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.