Synthesis and crystal structures of 3,6-dihydroxy-picolinic acid and its labile intermediate dipotassium 3-hydroxy-6-(sulfonatooxy)pyridine-2-carboxylate monohydrate

A simplified two-step synthesis of 3,6-dihydroxypicolinic acid (3-hydroxy-6-oxo-1,6-dihydropyridine-2-carboxylic acid), C₆H₅NO₄ (II), an intermediate in the metabolism of picolinic acid, is described. The crystal structure of II, along with that of a labile intermediate, dipotassium 3-hydroxy-6-(sulfonatooxy)-pyridine-2-carboxylate monohydrate, 2K⁺.C₆H₃NO₇S^2-.H₂O (I), is also described. Compound I comprises a pyridine ring with carboxylate, hydroxyl (connected by an intramolecular O—H. . .O hydrogen bond), and sulfate groups at the 2-, 3-, and 6-positions, respectively, along with two potassium cations for charge balance and one water molecule of crystallization. These components are connected into a three-dimensional network by O—H. . .O hydrogen bonds arising from the water molecule, C—H. . .O interactions and p–p stacking of pyridine rings. In II, the ring nitrogen atom is protonated, with charge balance provided by the carboxylate group (i.e., a zwitterion). The intramolecular O—H. . .O hydrogen bond observed in I is preserved in II. Crystals of II have unusual space-group symmetry of type Abm2 in which extended planar networks of O—H. . .O and N—H. . .O hydrogen-bonded molecules form sheets lying parallel to the ac plane, constrained to b = 0.25 (and 0.75). The structure was refined as a 50:50 inversion twin. A minor disorder component was modeled by reflection of the major component across a mirror plane perpendicular to c.