Conformations and structure studies of sugar lactones in the solid state. Part I. The molecular structure of L-rhamnono- and L-mannono-1,4-lactones

5-O-Acetyl-d-ribono-1,4-lactone

The title compound, C₇H₁₀O₆, was obtained from a regioselective enzyme-catalysed acyl­ation of d-ribono-1,4-lactone. The five-membered ring of the acyl­ated sugar shows an envelope conformation. In the crystal, the mol­ecules are linked by inter­molecular O—H⋯O hydrogen-bonds, forming a one-dimensional polymeric structure parallel to [010]. In addition, packing analysis shows stacking along the b axis.

Synthesis of 3,4-di-O-acetyl-2,5-anhydro-1,6-dideoxy-1,6-diiodo-D-mannitol. Comparison of NMR spectral results for the solid state and solution with those of the X-ray structural determination

3,4-Di-O-acetyl-2,5-anhydro-1,6-dideoxy-1,6-diiodo-D-mannitol (3) is prepared from 2,5-anhydro-D-mannitol (1) in three steps. The solution and solid-state NMR spectra of 3 indicate considerable variation in conformation. In solution, it adopts, on average, a symmetric 4T3 conformation, whereas in the solid state it adopts an asymmetric conformation as revealed by 13C NMR cross polarization and magic angle spinning techniques. A single-crystal X-ray structure analysis confirmed the asymmetric conformation of 3 in a monoclinic crystal, space group P2(1) with a = 8.9608(4), b = 8.6348(5), c = 9.6468(4) A, beta = 96.139(4) degrees, V = 742.1(1) A3, Dc = 2.085 g cm-3, mu (MoK alpha) = 4.2 mm-1, and Z = 2. The structure was refined to R = 0.039 and Rw = 0.047 for 5181 observed reflections. The furanoid ring of 3 adopts an envelope E5 conformation slightly distorted towards 4T5, with puckering parameters psi = 313.49 degrees and q = 0.37 A. The asymmetric conformation is rationalized in terms of the weak packing forces in the crystal.

Conformations and structure studies of sugar lactones. Part III. The composition and conformation of D-mannurono-gamma-lactone in solution, and the structural analysis of its beta anomer in the solid state

Complete analyses of the proton and carbon chemical-shift assignments of D-mannurono-gamma-lactone (1) have been achieved by 1D and 2D NMR spectral measurements. At equilibrium, the anomeric alpha and beta forms were present in the ratio of 34:66 in D2O and 72:28 in Me2SO-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 beta 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 2(1)T and 1E conformations with phase angle (P) and pseudorotation amplitude (tau m) of -44.23 degrees and 37.9 degrees, respectively, whereas the lactone ring adopts an envelope E5 conformation slightly distorted towards 6T5 with a phase angle (P) of -22.3 degrees and a pseudorotation amplitude (tau m) of 18.4 degrees. The molecules are linked in the crystal through a hydrogen-bonding network that involves all hydroxyl groups as well as the ring oxygen atoms.