°. In the crystal, the components are linked by N—H⋯O, O—H⋯N and C—H⋯O hydrogen bonds, forming sheets lying parallel to the ac plane.

The asymmetric unit of the title compound, C9H7N3O2, is composed of two independent mol­ecules. The crystal structure is stabilized by C—H⋯O and C—H⋯N hydrogen bonds, forming a three-dimensional network. The crystal structure also features pyrazine–pyrazine π–π inter­actions [centroid–centroid distance = 3.6994 (5) Å] and also pyridine–pyrazine π–π inter­actions [centroid–centroid distance = 3.6374 (5) Å].

In the title compound, C18H17N3, the dihedral angles between the central pyrimidine ring and its directly-bonded and N-bonded pendant phenyl rings are 25.48 (6) and 80.33 (6)°, respectively. The dihedral angle between the phenyl rings is 79.66 (6)°. In the crystal, inversion dimers linked by pairs of N—H⋯N hydrogen bonds generate R 2 2(8) loops. The crystal structure also features weak π–π [centroid–centroid separation = 3.6720 (7) Å] and C—H⋯π inter­actions.

In the title compound, C20H21N3, the pyrimidine ring is inclined at dihedral angles of 51.57 (4) and 2.49 (4)° to the two phenyl rings. The dihedral angle between the two terminal phenyl rings is 50.44 (4)°. In the crystal, adjacent mol­ecules are linked via a pair of N—H⋯N hydrogen bonds, forming an inversion dimer with an R 2 2(8) ring motif. Furthermore, the crystal structure is stabilized by a weak π–π inter­action, with a centroid–centroid distance of 3.6065 (5) Å.

In the title compound, C15H19N3, the pyrimidine ring is approximately planar [maximum deviation = 0.007 (1) Å] and forms a dihedral angle of 3.15 (6)° with the benzene ring. In the crystal packing, inter­molecular N—H⋯N hydrogen bonds link pairs of neighbouring mol­ecules into dimers with R 2 2(8) ring motifs. These dimers are stacked along the b axis.

The asymmetric unit of the title compound, C14H23N5O3·2H2O, contains two crystallographically independent 6-amino-2,5-bis­(pivaloylamino)pyrimidin-4(3H)-one mol­ecules (A and B) with similar geometry and four water mol­ecules. In both independent mol­ecules, one of the amide groups is almost coplanar with the pyrimidine ring [dihedral angle of 12.85 (9) in A and 12.30 (10)° in B], whereas the other amide group is significantly twisted away from it [dihedral angle is 72.18 (7) in A and 71.29 (7)° in B]. In each independent mol­ecule, an intra­molecular N—H⋯O hydrogen bond generates an S(6) ring motif. Mol­ecules A and B are linked into chains along the a axis by N—H⋯O and C—H⋯O hydrogen bonds. Adjacent chains are linked into a two-dimensional network parallel to the ac plane by water mol­ecules via N—H⋯O and O—H⋯O hydrogen bonds.

The asymmetric unit of the title compound, C20H17N3, contains two crystallographically independent mol­ecules (A and B). In mol­ecule A, the two benzene rings form dihedral angles of 74.12 (7) and 7.83 (7)° with the pyridine ring, while in mol­ecule B these angles are 77.48 (7) and 21.50 (7)°. The seven-membered heterocyclic ring adopts a boat conformation in both mol­ecules. In the crystal structure, each of the independent mol­ecules forms a centrosymmetric R 2 2(8) dimer linked by paired N—H⋯N hydrogen bonds. The crystal structure is further stabilized by inter­molecular C—H⋯N hydrogen bonds and C—H⋯π inter­actions.

The organic mol­ecule of the title compound, C16H12N6O2·2H2O, lies across a crystallographic inversion centre. The dihedral angle between the pyrimidine and benzene rings is 80.78 (6)°. The two pyrimidine rings are parallel by virtue of the centre of symmetry. The pyrimidine and benzene rings form dihedral angles of 41.41 (7) and 40.26 (7)°, respectively, with the amide plane. The mol­ecules are linked by N—H⋯N and C—H⋯N hydrogen bonds into a two-dimensional network parallel to the (11) plane. O—H⋯O and C—H⋯O hydrogen bonds involving the water mol­ecules link the adjacent layers into a three-dimensional network. In addition, a C—H⋯π inter­action involving the benzene ring is observed.

In the mol­ecular structure of the title hydrazide derivative, C18H26N4O4, the conformations of the two units of 2-(2,2-dimethyl-1-oxoprop­yl)hydrazide substituents are not planar; these two units are attached axially to the benzene ring with C(ortho)—C—C(=O)—N torsion angles of 28.1 (2) and 31.0 (2)° [where C(ortho) is the C atom at position 4 of the benzene ring relative to the substituent at position 3 or the C atom at position 6 of the benzene ring relative to the substituent at position 1, as appropriate]. The dihedral angles between the hydrazide units and the benzene ring are 62.66 (7) and 63.84 (7)°. In the crystal structure, mol­ecules are arranged in an anti-parallel manner and are linked by N—H⋯O inter­molecular hydrogen bonds and weak C—H⋯O inter­molecular inter­actions into a three-dimensional network. The structure is further stabilized by a weak C—H⋯N intra­molecular inter­action.