Pseudopolymorphs of chelidamic acid and its dimethyl ester

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 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, 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—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.