Transition-metal catalysis in michael addition of β-dicarbonyls : Tuning of the reaction conditions

(2,4- Pentanedionato) copper(I) and bis(2,4-pentanedionato)copper(II) together with boron trifluoride etherate catalyze Michael addition of β-dicarbonyls to cyclohexenone, cyclopentenone, 2-methylcyclopentenone, and 3-thienylidenemalonaldehyde.     

Stannous triflate mediated glycosidations. A stereoselective synthesis of 2-amino 2-deoxy-β-D-glucopyranosides directly with the natural N-acetyl protecting group.

Various 2-acetamido 2-deoxy β-D-glucopyranosides have been prepared in good yield from the crystalline 2-acetamido 3,4,6-tri-O-acetyl 2-deoxy α-D-glucopyranosyl chloride using stannous triflate as promoter.     

An improved method for determination of the activity of β-galactosidase in yoghurt by high-performance liquid chromatography

Summary A new method has been developed for the HPLC determination of the activity of lactase from the microorganisms in yoghurt. The method is based upon the ability of -galactosidase to hydrolyze lactose bonds in glucose and galactose. To determine such activity in yoghurt, 1 mL of yoghurt was diluted 110 with 0.1 M phosphate buffer (pH 7) containing 2 % (w/v) lactose and 5 mM dithioerythritol as reducing agent; the mixture was the incubated and injected into the HPLC. Lactase activity remained high as long as the number of viable microorganisms did not fall below the minimum CFU·mL^–1 (10⁷). This method is more repeatable than conventional colorimetric determination, and may also be automated for routine analysis.     

Does neighboring group participation by non-vicinal esters play a role in glycosylation reactions? Effective probes for the detection of bridging intermediates

Neighboring group participation in glycopyranosylation reactions is probed for esters at the 3-O-axial and -equatorial, 4-O-axial and -equatorial, and 6-O-sites of a range of donors through the use tert-butoxycarbonyl esters. The anticipated intermediate cyclic dioxanyl cation is interrupted for the axial 3-O-derivative, leading to the formation of a 1,3-O-cyclic carbonate ester, with loss of a tert-butyl cation, providing convincing evidence of participation by esters at that position. However, no evidence was found for such a fragmentation of carbonate esters at the 3-O-equatorial, 4-O-axial and -equatorial, and 6-O positions, indicating that neighboring group participation from those sites does not occur under typical glycosylation conditions. Further probes employing a 4-O-(2-carboxy)benzoate ester and a 4-O-(4-methoxybenzoate) ester, the latter in conjunction with an (18)O quench designed to detect bridging intermediates, also failed to provide evidence for participation by 4-O-esters in galactopyranosylation.     

Syntheses of Retinol Glycosides Using β-glucosidase in SCCO<Subscript>2</Subscript> Media

β-Glucosidase isolated from sweet almond catalyzed syntheses of water soluble retinol glycosides were carried out in SCCO₂ media with carbohydrates—D-glucose 2, D-galactose 3, D-mannose 4, D-fructose 5, and D-sorbitol 6. Retinol glycosides yields were in the 9–34% range. Reaction with D-fructose 5 gave a highest yield of 34%. Excellent regioselectivity was observed with D-mannose 4 and D-sorbitol 6 which gave exclusively C1β-mannoside and C1-D-sorbitolide.     

Nickel-catalyzed C−H arylation of benzoxazoles and oxazoles: Benchmarking the influence of electronic,steric and leaving group variations in phenolic electrophiles

Electronic, steric and leaving group effects for Ni-catalyzed direct arylations using C?O electrophiles were benchmarked. The scope of arylations with pivalates was general with respect to both the electronics on the electrophile and the azoles. Furthermore, the arylation of azoles with tosylates, mesylates and carbamates with varying electronics was explored, and showed electronic trends similar to those of the pivalate reactions. Finally, the relative rate of arylation of 5-methyl benzoxazole with two electronically-similar electrophiles bearing different leaving groups was established. The results from these studies implicate the following order of relative reactivity: mesylates>pivalates>carbamates.     

A cooperative participation of the amido group in the organocatalytic construction of all-carbon quaternary stereocenters by Michael addition with β-ketoamides

The secondary amido group of α-substituted β-ketoamides plays a crucial role in the control of the reactivity and spatial arrangement (selectivity) in the organocatalyzed Michael addition to unsaturated carbonyls. This results in an unprecedented activation mode of substrates through H-bonding interactions allowing the construction of enantiomerically enriched functionalized all-carbon quaternary centers and spiroaminals of high synthetic potential.     

