Heger Dominik, Eugene Alexis J, Parkin Sean R, Guzman Marcelo I
Department of Chemistry, University of Kentucky, Lexington, Kentucky 40506, USA.
Department of Chemistry, Faculty of Science, Masaryk University, Kamenice 5, 625 00 Brno, Czech Republic.
Acta Crystallogr E Crystallogr Commun. 2019 May 24;75(Pt 6):858-862. doi: 10.1107/S2056989019007072. eCollection 2019 Jun 1.
The structure of zymonic acid (systematic name: 4-hy-droxy-2-methyl-5-oxo-2,5-di-hydro-furan-2-carb-oxy-lic acid), CHO, 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), CHO, at low temperature (90 K) and with increased precision, is also presented [for the previous structure, see: Harata (1977 ▸). B, 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 (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 2/, 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 2-screw axis, and generate an (9) motif. These hydrogen-bonding inter-actions propagate to form extended pleated sheets in the plane. Stacking of these zigzag sheets along involves only van der Waals contacts. In pyruvic acid, inversion-related mol-ecules are linked into (8) dimers, with van der Waals inter-actions between dimers as the only other inter-molecular contacts.
酒酮酸（系统命名：4-羟基-2-甲基-5-氧代-2,5-二氢呋喃-2-羧酸），化学式为CHO，其结构此前一直未能通过晶体学测定确定，本文首次给出其结构。它由仲丙酮酸分子内缩合形成，仲丙酮酸是丙酮酸羟醛缩合的产物。本文还给出了丙酮酸（系统命名：2-氧代丙酸），化学式为CHO，在低温（90 K）下且精度提高后的晶体结构重新测定结果[关于之前的结构，见：Harata (1977 ▸). B, 210 - 212]。在酒酮酸中，羟基内酯环接近平面（均方根偏差 = 0.0108 Å），环与由甲基和羧酸碳原子与环相连的键所形成平面之间的二面角为88.68 (7)°。羧酸基团相对于环的扭转角为12.04 (16)°。丙酮酸分子几乎是平面的，羧酸基团与甲基酮基团之间的二面角为3.95 (6)°。两种晶体结构中的分子间相互作用均以氢键为主导。常见的（8）氢键模式在两种结构中都将相邻分子上的羧酸基团连接起来。在酒酮酸中，这导致围绕空间群2/的一个晶体学二重轴形成二聚体，这使得羧酸基团精确地以50:50的比例无序排列，从而使羰基和羟基基团混乱，并使C - O键长明显平均化[1.2568 (16) 和1.2602 (16) Å]。酒酮酸中的其他氢键（O - H⋯O和弱C - H⋯O），跨越一个2 - 螺旋轴连接分子，并形成一个（9）模式。这些氢键相互作用传播形成平面内的延伸褶皱片层。这些锯齿状片层沿着方向堆叠仅涉及范德华接触。在丙酮酸中，与反演相关的分子连接成（8）二聚体，二聚体之间的范德华相互作用是唯一的其他分子间接触。
