title: Pseudopolymorphs of chelidamic acid and its dimethyl ester.
abstract: Different tautomeric and zwitterionic forms of chelidamic acid (4-hydroxypyridine-2,6-dicarboxylic acid) are present in the crystal structures of chelidamic acid methanol monosolvate, C(7)H(5)NO(5)·CH(4)O, (Ia), dimethylammonium chelidamate (dimethylammonium 6-carboxy-4-hydroxypyridine-2-carboxylate), C(2)H(8)N(+)·C(7)H(4)NO(5)(-), (Ib), and chelidamic acid dimethyl sulfoxide monosolvate, C(7)H(5)NO(5)·C(2)H(6)OS, (Ic). While the zwitterionic pyridinium carboxylate in (Ia) can be explained from the pK(a) values, a (partially) deprotonated hydroxy group in the presence of a neutral carboxy group, as observed in (Ib) and (Ic), is unexpected. In (Ib), there are two formula units in the asymmetric unit with the chelidamic acid entities connected by a symmetric O-H···O hydrogen bond. Also, crystals of chelidamic acid dimethyl ester (dimethyl 4-hydroxypyridine-2,6-dicarboxylate) were obtained as a monohydrate, C(9)H(9)NO(5)·H(2)O, (IIa), and as a solvent-free modification, in which both ester molecules adopt the hydroxypyridine form. In (IIa), the solvent water molecule stabilizes the synperiplanar conformation of both carbonyl O atoms with respect to the pyridine N atom by two O-H···O hydrogen bonds, whereas an antiperiplanar arrangement is observed in the water-free structure. A database study and ab initio energy calculations help to compare the stabilities of the various ester conformations.
authors.affiliation: Institut für Organische Chemie und Chemische Biologie, Goethe-Universität Frankfurt, Max-von-Laue-Strasse 7, 60438 Frankfurt am Main, Germany.
authors.forename: Maya
authors.initials: M
authors.lastname: Tutughamiarso
authors.forename: Thorsten
authors.initials: T
authors.lastname: Pisternick
authors.forename: Ernst
authors.initials: E
authors.lastname: Egert
mesh_terms: D018360:Crystallography, X-Ray; D003998:Dicarboxylic Acids / Q000737:chemistry*; D004952:Esters; D006860:Hydrogen Bonding; D015394:Molecular Structure; D011728:Pyridones / Q000737:chemistry*
chemical_list: D003998:Dicarboxylic Acids; D004952:Esters; D011728:Pyridones; C042436:chelidamic acid