Liquid Chromatographic Resolution of Racemic α-Hydroxycarboxylic Acids on Ligand Exchange Chiral Stationary Phases

Two ligand exchange chiral stationary phases (CSPs) based on (S)-leucinol derivative, sodium N-((S)-1-hydroxymethyl-3-methylbutyl)-N-undecylaminoacetate, and (R)-phenylglycinol derivative, sodium N-((R)-2-hydroxy-1-phenylethyl)-N-undecylaminoacetate, covalently bonded to silica gel have been successfully applied in the resolution of nine -hydroxycarboxylic acids. The latter was more effective than the former, the separation factors () being 1.05 to 2.12 while the resolution factors (R_S) varying from 0.18 to 5.29 on the latter. The chromatographic resolution behaviors were dependent on the type and the content of organic modifier and the content of CuSO₄ in aqueous mobile phase and the column temperature. A possible chiral recognition mechanism was also proposed based on the chromatographic resolution behaviors.     

Darstellung,Kristallstruktur und spektroskopische Eigenschaften der Clusteranionen [(Mo6Br)X]2− mit Xa = F,Cl, Br,I

Synthesis, Crystal Structure and Spectroscopic Properties of the Cluster Anions [(Mo₆Br )X ]^2? with X^a = F, Cl, Br, I The tetrabutylammonium (TBA), tetraphenylphosphonium (TPP) and tetraphenylarsonium (TPAs) salts of the octa-μ₃-bromo-hexahalogeno-octahedro-hexamolybdate(2?) anions [(Mo₆Br)X]^2? (X^a = F, Cl, Br, I) are synthesized from solutions of the free acids H₂[(Mo₆Br)X] · 8 H₂O with X^a = Cl, Br, I. The crystal structures show systematic stretchings in the Mo? Mo bond length and a slight compression of the Brⁱ₈ cube in the F^a to I^a series. The cations do not change much. The i.r. and Raman spectra show at 10 K almost constant frequencies of the (Mo₆Brⁱ₈) cluster vibrations, whereas all modes with X^a ligand contribution are characteristically shifted. The most important bands are assigned by polarization measurements and the force constants are derived from normal coordinate analysis. The ⁹⁵Mo nmr signals are shifted to lower field with increasing electronegativity of the X^a ligands. The fluorine compound shows a sharp ¹⁹F nmr singlet at ?184.5 ppm.     

Ion mobility,phase transitions,and ion transport in potassium-ammonium hexafluorozirconates

The mobility of fluoride and ammonium ions (180–480 K) in compounds K_{2? n} (NH4)_nZrF₆ (0.2 ≤ n ≤ 1.70) was investigated by ¹⁹F and ¹H NMR. Correlations have been found between the composition of the cation sublattice, the character of ion motions, and phase transition temperature in these compounds. The hightemperature modifications with n ≥ 0.85 of the compounds are characterized by translational diffusion of fluoride and ammonium ions and by uniaxial anisotropy of the ¹⁹F magnetic shielding tensor. The electrophysical characteristics of these compounds were studied in the temperature range 300–480 K.     

Synthesis of an optically active intermediate for A-ring of 19-nor- 1α,25-dihydroxyvitamin D3

An optically active intermediate 5 for A-ring of 19-nor-1a,25-dihydroxyvitamin D3 2 has been synthesized in five steps, starting from readily available, inexpensive D( )-xylose 6 with good yield.     

Electronic Structures, (d-p)π Conjugation Effects, and Spectroscopic Properties of Polyoxometalates: M6O19 2− (M=Cr, Mo, W)

The electronic structures and bonding of isopoly oxometalates M₆O₁₉ ^2– (M=Cr, Mo, W) have been investigated by using ab initio and relativistic density functional methods. We have discussed the role of the central oxygen atom and the (d-p) conjugation interactions between the metal and bridging oxygen atoms. It is found that there exist 12 three-centered two-electron (d-p-d) bonds for the three M₄(-O)₄ planar rings in M₆O₁₉ ^2– ions and these hexametalates are considered to have quasi-aromaticity. The (d-p) conjugation effects play essential role in stabilizing these cluster compounds, and the reduced (d-p) conjugation effects account for the instability of the isopoly oxochromate ion, Cr₆O₁₉ ^2–. The vibrational spectra and electronic spectra of M₆O₁₉ ^2– ions are evaluated and assigned theoretically and the calculated spectra are in fairly good agreement with the measured experimental results.     

19F-NMR-spektroskopischer Nachweis und Berechnung der statistischen Bildung der gemischten Clusteranionen [(Mo6BrCl)F]2−, n = 0 – 8

¹⁹F NMR Spectroscopic Evidence and Calculation of the Statistical Formation of Mixed Cluster Anions [(Mo₆Br Cl )F ]^2?, n = 0 – 8 The complete system of the innersphere mixed clusters (Mo₆BrCl)⁴⁺ is formed by exchange of innersphere bound Clⁱ against outersphere bound Br^a on tempering the solid [(Mo₆Cl)Br] at 500°C for 16 h. After conversion with conc. HCl into (H₃O)₂[(Mo₆BrCl)Cl] and precipitation of the outer Cl^a with AgBF₄ in ethanol, treatment with tetrabutylammonium(TBA)fluoride yields (TBA)₂ [(Mo₆BrCl)F], a mixture of 22 different species. According to the sets of chemical equivalent fluorine atoms in total 55 ¹⁹F nmr signals are expected, which are really observed in the high resolution 1D-¹⁹F-nmr spectrum. Using increments of chemical shifts, peak intensities and multiplet structures as well as the 2D-¹⁹F/¹⁹F-COSY spectrum the complete and unambiguous assignment of all resonances is achieved. From the measured integral intensities the distribution of the different compounds is determined, revealing statistical formation of the geometrical isomers.     

Reaction of a 5α-bromo-6β,19-epoxysteroid with BF3·Et2O/Ac2O: An evidence of a cyclic bromonium cation

Treatment of 3β-acetoxy-5-bromo-6β,19-epoxy-5α-androstan-17-one with Ac₂O and BF₃·OEt₂, produced the cleavage of the epoxy moiety and migration of the bromine atom to afford 3β,19-diacetoxy-6α-bromo-5-hydroxy-5β-androst-17-one in high yield.     

Evaluation of the reactivity of new activated difluoroaromatic compounds

To evaluate the reactivity of new difluroroaromatic compounds in nucleophilic substitution, the positive charges on carbon atoms of C−F bonds were calculated using the quantum-chemical semiempirical PM3 method. A correlation between the charges calculated and the chemical shifts in the¹⁹F NMR spectra was established. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 4, pp. 623–625, April, 1998.     

Monofluoroalkene-Isostere as a 19F NMR Label for the Peptide Backbone: Synthesis and Evaluation in Membrane-Bound PGLa and (KIGAKI)3

Solid-state ¹⁹F NMR is a powerful method to study the interactions of biologically active peptides with membranes. So far, in labelled peptides, the ¹⁹F-reporter group has always been installed on the side chain of an amino acid. Given the fact that monofluoroalkenes are non-hydrolyzable peptide bond mimics, we have synthesized a monofluoroalkene-based dipeptide isostere, Val-Ψ[(Z)-CF=CH]-Gly, and inserted it in the sequence of two well-studied antimicrobial peptides: PGLa and (KIGAKI)₃ are representatives of an α-helix and a β-sheet. The conformations and biological activities of these labeled peptides were studied to assess the suitability of monofluoroalkenes for ¹⁹F NMR structure analysis.