Synthesis and crystal structures of 3,6-di-hydroxy-picolinic acid and its labile inter-mediate dipotassium...

A simplified two-step synthesis of 3,6-di-hydroxy-picolinic acid (3-hy-droxy-6-oxo-1,6-di-hydro-pyridine-2-carb-oxy-lic acid), C₆H₅NO₄ (II), an inter-mediate in the metabolism of picolinic acid, is described. The crystal structure of II, along with that of a labile inter-mediate, dipotassium 3-hy-droxy-6-(sulfonato-oxy)pyridine-2-carboxyl-ate monohydrate, 2K⁺·C₆H₃NO₇S^2-·H₂O (I), is also described. Compound I comprises a pyridine ring with carboxyl-ate, hydroxyl (connected by an intra-molecular 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 mol-ecule of crystallization. These components are connected into a three-dimensional network by O-H⋯O hydrogen bonds arising from the water mol-ecule, C-H⋯O inter-actions and π-π stacking of pyridine rings. In II, the ring nitro-gen atom is protonated, with charge balance provided by the carboxyl-ate group (i.e., a zwitterion). The intra-molecular 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 mol-ecules 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.