The crystal and molecular structure of 1,2-O-isopropylidene-alpha-D-xylo-pentodialdo-1,4-furanose. A dimeric form in the crystalline state.

1,2-O-isopropylidene-alpha-D-xylo-pentodialdo-1,4-furanose (1), C16H24O10, M(r) = 376.4, is orthorhombic, space group P2(1)2(1)2(1) with a = 10.3028(10), b = 11.1875(3), c = 15.7484(13) A, V = 1815.2(4) A3, Dc = 1.377 gcm-3, mu(CuK alpha) = 9.5 cm-1 and Z = 4. The structure was refined to R = 0.033 and Rw = 0.045 for 1984 observed reflections. Crystalline 1 has a dimeric cyclic acetal-hemiacetal structure, formed by self aldol condensation of two monomers. The absolute configuration at the condensation centers, C-5 and C-5', were assigned as 5R and 5S, respectively. In the dimer 1, the xylofuranose rings adopt different conformations, one is a twist 3T4, whereas the second is an envelope E4 slightly distorted towards the 3T4 conformation; their fused 1,2-O-isopropylidene rings adopt O-2E and O-2TC-6 conformations, respectively. The 1,3-dioxane ring has a distorted chair conformation with puckering parameters Q = 0.516 A, phi = 90.9, and theta = 11.0 degrees. The molecules are linked in the crystal through intermolecular hydrogen-bonding interactions that involve the two hydroxyl groups, OH-3 and OH-5', and the isopropylidene ring oxygen atoms, O-2 and O-1', as donor and acceptor, respectively.