Extending the hydrophobic cavity of β-cyclodextrin results in more negative heat capacity changes but reduced binding affinities

The formation of inclusion complexes of hydroxypropylated β-cyclodextrins (CDs) with three bile salts are investigated to shed light on the role played by the hydroxypropyl (HP) substituents. The HP-chains are situated at the rim of the CD and may thus extend the hydrophobic cavity of the CD. Calorimetric titrations in a broad temperature range and molecular dynamics simulations confirm previous speculations that the HP-chains cause an increase in dehydrated nonpolar surface area upon formation of the complexes. This additional burial of nonpolar surface area, 12–16 Å² per HP-chain according to the MD simulations, results in more negative values of ΔC _p °, which are in quantitative agreement with what is expected for hydrophobic dehydration. Although these observations support the picture of an extended hydrophobic cavity, HPβCD complexes were less stable than their unsubstituted counterparts. This indicates that increased hydrophobic contacts are not always accompanied by increased binding strength. The linear dependence of ΔC _p °, ΔH° and ΔS° on the number of HP-chains give rise to isoentropic and isoenthalpic temperatures at which ΔH° and ΔS° are independent of the number of HP-chains on the host CD (but depend on the type of bile salt). Interestingly, these convergence temperatures are close to what is observed for unfolding of proteins and may be a common feature of hydrophobic dehydration.     

Synthesis of bi- and tervalent ligands of galectins,derivatives of β-lactosylamine,with the amino group in the terminal position of the spacer

A method for the synthesis of glycoclusters by N-alkylation of N-glycyl-β-lactosylamine with N-chloroacetyl derivatives of β-lactosylamine and N,N’-iminodiacetyldilactosylamine has been developed. The glycoclusters obtained with two and three lactosylamine residues with the amino group in the spacer were found to be suitable for further conjugation with carboxy-containing physiologically active compounds. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 3, pp. 647–651, March, 2008.     

Behaviour of β2-microglobulin in the electrical double layer of the mercury/solution interface

β₂-Microglobulin (β₂-m) is a small globular protein (12000 Da) which exists as a monomer in numerous body fluids. Data published on the electrochemical behaviour of β₂-m are scarce. In this paper an attempt was made to ge more information on the structure of the double layer at mercury/solution interface in the presence of β₂-m. Using phase selective ac polarography the capacitive current-potential (C-E) curve was analysed as the function of various β₂-m concentration, pH of medium, temperature, denaturation agent concentration etc. The data obtained are discussed particularly from the aspect of folding/unfolding state of the protein studied.     

Thermal stability of the β″ phase in the ZrO2-Zr3N4 system

Annealing of ZrO₂ in a nitrogen atmosphere leads to a nitrogen containing phase called phase having a trigonally distorted fluorite structure with ordered anion vacancies. DTA/TG investigations indicated that the phase decomposes intom-ZrO₂ above 500C releasing nitrogen during the reaction with oxygen. Prevention of oxygen uptake by use of closed tantalum tubes instead of open platinum crucibles enabled the detection of a new reversible phase transition at 965C. The thermoanalytical results have been confirmed by high temperature-X-ray investigation.Dedicated to Professor Dr. Roger Blachnik on the occasion of his 60^th birthday     

C2 hydroxyl group governs the difference in hydrolysis rates of methyl-α-D-glycero-D-guloseptanoside and methyl-β-D-glycero-D-guloseptanoside

A computational investigation into the hydrolysis of two methyl septanosides, methyl-α-D-glycero-D-guloseptanoside and methyl-β-D-glycero-D-guloseptanoside was undertaken. These septanosides were chosen as model compounds for comparison to methyl pyranosides and allowed direct comparison of α versus β hydrolysis rates for a specific septanoside isomer. Results suggest that hydrolysis takes place without proceeding through a transition state, an observation that was suggested in previous computational studies on exocyclic bond cleavage of carbohydrates. A conformational analysis of α- and β-anomers 1 and 2 and their corresponding oxocarbenium 3, coupled with relaxed potential energy surface (PES) scans (M06-2X/6-311+G**, implicit methanol), indicated that hydrolysis of the α-anomer is favored by 1-2 kcal/mol over the β-anomer, consistent with experiment. Model systems revealed that the lowest energy conformations of the septanoside ring system destabilize the β-anomer by 2-3 kcal/mol relative to the α-anomer, and the addition of a single hydroxyl group at the C2-position on a minimal oxepane acetal can reproduce the PES for the septanoside 1. These results suggest that the C2 hydroxyl plays a unique role in the hydrolysis mechanism, destabilizing the septanoside via its proximity to the anomeric carbon and also through its interaction with the departing methanol from the α-anomer via hydrogen-bonding interactions.     

Concise synthesis of glyconoamidines as affinity ligands for the purification of β-glucosidase involved in control of some biological events including plant leaf movement

Glycosidases are involved in deactivation or storage of some endogenous bioactive substances through biologically intriguing processes. For example, nyctinastic leaf movement is controlled by a biological clock through the regulation of β-glucosidase activity. Ganem’s glyconoamidine (1) is used as a micromolar inhibitor of glucosidase in biochemical studies and would be useful as an affinity ligand for purification of glycosidase. However, its use for the specific inhibition of glucosidase which is highly specific to a glycoside with voluntary aglycon is seriously restricted because no universal method for the synthesis of N-alkylated glyconoamidine has been reported. Here, we report a concise synthesis of N-alkylated Ganem’s glyconoamidine with voluntary aglycon using a non-protected sugar derivative.     

