Hydrogen bonds and local symmetry in the crystal structure of gibbsite

First‐principles quantum mechanical calculations of NMR chemical shifts and quadrupolar parameters have been carried out to assign the ²⁷Al MAS NMR resonances in gibbsite. The ²⁷Al NMR spectrum shows two signals for octahedral aluminum revealing two aluminum sites coordinated by six hydroxyl groups each, although the crystallographic positions of the two Al sites show little difference. The presence of two distinguished ²⁷Al NMR resonances characterized by rather similar chemical shifts but quadrupolar coupling constants differing by roughly a factor of two is explained by different character of the hydrogen bonds, in which the hydroxyls forming the corresponding octahedron around each aluminum site, are involved. The Al‐I site characterized by a C_Q = 4.6 MHz is surrounded by OH? groups participating in four intralayer and two interlayer hydrogen bonds, while the Al‐II site with the smaller quadrupolar constant (2.2 MHz) is coordinated by hydroxides, of which two point toward the intralayer cavities and four OH‐bonds are aligned toward the interlayer gallery. In high‐resolution solid‐state ¹H CRAMPS (combination of rotation and multiple‐pulse spectroscopy) four signals with an intensity ratio of 1:2:2:1 are resolved which allow to distinguish six nonequivalent hydrogen sites reported in the gibbsite crystal structure and to ascribe them to two types of structural OH groups associated with intralayer and interlayer hydrogen bonds. This study can be applied to characterize the gibbsite‐like layer—intergallery interactions associated with hydrogen bonding in the more complex systems, such as synthetic aluminum layered double hydroxides. Copyright © 2010 John Wiley & Sons, Ltd.     

TiF4-mediated biomimetic alkylation-cyclization cascade reaction of 2-trimethylsilylmethyl-1,5-dienes with aldehydes

TiF₄ has proven to be the Lewis acid of choice for promoting the biomimetic addition of 2-trimethylsilylmethyl-1,5-dienes to aliphatic aldehydes with concomitant cyclization. 1,3-cis-Disubstituted methylenecyclohexanes are thus produced in good yields and high diastereoselectivity. The reaction appears to proceed via a highly concerted mechanism involving a chair-like transition state.     

Exact parametric analytic solutions of the elastica ODEs for bars including effects of the transverse deformation

We succeed in constructing exact parametric analytic solutions for the non-linear ordinary differential equations governing the elastica response of a cantilever due to a generalized end loading by taking into account the effects of transverse deformation. Application to the case of the eccentric buckling of a cantilever by taking into account the above influences is developed.     

A cationic cysteine-hydrazide as an enrichment tool for the mass spectrometric characterization of bacterial free oligosaccharides

In Campylobacterales and related ε-proteobacteria with N-linked glycosylation (NLG) pathways, free oligosaccharides (fOS) are released into the periplasmic space from lipid-linked precursors by the bacterial oligosaccharyltransferase (PglB). This hydrolysis results in the same molecular structure as the oligosaccharide that is transferred to a protein to be glycosylated. This allowed for the general elucidation of the fOS-branched structures and monosaccharides from a number of species using standard enrichment and mass spectrometry methods. To aid characterization of fOS, hydrazide chemistry has often been used for chemical modification of the reducing part of oligosaccharides resulting in better selectivity and sensitivity in mass spectrometry; however, the removal of the unreacted reagents used for the modification often causes the loss of the sample. Here, we develop a more robust method for fOS purification and characterize glycostructures using complementary tandem mass spectrometry (MS/MS) analysis. A cationic cysteine hydrazide derivative was synthesized to selectively isolate fOS from periplasmic fractions of bacteria. The cysteine hydrazide nicotinamide (Cyhn) probe possesses both thiol and cationic moieties. The former enables reversible conjugation to a thiol-activated solid support, while the latter improves the ionization signal during MS analysis. This enrichment was validated on the well-studied Campylobacter jejuni by identifying fOS from the periplasmic extracts. Using complementary MS/MS analysis, we approximated data of a known structure of the fOS from Campylobacter concisus. This versatile enrichment technique allows for the exploration of a diversity of protein glycosylation pathways.     

Synthesis and Biological Evaluation of Several Dephosphonated Analogues of CMP‐Neu5Ac as Inhibitors of GM3‐Synthase

Previous studies demonstrated that reducing the GM3 content in myoblasts increased the cell resistance to hypoxic stress, suggesting that a pharmacological inhibition of the GM3 synthesis could be instrumental for the development of new treatments for ischemic diseases. Herein, the synthesis of several dephosphonated CMP‐Neu5Ac congeners and their anti ‐ GM3‐synthase activity is reported. Biological activity testes revealed that some inhibitors almost completely blocked the GM3‐synthase activity in vitro and reduced the GM3 content in living embryonic kidney 293A cells, eventually activating the epidermal growth factor receptor (EGFR) signaling cascade.     

