Conformations and structure studies of sugar lactones. Part 111. The composition and conformation of d-mannurono-γ lactone in solution, and the structural analysis of its β anomer in the solid state

Complete analyses of the proton and carbon chemical-shift assignments of d-mannurono-γ lactone (1) have been achieved by 1D and 2D NMR spectral measurements. At equilibrium, the anomeric α and β forms were present in the ratio of 34:66 in D2O and 72:28 in Me2 SO-d6. The solution data indicated that the dienvelope conformation 2E:E4 to be the favored conformation of 1 in solution. The crystal structure of 1 was determined, and it showed that the crystalline form is the β anomer, a bicyclic structure, consisting of fused five-membered furanose and lactone rings, in agreement with an earlier deduction from chemical evidence. In contrast to the solution conformation, the furanose ring adopts a twist conformation lying between the 12T and 1E conformations with phase angle (P) and pseudorotation amplitude (τ,m) of -44.23° and 37.9°, respectively, whereas the lactone ring adopts an envelope E5 conformation slightly distorted towards 6T5 with a phase angle (P) of -22.3° and a pseudorotation amplitude (τm) of 18.4°. The molecules are linked in the crystal through a hydrogen- bonding network that involves all hydroxyl groups as well as the ring oxygen atoms.