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

A two-step synthesis of 3,6-dihydroxypicolinic acid, its crystal structure and that of a labile intermediate, are described.

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 + ÁC 6 H 3 NO 7 S 2À ÁH 2 O (I), is also described. Compound I comprises a pyridine ring with carboxylate, hydroxyl (connected by an intramolecular O-HÁ Á ÁO 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-HÁ Á ÁO hydrogen bonds arising from the water molecule, C-HÁ Á ÁO interactions andstacking 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-HÁ Á ÁO 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-HÁ Á ÁO and N-HÁ Á ÁO 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.
We report here a two-step synthesis also starting with 3-hydroxypicolinic acid by an Elbs oxidation (Behrman, 1988;2021) and crystal structures of both the intermediate sulfate ester (I) and of 3,6-dihydroxypicolinic acid (II). We considered two routes, as shown in Fig. 1. Both give the desired product but we chose the pathway from 3-hydroxypicolinic acid because in the first step, the dipotassium salt of the 6-sulfate ester precipitates from the mixture in an almost pure state. ISSN 2056-9890 This sulfate ester is extraordinarily sensitive to acid-catalyzed hydrolysis; acidification at room temperature (RT) gives complete hydrolysis in a few minutes. The isomeric 6-oxopicolinic acid-3-sulfate is much more stable although subject to anchimeric assistance (Benkovic, 1966); it is not completely hydrolyzed after 22 h, RT, pH 2. Nantka-Namirski & Rykowski (1972a,b) used boiling 20% sulfuric acid for four hours to effect the hydrolyses of the 5-sulfate esters of two dihydroxynicotinic acids. For the rapid hydrolysis of pyridyl-4sulfate see Jerfy & Roy (1970) and Goren & Kochansky (1973) for the effects of impurities. A reasonable representation of the mechanism of the hydrolysis for the 4-sulfate is shown in Fig. 2a. The 2-sulfate should behave similarly (Fig. 2b). Jerfy & Roy (1970) also showed that sulfation of 2-pyridone gives the sulfamate, so that it has not yet been possible to prepare the pyridyl-2-sulfate for comparative purposes. However, a route to 5-hydroxypyridine-2-sulfate together with the isomeric 4-and 6-sulfate esters is known (Behrman & Pitt, 1958). Examination of these mixed esters by electrophoresis shows that all three are rapidly hydrolyzed under acid catalysis, as predicted by the model. The reaction between potassium peroxydisulfate and 3-hydroxypicolinic acid was carried out as usual in KOH solution except that if there is excess peroxydisulfate, the sulfate ester and the peroxide precipitate from the reaction mixture together: to avoid this the persulfate was used as the limiting reagent. The ester was crystallized from water and 3,6-dihydroxypicolinic acid obtained by hydrolysis.

Structural commentary
The asymmetric unit in I (Fig. 3) contains a single pyridine ring with a carboxylate group at the 2-position, a hydroxyl group at the 3-position, and a sulfate group attached to the 6-position. Charge balance is provided by a pair of K + cations. There is also a single water molecule of crystallization present. The carboxylate C-O distances are 1.2510 (16) and 1.2758 (16) Å for C7-O3 and C7-O4, respectively. The longer of these is part of an S(6) intramolecular hydrogen-bonded ring with the hydroxyl group. In the sulfate group, the oxygen atom bound at the 6-position [C6-O2 = 1.3968 (14) Å ] is longer [S1-O2 = 1.6343 (9)     An ellipsoid (50% probability) plot of the asymmetric unit of I. The intramolecular hydrogen bond is shown by the thick dashed line. Dotted lines indicate close contacts of the K + cations.
In II (Fig. 4), the nitrogen atom of the ring is protonated. Charge balance is provided by the deprotonated 2-carboxylate group [C7-O3 = 1.262 (3), C7-O4 = 1.259 (3) Å ], leading to a zwitterionic molecule. The intramolecular S(6) hydrogenbonded ring from I is preserved in II. The 6-position of the ring is occupied by a second hydroxyl group. As discussed in more detail in section 6 (Refinement), there is a small minor disorder component [refined occupancy 4.7 (3)%] and probable inversion twinning.