TOF MS Investigation of Nickel Oxide CVD

NiO layers were deposited by metal-organic chemical vapor deposition using bis-(ethylcyclopentadienyl) nickel (EtCp)₂Ni and oxygen or ozone. As a continuation of kinetic study of NiO MOCVD the gas-phase, transformations of (EtCp)₂Ni were studied in the temperature range of 380–830 K. Time of reactions corresponding to the residence time of the gas stream in hot zone of the reactor was about 0.1 s under conditions studied. The interaction of (EtCp)₂Ni with oxygen started at 450 K and its conversion rate reached the maximum at 700 K. The interaction of (EtCp)₂Ni with ozone started at 400 K and its conversion rate reached the maximum at 600 K. Transformations of the gas phase with the temperature in the reaction zone were studied, the model reaction schemes illustrating (EtCp)₂Ni transformations in the reaction systems containing oxygen and ozone have developed. In the reaction system (EtCp)₂Ni–O₂–Ar the main gas-phase products at 380–500 K were CO, CO₂, HCO, C₂H₅OH, CpCOOH, and CpO. Formation of the C₂H₂O, C₃H₄O, and C₅H₈O was found at 630–830 K. The same gas-phase species, (C₄H₃O)₂Ni and dialdehydes was formed in the reaction system (EtCp)₂Ni–O₃–O₂–Ar.

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Nucleophile-mediated ring expansion of 4-chloromethyl- and 4-mesyloxymethyl-5-tosyl-1,2,3,4-tetrahydropyrimidin-2-ones to 6-tosyl-2,3,4,5-tetrahydro-1H-1,3-diazepin-2-ones: effect of the leaving group and the substituent at C6

A five-step synthesis of 4-chloromethyl- and 4-mesyloxymethyl-5-tosyl-1,2,3,4-tetrahydropyrimidin-2-ones has been developed. The reaction of N-[(2-benzoyloxy-1-tosyl)ethyl]urea with sodium enolates of α-tosylketones followed by cyclization-dehydration, and debenzoylation gave 4-hydroxymethyl-5-tosyl-1,2,3,4-tetrahydropyrimidin-2-ones, which were transformed into the 4-chloromethyl- or 4-mesyloxymethyl-derivatives. Treatment of the latter with nucleophilic reagents, such as sodium cyanide, sodium diethyl malonate, sodium thiophenolate, or potassium phthalimide, afforded the corresponding 4,7-disubstituted 6-tosyl-2,3,4,5-tetrahydro-1H-1,3-diazepin-2-ones as a result of ring expansion. The effect of the leaving group and the substitution at the position C6 on the reactivity of the pyrimidines is discussed.     

Triangular Ni2Ln and Ni2Y complexes derived from di-2-pyridyl ketone: Synthesis, structures and magnetic properties

The synthetic investigation of the Ni^II/M(NO₃)₃·6H₂O/di-2-pyridyl ketone [(py)₂CO] tertiary reaction system in EtOH has yielded triangular Ni₂M cationic complexes (M = lanthanide, Y). The reaction between Ln(NO₃)₃·6H₂O, Ni(ClO₄)₂·6H₂O, (py)₂CO and base (1:3:3:3) in EtOH under gentle heating gave the isostructural complexes [Ni₂Ln{(py)₂C(OEt)(O)}₃{(py)₂C(OH)(O)}(NO₃)(H₂O)](ClO₄)₂ (Ln = Gd, 2; Ln = Tb, 3) in high yields. The ligands (py)₂C(OEt)(O)⁻ and (py)₂C(OH)(O)⁻ are the monoanions of the hemiketal and gem-diol derivatives of (py)₂CO, respectively, formed in situ in the presence of the metal ions. The cations of 2 and 3 consist of one 8-coordinate Ln^III and two distorted octahedral Ni^II atoms in an essentially isosceles, triangular arrangement capped by a central μ₃-Ο⁻ atom of the unique 3.3011 (Harris notation) (py)₂C(OH)(O)⁻ ligand. Each metal-metal edge is bridged by the deprotonated O atom of one 2.2011 (py)₂C(OEt)(O)⁻ ligand. The isostructural complexes [Ni₂M{(py)₂C(OEt)(O)}₄(NO₃)(H₂O)]₂[M(NO₃)₅](ClO₄)₂ (M = Y, 4 ; M = Tb, 5 ; M = Dy, 6) were prepared by the 1:1 reaction of the mononuclear “metalloligand” [Ni(O₂CMe){(py)₂CO}{(py)₂C(OH)₂}](ClO₄) (1) and M(NO₃)₃·6H₂O in EtOH under mild heating in moderate to good yields. The structures of the dications of 4-6 are similar to those in 2 and 3, the only difference being the replacement of the unique 3.3011 (py)₂C(OH)(O)⁻ ligand of the latter by one 3.3011 (py)₂C(OEt)(O)⁻ group in the former. The Y^III, Tb^III and Dy^III atoms in [M(NO₃)₅]²⁻ are coordinated by five bidentate chelating nitrato groups. Characteristic IR bands of the complexes are discussed in terms of the known structures and the coordination modes of the ligands. Variable temperature, solid-state direct current magnetic susceptibility and magnetization studies were carried out on dried samples of 2-4. The data indicate ferromagnetic Ni?Ni and Ni?Gd exchange interactions, and an S_T = 11/2 ground state for 2. Complex 3 is characterized by a high-spin ground state while the ferromagnetic Ni?Ni interaction for 2 is independently supported by the study of 4. No out-of-phase, alternating current susceptibility signals have been detected for 3 that would be indicative of SMM behavior.