4-Methyl-2H-1,3-oxazine-2,6(3H)-dione.

In the title compound, C(5)H(5)NO(3), the planar (maximum deviation = 0.075 Šfor the ring O atom) mol-ecules form N-H⋯O hydrogen bonds in a zigzag chain (C-O⋯N bond angle ≃ 140°) between glide-related mol-ecules.

4,5-Dichloro-2H-1,3-oxazine-2,6(3H)-dione.

In the title compound, C(4)HCl(2)NO(3), the essentially planar (maximum deviation = 0.023 Šfor the ring O atom) mol-ecules form N-H⋯O hydrogen bonds between mol-ecules lying about inversion centers, forming eight-membered rings with an R(2) (2)(8) motif in graph-set notation.

4-Bromo-2H-1,3-oxazine-2,6(3H)-dione.

The title compound, C(4)H(2)BrNO(3), is one of a series of three substituted oxauracils prepared as precursors in the preparation of 1-aza-1,3-butadienes. Although each structure has identical potential for N-H⋯O inter-molecular hydrogen bonds, each forms a distinctive inter-molecular network. In the title compound, there are two independent mol-ecules in the asymmetric unit, with a non-crystallographic twofold screw-like relationship between them. The two indpendent mol-ecules are linked by an inter-molecular N-H⋯O hydrogen bond. In the crystal structure, this hydrogen-bonded pair is linked to translationally related mol-ecules through further inter-molecular N-H⋯O hydrogen bonds, forming one-dimensional chains along [100]. The crystal structure also has short Br⋯O=C inter-molecular contacts with distances of 2.843 (4) and 2.852 (4) Å.