Supramolecular features
In the packing of I, strong O w -HÁ Á ÁO (w = water) hydrogen bonds exist in which the water molecule acts as a linker between c-glide-related anions. The water oxygen is also coordinated to the K + cations, both of which are seven coordinate. Around K1, coordination distances range from 2.7376 (9)-2.9102 (10) Å (for KÁ Á ÁO) and 2.7869 (11) Å (K1Á Á ÁN). For K2, coordination distances range from 2.6769 (9) to 2.9525 (10) Å (all KÁ Á ÁO). The coordination geometry about each K + cation is very roughly pentagonal bipyramidal, with K1 much more distorted than K2. As each K + cation is coordinated to the water molecule, the KO 6 N and KO 7 polyhedra augment the extended chains that propagate parallel to c (Fig. 5, Table 1). In addition, there are weaker C-HÁ Á ÁO contacts: pairs of C5-H5Á Á ÁO6 ii interactions form R 2 2 (12) inversion-related dimers and C4-H4Á Á ÁO5 i contacts link molecules via c-glide symmetry (symmetry codes as per Table 1). This in turn joins c-glide-related pyridine rings into an extendedstack along the c-axis direction. Adjacent rings within the stacks are almost parallel; the dihedral angle being 0.38 (4) with a centroid-centroid distance of 3.698 (1) Å . These columns of hydrogen-bonded andstacked molecules are interlinked by the aforementioned chains of K + cations and water molecules into a threedimensional network (Fig. 5).

Figure 4
An ellipsoid (50% probability) plot of the asymmetric unit of II with the intramolecular hydrogen bond shown as a thick dashed line.

