Absolute Structural Configuration Research Articles

Hypoxylonoids A−G: Isopimarane diterpene glycosides from Xylaria hypoxylon

Seven undescribed isopimarane diterpene glycosides hypoxylonoids A−G, along with five known analogues were obtained from the fungus Xylaria hypoxylon. The structures and absolute configurations of hypoxylonoids A−G were confirmed by extensive spectroscopic and single-crystal X-ray diffraction analyses. Among these compounds, the γ-lactone moiety formed between C-19 and C-6 of hypoxylonoid A; the 1,2-methyl shift of Me-18 of hypoxylonoids B and E; and the decarboxylation of C-19 of hypoxylonoid E, make them outstanding from the isopimarane family. Single crystal X-ray diffraction analyses of hypoxylonoids A, C, F, and 15-hydroxy-16-α-D-mannopyranosyloxyisopimar-7-en-19-oic acid was performed to determine their absolute structural configuration.

A Novel 8-Carbon Keto Saccharide Backbone from Roman Chamomile (Chamaemelum nobile L.)

Chamaemelum nobile (L.) Allioni, (syn. Anthemis nobilis L. or Chamomilla nobilis G.) is a perennial herb of the Asteraceae family, with the common name Roman chamomile. The plant has been widely used as household medicine for the treatment of a variety of health conditions [1]. The flowering tops of the plant are used to make teas, liquid extracts, capsules, or tablets sold as dietary supplements. A phytochemical investigation to explore the unknown constituents in the flowers of C. nobile led us to isolate and identify an 8-carbon keto saccharide acid and six of its derivatives. The structure of this 8-carbon sugar backbone was elucidated as β-3-deoxy-L-manno-2-octulopyranosonic acid, and its absolute structural configuration was completely assigned using 1D and 2D NMR techniques. This 8-carbon L-sugar skeleton has never been reported or isolated from a natural source. More interestingly, its D-isomer, namely 2-keto-3-deoxy-D-manno-octulosonic acid (KDO), is an essential component of the outer-cell membrane lipopolysaccharide (LPS) of gram-negative bacteria and plays critical roles in the biological system [2]. Inhibitors of KDO biosynthesis have the potential to be a new class of antibacterial agents [3].

Electronic Ferroelectricity in a Molecular Crystal with Large Polarization Directing Antiparallel to Ionic Displacement

Ferroelectric polarization of 6.3 μC cm(-2) is induced by the neutral-to-ionic transition, upon which nonpolar molecules of electron donor tetrathiafulvalene (TTF) and acceptor p-chloranil (CA) are incompletely ionized to ±0.60e and dimerized along the molecular stacking chain. We find that the ferroelectric properties are governed by intermolecular charge transfer rather than simple displacement of static point charge on molecules. The observed polarization and poling effect on the absolute structural configuration can be interpreted in terms of electronic ferroelectricity, which not only exhibits antiparallel polarity to the ionic displacement but also enhances the polarization more than 20 times that of the point-charge model.

Crystallization of an Achiral Cyclohexanone Ethylene Ketal in Enantiomorphs and Determination of the Absolute Structure

The achiral 4-methoxy-4-(p-methoxyphenyl)-cyclohexanone ethylene ketal (1) resolves spontaneously. The crystal structure is solved in chiral spatial group P2(1). Because compound 1 is composed of only light atoms (C, H, O) it is not possible to determine its absolute structure configuration. An efficient procedure for the absolute structure configuration determination of flexible molecules containing only light atoms is proposed, based on the combination of X-ray diffraction, solid-state VCD, and DFT calculations.

Structural Investigation of P4W24O80: A New Monophosphate Tungsten Bronze

The crystallographic study of the tungsten phosphate bronze P4W24O80 was performed from three-dimensional single-crystal X-ray diffraction data and electron microscopy. This compound crystallizes in the orthorhombic system with unit-cell dimensions a = 5.312 (1), b = 6.5557 (8), c = 42.196 (8) Å and space group P212121. The crystal structure was solved by direct methods and Fourier techniques, and refined to the reliability factor R = 0.0284 (wR = 0.0272). Its determination confirms that it belongs to the monophosphate tungsten bronze family of the general formula (PO2)4(WO3)2m with the value m = 12. Large empty cages surrounded by 18 O atoms are built up of eight WO6 octahedra and four PO4 tetrahedra sharing corners. They are located between two WO3-type slabs forming pentagonal-shaped tunnels running in the a direction. Electron microscopy investigations confirm that the studied crystal does not imply modulation phenomena when other crystals of the same composition exhibit satellite reflections with a modulation vector q* involving a doubling of a. The observations also reveal the existence of a monoclinic form of the m = 12 compound, which is a regular intergrowth of m = 11 and m = 13 members in a similar way to the m = 5 member of the series where the same feature has already been observed. On each edge of a WO3-type slab, a large variation (from 1.73 to 2.09 Å) of the six W—O distances within the WO6 octahedron is noted, which yields an oxidation state of W near 6, whereas for the WO6 octahedra located in the middle part of the slab the six W—O distances are gathered about their mean value (1.92 Å), which involves a more important electronic delocalization. The thermal motion of the W atoms is described. The absolute structural configuration is tested on the basis of some calculated structure factors, which are more sensitive to the x, y, z → −x, −y, −z change of atomic positions.

The Crystal Structure and Optical Activity of Tellurium

The element tellurium has a crystal structure made up of spiral chains of bonded atoms packed in a hexagonal array. Its symmetry leads to the existence of enantiomorphic forms containing spirals of opposite handedness, the right-handed one belonging to space group P3121 and the other to P3221, which have opposite optical rotatory powers. The normal methods of crystal structure determination cannot distinguish between the enantiomorphs, nor is this feasible using anomalous dispersion unless there is sufficient asphericity in the tellurium electron density due to bonding. Such asphericity also gives rise to small but measurable differences from unity in the flipping ratios for polarized neutron scattering due to the polarization dependence of the Schwinger scattering. This effect is easier to measure than is the intensity difference between Bijvoet pairs and it has been used to determine the absolute structural configuration that corresponds to a particular sense of optical rotation in a tellurium single crystal. The plane of polarization of the transmitted light rotates in the same sense as the bonded atoms in the spiral chains. This observation disagrees with a previous theoretical calculation based on the single polarizable ion model.