Acta Cryst. (2012). C68, o344-o350 [ doi:10.1107/S0108270112031691 ]
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, C7H5NO5·CH4O, (Ia), dimethylammonium chelidamate (dimethylammonium 6-carboxy-4-hydroxypyridine-2-carboxylate), C2H8N+·C7H4NO5-, (Ib), and chelidamic acid dimethyl sulfoxide monosolvate, C7H5NO5·C2H6OS, (Ic). While the zwitterionic pyridinium carboxylate in (Ia) can be explained from the pKa 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-HO hydrogen bond. Also, crystals of chelidamic acid dimethyl ester (dimethyl 4-hydroxypyridine-2,6-dicarboxylate) were obtained as a monohydrate, C9H9NO5·H2O, (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-HO 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.
Formula: C7H5NO5·CH4O, C2H8N+·C7H4NO5-, C7H5NO5·C2H6OS and C9H9NO5·H2O
Structure factor file (CIF format) (84.4 kbytes)
Structure factor file (CIF format) (190.4 kbytes)
Structure factor file (CIF format) (125.3 kbytes)
Structure factor file (CIF format) (96.1 kbytes)
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