Database survey
A search of the Cambridge Structure Database (CSD v5.42, Nov. 2020; Groom et al., 2016) on a fragment composed of 3-picolinic acid with 'any non-H' at the 6-position gave only seven hits. None of these have much in common with I or II, but the most similar were AGUMEV (ammonium 2,4,6-tricarboxypyridine-3-olate monohydrate; Li et al., 2010) and MAFTEU (6-chloro-3-trifluoromethoxy pyridine-2-carboxylic acid; Manteau et al., 2010) in that their main components consist of a single pyridine ring with a carboxylic acid group adjacent to the ring nitrogen and an oxygen (O À in AGUMEV; O-CF 3 in MAFTEU) at the 3-position. Space group Abm2 is not common, with only 62 entries listed in v5.42 of the CSD. Excluding polymers, entries flagged with known errors, and those without deposited coordinates left only 47 hits, i.e. <0.005% of known structures. Of these, only 29 have R 1 5% and none form planar networks in a manner similar to II. By any measure, the crystal structure of II is unusual.
3,6-Dihydroxypicolinic acid (II): The crude sulfate (I, 150 mg), was suspended in 2 ml of water and then heated to about 313 K to dissolve it. HCl was then added to about pH 2. A heavy precipitate formed immediately. After cooling, the colorless solid was filtered and washed with cold water to yield 50-60 mg of the product (yield 67-80%), R p = 1. The proton NMR spectrum of II agreed with that reported by Qiu et al. (2019) except that our spectrum was taken in D 2 O, 600 MHz, 6.69 (d, J = 9.65 Hz) and 7.45 (d, J = 9.65 Hz). These are shifted because of the solvent difference from Qiu et al. (2019), but the couplings are the same, as is the difference between the two resonances ( 0.76). 50 mg of II was crystallized from 11 ml of hot water under argon to form 35 mg of crystals. Analytical results show that the precipitate is very nearly as clean as the crystals. Analysis (%): calculated for C 6 H 5 NO 4 : C, 46.46;H, 3.25;N, 9.03. Found (crystals), C, 46.22;H, 3.20;N, 9.07. (precipitate), C, 45.65;H, 3.16;N, 9.10. IR: Nujol, 3120, 1614, 1540, 1360, 1269, 1100, 621 cm À1 . Later, larger crystals were obtained using 88% formic acid as solvent. Upon heating, II carbonizes above 473 K without melting. UV (in water): max , (nm), " (l mol À1 cm À1 ) at pH 7: 346, 5970;243, 4350;221.5, 9450. Shukla & Kaul (1973) reported the pHdependence of the spectrum. [See also Qiu et al. (2019), but on p. S2, line 3, read '240' for '360 '.] 6. Data collection, structure solution and refinement The crystals were mounted using polyisobutene oil on the tip of fine glass fibers, which were fastened in copper mounting pins with electrical solder. Crystals of I were placed directly into the cold gas stream of a liquid-N 2 based cryostat, while crystals of II were handled using methods developed for macromolecular cryocrystallography (Parkin & Hope, 1998b). Diffraction data were collected with the crystals at 90 K. Crystal data, data collection and refinement details are summarized in Table 3. In II, a small minor disorder component was apparent in a difference map. It was modeled by reflection of the major component across a mirror plane perpendicular to its c axis, with coordinates related by the mirror plane and constrained by mapping its coordinates to the major component via SHELXL  The Hooft (Hooft et al., 2008) and Parsons (Parsons et al., 2013) parameters [y = 0.63 (7); z = 0.62 (10), respectively], however, each gave a much stronger suggestion of twinning by inversion. Since the space group itself has a twofold parallel to its c axis, this results in a mirror operation (i.e. a twofold rotation combined with inversion). Thus, the twinning and disorder in II may effectively be treated by the same operation, not unlike that in uric acid dihydrate (Parkin & Hope, 1998a;Parkin, 2000). To ensure satisfactory refinement of disorder, constraints (SHELXL command EADP) were used to equalize displacement parameters of superimposed groups. Full occupancy (and major component for II) hydrogen atoms were found in difference-Fourier maps. Carbon-bound hydrogen atoms were included using riding models with constrained distances set to 0.95 Å (Csp 2 H). In I, the water H atoms were refined but subject to distance and angle-distance restraints, while the hydroxyl H atom was refined freely. In II, for O-H and N-H groups, riding models that allowed the bond distance to refine were used. U iso (H) parameters were set to values of either 1.2U eq or 1.5U eq (OH only) of the attached atom. The structures were validated using PLATON and checkCIF (Spek, 2020). For both structures, data collection: APEX3 (Bruker, 2016); cell refinement: APEX3 (Bruker, 2016); data reduction: APEX3 (Bruker, 2016); program(s) used to solve structure: SHELXT (Sheldrick, 2015a); program(s) used to refine structure: SHELXL2018/3 (Sheldrick, 2015b); molecular graphics: XP in SHELXTL (Sheldrick, 2008); software used to prepare material for publication: SHELX (Sheldrick, 2008) and CIFFIX (Parkin, 2013).  (12) Special details Experimental. The crystal was mounted using polyisobutene oil on the tip of a fine glass fibre, which was fastened in a copper mounting pin with electrical solder. It was placed directly into the cold gas stream of a liquid-nitrogen based cryostat (Hope, 1994;Parkin & Hope, 1998). Diffraction data were collected with the crystal at 90K, which is standard practice in this laboratory for the majority of flash-cooled crystals. Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes. Refinement. Refinement progress was checked using Platon (Spek, 2020) and by an R-tensor (Parkin, 2000 (2) O7-S1-O2-C6 55.28 (10) K1 ii -S1-O7-K2 142.06 (18) O6-S1-O2-C6 175.77 (9) C6-N1-C2-C3 −0.18 (17) O5-S1-O2-C6 −66.14 (10) K1 ii -N1-C2-C3 −161.62 (9) K2 vii -S1-O2-C6 167.38 (11) C6-N1-C2-C7 −179.2 (1) K2 ix -S1-O2-C6 −144.57 (8 where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max = 0.024 Δρ max = 0.20 e Å −3 Δρ min = −0.25 e Å −3 Absolute structure: Refined as a perfect inversion twin Absolute structure parameter: 0.5

Special details
Experimental. The crystal was mounted using polyisobutene oil on the tip of a fine glass fibre, which was fastened in a copper mounting pin with electrical solder. It was placed directly into the cold gas stream of a liquid-nitrogen based cryostat (Hope, 1994;Parkin & Hope, 1998). Diffraction data were collected with the crystal at 90K, which is standard practice in this laboratory for the majority of flash-cooled crystals. Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes. Refinement. Refined as a two-component perfect inversion twin.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq Occ.