RESUMO
The title compound, C22H33NO12·CHCl3, was obtained as a product of a double aza-Michael addition of hydroxyl-amine on a Chiron with a known absolute configuration. The enanti-opure compound crystallized as a chloro-form solvate, in space group P1, and diffraction data were collected at room temperature with Ag Kα radiation. The Flack parameter refined to x = -0.01â (16); however, the Flack and Watkin 2AD plot clearly shows that differences between Friedel opposites (the D component of the plot) do not carry any reliable information about resonant scattering of Cl atoms, and are rather dominated by random and systematic errors. The RD factor calculated using 1941 acentric Friedel pairs is RD = 0.995. On the other hand, the 2A component of the plot, related to average intensities of Friedel pairs, shows that data are of good quality (RA = 0.069). This example illustrates that while using Ag Kα radiation (λ = 0.56083â Å), scatterers heavier than Cl should be present in a chiral crystal in order to determine confidently the absolute structure of the crystal.
RESUMO
The crystal structure of triphenylmethanol, C19H16O, has been redetermined using data collected at 295 and 153â K, and is compared to the model published by Ferguson et al. over 25 years ago [Ferguson et al. (1992). Acta Cryst. C48, 1272-1275] and that published by Serrano-González et al., using neutron and X-ray diffraction data [Serrano-González et al. (1999). J. Phys. Chem. B, 103, 6215-6223]. As predicted by these authors, the hydroxy groups are involved in weak intermolecular hydrogen bonds in the crystal, forming tetrahedral tetramers based on the two independent molecules in the asymmetric unit, one of which is placed on the threefold symmetry axis of the R-3 space group. However, the reliable determination of the hydroxy H-atom positions is difficult to achieve, for two reasons. Firstly, a positional disorder affects the full asymmetric unit, which is split over two sets of positions, with occupancy factors of ca 0.74 and 0.26. Secondly, all hydroxy H atoms are further disordered, either by symmetry, or through a positional disorder in the case of parts placed in general positions. We show that the correct description of the hydrogen-bonding scheme is possible only if diffraction data are collected at low temperature. The prochiral character of the hydrogen-bonded tetrameric supramolecular clusters leads to enantiomorphic three-dimensional graphs in each tetramer. The crystal is thus a racemic mixture of supS and supR motifs, consistent with the centrosymmetric nature of the R-3 space group.
RESUMO
The title compound, C(25)H(29)NO(4)S(2) 0.5CH(2)Cl(2), was obtained as a racemate. The pyridine and phenyl rings are arranged face-to-face, giving a weak intra-molecular π-π inter-action [centroid-centroid separation = 3.759â (3)â Å]. These inter-actions are extended inter-molecularly, forming chains of stacked rings along [001] with separations of 3.859â (3) and 3.916â (3)â Å. The solvent used for crystallization, CH(2)Cl(2), is located in voids between the chains of mol-ecules, with a site occupancy of 0.5.
RESUMO
THE TITLE COMPOUND [SYSTEMATIC NAME: (R)-2-trichloro-methyl-3a,3b,7a,8a-tetra-hydro-5H-pyrano[2',3':4,5]furano[2,3-d][1,3]dioxol-5-one], C(9)H(7)Cl(3)O(5), a triyclic system that contains a central α-d-furan-ose ring cis-fused with a dioxolane ring as well as a δ-lactone ring, exhibits a twisted conformation. The CCl(3) group has an axial orientation. The furan-ose ring approximates an envelope conformation due to the α,ß-unsaturated lactone functionality. The asymmetric unit contains two independent mol-ecules with almost identical geometries.
