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.
Keywords: crystal structure; hydrogen bonding; low temperature; pyruvic; zymonic.