Conformational studies of hydantoin-5-acetic acid and orotic acid

Hydantoin-5-acetic acid [2-(2,5-dioxoimidazolidin-4-yl)acetic acid] and orotic acid (2,6-dioxo-1,2,3,6-tetra­hydro­pyrimidine-4-carb­oxy­lic acid) each contain one rigid acceptor-donor-acceptor hydrogen-bonding site and a flexible side chain, which can adopt different conformations. Since both com­pounds may be used as coformers for supra­molecular com­plexes, they have been crystallized in order to examine their conformational preferences, giving solvent-free hydantoin-5-acetic acid, C₅H₆N₂O₄, (I), and three crystals containing orotic acid, namely, orotic acid dimethyl sulfoxide monosolvate, C₅H₄N₂O₄·C₂H₆OS, (IIa), dimethyl­ammonium oro­tate-orotic acid (1/1), C₂H₈N⁺·C₅H₃N₂O₄^-·C₅H₄N₂O₄, (IIb), and dimethyl­ammonium orotate-orotic acid (3/1), 3C₂H₈N⁺·3C₅H₃N₂O₄^-·C₅H₄N₂O₄, (IIc). The crystal structure of (I) shows a three-dimensional network, with the acid function located perpendicular to the ring. Inter­estingly, the hy­droxy O atom acts as an acceptor, even though the carbonyl O atom is not involved in any hydrogen bonds. However, in (IIa), (IIb) and (IIc), the acid functions are only slightly twisted out of the ring planes. All H atoms of the acidic functions are directed away from the rings and, with respect to the carbonyl O atoms, they show an anti­periplanar conformation in (I) and synperiplanar conformations in (IIa), (IIb) and (IIc). Furthermore, in (IIa), (IIb) and (IIc), different con­formations of the acid O=C-C-N torsion angle are observed, leading to different hydrogen-bonding arrangements depending on their conformation and composition.