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There are two molecules in the asymmetric unit of the title compound, C15H17N3O6. The 2,3-dioxopiperazine ring adopts a half-chair conformation with torsion angles of −7.6 (4) and 35.1 (4)° in one molecule, and 5.3 (4) and 45.4 (4)° in the other molecule. In the crystal structure, the carboxy groups are involved in classical inversion-related O—H...O hydrogen bonds, which link the molecules into centrosymmetric dimers. These dimers are further linked by intermolecular O—H...O and C—H...O hydrogen bonds. Each independent molecule also exhibits an intramolecular N—H...O hydrogen bond. The H atoms of the carboxy groups are disordered over two positions, with refined site-occupancy factors of 0.5.