Conformations and lipophilicity profiles of some cyclic β-(1→3)- and β-(1→6)-linked oligogalactofuranosides

Conformations of methyl 2,5-di-O-acetyl-beta-D-glucofuranosidurono-6,3-lactone and 1,2,5-tri-O-acetyl-beta-D-glucofuranurono-6,3-lactone in the crystal structure and in solution

The single-crystal X-ray diffraction and high-resolution 1H and 13C NMR spectral data for methyl 2,5-di-O-acetyl-beta-D-glucofuranosidurono-6,3-lactone and 1,2,5-tri-O-acetyl-beta-D-glucofuranurono-6,3-lactone are reported. The lactones were synthesized as byproducts of reactions carried out to obtain methyl 1,2,3,4-tetra-O-acetyl-D-glucopyranuronate. The conformations of these lactones in the crystal structure and in solution are discussed. A 1T2-like conformation was found to be the preferred form for these lactones in both the crystal lattice and in solution.

Conformational studies of methyl 3-O-methyl-alpha-D-arabinofuranoside: an approach for studying the conformation of furanose rings

A computational method for probing furanose conformation has been developed using a methylated monosaccharide derivative 1. First, a large library of conformers was generated by a systematic pseudo Monte Carlo search followed by optimization with the AMBER molecular mechanics force field. A subset of these conformers was then subjected to ab initio and density functional theory calculations in both the gas and aqueous phases. These calculations indicate that entropic contributions to the Gibbs free energy are important determinants of the Boltzmann distribution for the conformational preferences of 1 in the gas phase. The results obtained at each level of theory are discussed and compared with the experimentally determined conformer distribution from NMR studies in aqueous solution. In addition, the ability of each level of theory to reproduce the experimentally measured 1H-1H coupling constants in 1 is discussed. Empirical Karplus equations and density functional theory methods were used to determine average 3J(H1,H2), 3J(H2,H3), and 3J(H3,H4) for each level of theory. On the basis of this comparison, consideration of solvation with the MN-GSM model provided good agreement with the experimental data.

Probing furanose ring conformation by gas-phase computational methods: energy profile and structural parameters in methyl beta-D-arabinofuranoside as a function of ring conformation

The potential energy surface of methyl beta-D-arabinofuranoside (3) has been studied by ab initio molecular orbital (HF/6-31G) and density functional theory (B3LYP/6-31G) calculations via minimization of the 10 possible envelope conformers. The partial potential energy surface identified that the global minimum and lowest energy northern conformer was E(2). In the HF calculations, (2)E was the most stable southern conformer, while the density functional theory methods identified (4)E as the local minimum in this hemisphere. Additional calculations at higher levels of theory showed that the B3LYP-derived energies of many of the envelope conformers of 3 are dependent upon the basis set used. It has also been demonstrated that B3LYP/6-31+G//B3LYP/6-31G single point energies are essentially the same as those obtained from full geometry optimizations at the B3LYP/6-31+G level. The northern and southern minima of the B3LYP/6-31+G surface are, respectively, the E(2) and (2)E conformers. The B3LYP/6-31G geometries were used to study the relationship between ring conformation and various structural parameters including bond angles, dihedral angles, bond lengths, and interatomic distances.

Topography of the 1:1 alpha-cyclodextrin-nitromethane inclusion complex

Dissolution of alpha-cyclodextrin (alpha-CD) in 9:1 water-nitromethane smoothly generates the title compound, which crystallizes as the pentahydrate in the orthorhombic space group P2(1)2(1)2(1) with a = 9.452(4), b = 14.299(3), c = 37.380(10) A, and Z = 4. Its crystal structure analysis revealed the alpha-CD macrocycle in an unstrained conformation stabilized through a ring of O-2...O-3' hydrogen bonds between five of the six adjacent glucose residues. The nitromethane is located in the alpha-CD cavity in an orientation parallel to the plane of the macrocycle, and assumes two sites of equal population with the nitro group in excessive thermal motion; the guest is held by van der Waals contacts and C-H...O-type hydrogen bonds to the pyranose H-3 and H-5 protons. The packing of the macrocycles in the crystal lattice is of cage herringbone-type with an extensive intra- and intermolecular hydrogen bonding network. The ready formation of a nitromethane inclusion complex in aqueous nitromethane, and the subtleties of its molecular structure amply demonstrate the ease with which water is expelled from the alpha-CD cavity by a more hydrophobic co-solvent.