Abstract
The structure of zymonic acid (systematic name: 4-hy-droxy-2-methyl-5-oxo-2,5-di-hydro-furan-2-carb-oxy-lic acid), C6H6O5, which had previously eluded crystallographic determination, is presented here for the first time. It forms by intra-molecular condensation of parapyruvic acid, which is the product of aldol condensation of pyruvic acid. A redetermination of the crystal structure of pyruvic acid (systematic name: 2-oxo-propanoic acid), C3H4O3, at low temperature (90 K) and with increased precision, is also presented [for the previous structure, see: Harata et al. (1977 ▸). Acta Cryst. B33, 210-212]. In zymonic acid, the hy-droxy-lactone ring is close to planar (r.m.s. deviation = 0.0108 Å) and the dihedral angle between the ring and the plane formed by the bonds of the methyl and carb-oxy-lic acid carbon atoms to the ring is 88.68 (7)°. The torsion angle of the carb-oxy-lic acid group relative to the ring is 12.04 (16)°. The pyruvic acid mol-ecule is almost planar, having a dihedral angle between the carb-oxy-lic acid and methyl-ketone groups of 3.95 (6)°. Inter-molecular inter-actions in both crystal structures are dominated by hydrogen bonding. The common R 2 2(8) hydrogen-bonding motif links carb-oxy-lic acid groups on adjacent mol-ecules in both structures. In zymonic acid, this results in dimers about a crystallographic twofold of space group C2/c, which forces the carb-oxy-lic acid group to be disordered exactly 50:50, which scrambles the carbonyl and hydroxyl groups and gives an apparent equalization of the C-O bond lengths [1.2568 (16) and 1.2602 (16) Å]. The other hydrogen bonds in zymonic acid (O-H⋯O and weak C-H⋯O), link mol-ecules across a 21-screw axis, and generate an R 2 2(9) motif. These hydrogen-bonding inter-actions propagate to form extended pleated sheets in the ab plane. Stacking of these zigzag sheets along c involves only van der Waals contacts. In pyruvic acid, inversion-related mol-ecules are linked into R 2 2(8) dimers, with van der Waals inter-actions between dimers as the only other inter-molecular contacts.
Highlights
The structure of zymonic acid, C6H6O5, which had previously eluded crystallographic determination, is presented here for the first time. It forms by intramolecular condensation of parapyruvic acid, which is the product of aldol condensation of pyruvic acid
The hydroxylactone ring is close to planar (r.m.s. deviation = 0.0108 A ) and the dihedral angle between the ring and the plane formed by the bonds of the methyl and carboxylic acid carbon atoms to the ring is 88.68 (7)
The generation of zymonic acid can proceed by condensation of parapyruvic acid, which itself forms by aldol condensation of pyruvic acid (IUPAC name 2oxopropanoic acid, C3H4O3; Bloomer et al, 1970)
Summary
The Human Metabolome Database (Wishart et al, 2007, 2009, 2013, 2018) lists the compound 4-hydroxy-2-methyl-5-oxofuran-2-carboxylic acid (C6H6O5), commonly named zymonic acid, with the metabocard HMDB0031210. As part of the process resulting in the aforementioned invention, the detailed 1H and 13C NMR spectra of pure zymonic acid have been reported (Hundshammer et al, 2017). We contribute new information to characterize zymonic acid by reporting for the first time its crystal structure, along with a low-temperature redetermination of pyruvic acid. In spite of increased precision resulting from much lower temperature (90 K versus 266 K) and data collection on modern equipment, the redetermined structure of pyruvic acid (Fig. 2) is largely unchanged from that reported by Harata et al (1977). The dihedral angle between the planes defined by atoms C1/C2/C3/O3 and C1/C2/O1/O2 is 3.95 (6) at 90.00 (2) K versus 3.5 at 266 (1) K
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