Synthesis and crystal structures of 3,6-di­hydroxy­picolinic 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), C(6)H(5)NO(4) (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(+)·C(6)H(3)NO(7)S(2−)·H(2)O (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 π–π 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.