Conformational analyses of 2,3-dihydroxypropanoic acid as a function of solvent and ionization state as determined by NMR spectroscopy

✍ Scribed by Michael D. Drake; Alex K. Harsha; Sergei Terterov; John D. Roberts

• Publisher: John Wiley and Sons
• Year: 2006
• Tongue: English
• Weight: 246 KB
• Volume: 44
• Category: Article
• ISSN: 0749-1581
• DOI: 10.1002/mrc.1758

✦ Synopsis

Abstract

Vicinal ^1^H^1^H coupling constants were used to determine the conformational preferences of 2,3‐dihydroxypropanoic acid (1) (DL‐glyceric acid) in various solvents and its different carboxyl ionization states. The stereospecific assignments of J~12~ and J~13~ were confirmed through the point‐group substitution of the C‐3 hydrogen with deuterium, yielding rac‐(2__SR__,3__RS__)‐[3‐^2^H]‐1, and the observation of only J~13~ in the ^1^H NMR spectra. While hydrogen bonding and steric strain may be expected to drive the conformational equilibrium, their role is overshadowed by a profound gauche effect between the vicinal hydroxyl groups that mimics other substituted ethanes, such as 1,2‐ethanediol and 1,2‐difluoroethane. At low pH, the conformational equilibrium is heavily weighted toward the gauche‐hydroxyl rotamers with a range of 81% in DMSO‐d~6~ to 92% in tert‐butyl alcohol‐d~10~. At high pH, the equilibrium exhibits a larger dependence upon the polarity and solvating capability of the medium, although the gauche effect still dominates in D~2~O, 1,4‐dioxane‐d~8~, methanol‐d~4~, and ethanol‐d~6~ (96, 89, 85, and 83% gauche‐hydroxyls respectively). The observed preference for the gauche‐hydroxyl rotamers is believed to stem primarily from hyperconjugative σ~CH~ → σ*~COH~ interactions. Copyright © 2006 John Wiley & Sons, Ltd.