New Fluoroaryl-containing β,β'-Dioxoesters in the Synthesis of Fluorobenzopyran-2(4)-ones

For the first time were obtained ethyl 2-(2-methoxy-3,4,5,6-tetrafluorobenzoyl)-3-oxobutanoate and ethyl 2-pentafluorobenzoyl-3-oxobutanoate and their copper chelates. The compounds were prepared by acylation of ethyl acetoacetate with 2-methoxy-3,4,5,6-tetrafluoro- and pentafluorobenzoyl chlorides. Cyclization of these ,'-dioxoesters afforded substituted chromones. 2-Methyl-5-methoxy-6,7,8-trifluoro-3-ethoxycarbonylchromone hydrolyzes depending on reaction conditions either to 5-hydroxy-2-methyl-6,7,8-trifluorochromone or to 5-hydroxy-2-methyl-6,7,8-trifluorochromone-3-carboxylic acid. Reaction with morpholine provided 7-substituted product, and with aqueous ammonia as a result of rearrangement forms 3-acetimidoyl-4-hydroxy-5-methoxy-6,7,8-trifluorocoumarin. Hydrolysis of the latter yields 3-acetyl-4-hydroxy-5-methoxy-6,7,8-trifluorocoumarin.     

Transformations of α- and β-ionones in the presence of Al2O3 in a supercritical solvent in a flow reactor

The reactivity of α- and β-ionones under the supercritical conditions in a flow type reactor in the presence of Al₂O₃ at 200–230°C was studied. α-Ionone was reduced to α-ionol, while β-ionol was unstable already at 200°C and underwent dehydration. The secondary reaction products were the corresponding megastigmatrienes.     

Effect of small amounts of Al on the surface silanol structure and their correlation to the improved catalytic performances of WOx/SiO2–Al2O3 in the propene self-metathesis

Surface silanol structures, acid properties, and tungsten dispersion of the sol-gel-derived 7W/SiO₂–xAl₂O₃ (x = 0.2–23 wt%) were investigated by means of ²⁹Si, ²⁷Al, and ¹H MAS NMR, NH₃-TPD, in-situ NH₃-IR spectroscopy, XRD, and Raman spectroscopy. The surface silanol structure changed upon Al and tungsten loadings; however, loading of 1 wt% Al₂O₃ appeared to be the threshold for preserving the Si(OH)Al with isolated bridge after impregnation of 7 wt% W. The 7W/SiO₂–1Al₂O₃ (1 wt% Al₂O₃) was also found to exhibit the lowest ratio of Bronsted to Lewis acid with the highest amount of Lewis acid sites and the best catalyst performances in propene self-metathesis at 550 °C in terms of both propene conversion and ethylene/butene selectivity. Despite its low tungsten dispersion, the metathesis activity was correlated well with the higher amount of tungsten carbene species, which were formed on the catalysts containing higher isolated bridge silanol and the presence of higher Lewis acid sites.     

Interplay between the structural and magnetic probes in the elucidation of the structure of a novel 2D layered [V4O4(OH)2(O2CC6H4CO2)4]·DMF

The title compound has been synthesized under solvothermal conditions by reacting vanadium(V) oxytriisopropoxide with terephthalic acid in N,N-dimethylformamide. A combination of synchrotron powder diffraction, infrared spectroscopy, scanning and transmission electron microscopy, and thermal and chemical analysis elucidated the chemical, structural and microstructural features of a new 2D layered inorganic-organic framework. Due to the low-crystallinity of the final material, its crystal structure has been solved from synchrotron X-ray powder diffraction data using a direct space global optimization technique and subsequent constraint Rietveld refinement. [V(4)O(4)(OH)(2)(O(2)CC(6)H(4)CO(2))(4)]·DMF crystallizes in the monoclinic system (space group P2/m (No. 10)); cell parameters: a = 20.923(4) ?, b = 5.963(4) ?, c = 20.425(1) ?, β = 123.70(6)°, V = 2120.1(9) ?(3), Z = 2. The overall structure can be described as an array of parallel 2D layers running along [-101] direction, consisting of two types of vanadium oxidation states and coordination polyhedra: face-shared trigonal prisms (V(4+)) and distorted corner-shared square pyramids (V(5+)). Both configurations form independent parallel chains oriented along the 2-fold symmetry crystallographic b-axis mutually interlinked with terephthalate ligands in a monodentate mode perpendicular to it. The morphology of the compound exhibits long nanofibers, with the growth direction along the layered [-101] axis. The magnetic susceptibility measurements show that the magnetic properties of [V(4)O(4)(OH)(2)(O(2)CC(6)H(4)CO(2))(4)]·DMF can be described by a linear antiferromagnetic chain model, with the isotropic exchange interaction of J = -75 K between the nearest V(4+) neighbours of S = 1/2.