Conformations of the d-glucarolactones and d-glucaric acid in solution

The conformations of d-glucaric acid (1), d-glucaro-1,4-lactone (2), d-glucaro-6,3-lactone (3), and d-glucaro-1,4:6,3-dilactone (4) in solution were investigated by 1H-n.m.r. and 13C-p.F.t., n.m.r. spectroscopy. The solvents used were deuterium oxide, methanol-d4, and dimethyl sulfoxide-d6, and praseodymium chloride was employed as a lanthanide shift-reagent. For 2, it was found that the conformational equilibrium 3E(d) E3(d) exists in solution, and that the OH-5 group tends to occupy the position over the lactone ring in the favored E3(d),gg conformation. The n.m.r. data for 3 indicated that the conformational equilibrium is shifted in favor of the 4E(d) E4(d),gt conformation in solution. The dienvelope conformation 3E:E4(d) was found to be the favored conformation of 4. For 1, a conformational equilibrium between one planar, zigzag form and two sickle forms was indicated by the n.m.r. data observed. 13C-N.m.r. spectroscopy proved to be a convenient method for monitoring the lactonization of 1, and the hydrolysis of its lactones. Lactones other than 2–4 were not found in solutions prepared from 1–4, either during their mutarotation or after equilibration at 30°.