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 center dot C2H6OS, (IIa), dimethylammonium orotate-orotic acid (1/1), C2H8N+center dot C5H3N2O4-center dot C5H4N2O4, (IIb), and dimethylammonium orotate-orotic acid (3/1), 3C(2)H(8)N(+)center dot 3C(5)H(3)N(2)O(4)(-)center dot 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.