Stereoselective conjugate addition of benzyl phenylsulfonyl carbanions to enoates derived from d-mannitol

The conjugate addition of benzylic phenylsulfonyl carbanions (2a'-d') to enoates derived from d-(+)-mannitol (E- or Z-1a-c) was studied using THF and THF/HMPA as solvent. Under kinetic conditions (-78 degrees C), enoate E-1a,b led to a mixture of syn-(R,S) and anti-(S,S) adducts (55/45), and syn-(R,S) adducts were the main product obtained ( approximately 90/10) from enoate Z-1a. Under thermodynamic conditions (-78 degrees C to room temperature) syn-(R,S) adducts were also preferentially formed ( approximately 90/10), despite the geometry at the double bond in the acceptor. Enoate 1c (E/Z = 57/43), bearing an additional benzyl group at the alpha-position, also reacted with carbanions 2'a,b, under thermodynamic conditions, leading to syn-adducts in excellent de (control at the three newly generated stereogenic centers). The adducts were quantitatively transformed into the corresponding beta-gamma-disubstituted gamma-butyrolactones and alpha,beta,gamma-trisubstituted gamma-butyrolactones. (1)H NMR studies (NOE and J-coupling) of these lactones allowed us to determine their configuration at the newly generated chiral centers. The reduction of the C-S bond in adducts syn-(R,S) with Na/Hg, followed by treatment of the resulting products in aqueous acid media, led to enantioenriched beta-benzyl-gamma-hydroxymethyl-gamma-butyrolactones. The conformational equilibrium of enoates E- and Z-1b was evaluated by theoretical calculations (ab initio, MP2/6-31G), and a mechanistic rationale was proposed to explain the observed stereoselectivities.     

Alkaline hydrolysis of diaryl ditellurides under phase transfer conditions; synthesis of alkyl aryl tellurides

The disproportionation reaction of diaryl ditellurides [(C₆H₅Te)₂, (p-CH₃C₆H₄Te)₂, (p-CH₃OC₆H₄Te)₂, (p-C₂H₅OC₄Te)₂, (2-naphthyl-Te)₂] with sodium hydroxide under phase transfer conditions at room temperature is described for the first time. The phase transfer catalyst used is 2HT-75, a trade name for a mixture of dialkyldimethylammonium chlorides. The intermediates aryl tellurolates react “in situ” with alkyl halides to give the corresponding alkyl aryl tellurides (ArTeR) in 52–72% yield. The following compounds were prepared: Ar  C₆H₅, R=CH₃(CH₂)₃CH₂, (CH₃)₂CHCH₂CH₂, (CH₃)₂CHCH₂, CH₃CHBrCH₂CH₂, CH₃(CH₂)₈CH₂, C₆H₅CH₂, ClCH₂, C₆H₅CH₂CH₂, CH₂CHCH₂, C₆H₅CHCHCH₂, C₆H₅SeCH₂, $\text{CH}{}_2\text{CH}{}_2\text{CH}{}_2\text{CHCHC}$H; Ar=p-CH₃C₆H₄, R = CH₃(CH₂)₂CH₂; Ar=p-CH₃OC₆H₄, R = CH₃(CH₂)₂CH₂; Ar = p-CH₂H₅OC₆H₄, R= CH₃(CH₂)₂CH₂; Ar = 2-naphthyl, R = CH₃(CH42)₂CH₂.     

Flow injection determination of cobalt after its sorption onto polyurethane foam loaded with 2-(2-thiazolylazo)-p-cresol (TAC)

An electrochemically stable monolayer of tris(2,2'-bipyridyl)ruthenium(II) was obtained for the first time. It was based on the electrostatic attachment of Ru(bpy)(3)(2+) to the benzene sulfonic acid monolayer film, which was covalently bound onto glassy carbon electrode by the electrochemical reduction of diazobenzene sulfonic acid. The surface-confined Ru(bpy)(3)(2+) underwent reversible surface process, and reacted with the coreactant, tripropylamine, to produce electrochemiluminescence. In view of the stability of the electrode, the results strongly suggested that light was emitted from the surface-confined Ru(bpy)(3)(2+), not from the detached Ru(bpy)(3)(2+). The Ru(bpy)(3)(2+) modified electrode was used to the determination of tripropylamine. It showed good linearity in the concentration range from 5 muM to 1 mM with a detection limit of 1 muM (S/N=4). The good stability of the Ru(bpy)(3)(2+) modified electrode also showed that the benzene sulfonic acid monolayer film prepared can be served as an excellent support to construct multilayers.     

Effects of Adsorbed Polyaniline on Redox Process on MoO3 Surface

The effects exhibited by adsorbed conducting polyaniline on the redox process on a molybdenum oxide surface were studied. Thermogravimetric results indicate a 4% polyaniline deposition. Cyclic voltammograms of the adsorbed polymer on MoO3 show that polyaniline exerts remarkable effects on the molybdenum blue oxidation-reduction process, with oxidation and reduction potentials of 0.33 and 0.18 V, respectively. This effect strongly enhances the electrode response, and can be used as an important tool in qualitative and/or quantitative determinations of molybdenum in solution as well as in any substrate. Copyright 1999 Academic Press.     

Cd(II) determination in the presence of aqueous micellar solutions

The equilibrium constants and molar absorptivities for the fast formation of a 1:3 complex between cadmium(II) (Cd(II)) and dithizonate anion, in the presence of cationic and non-ionic surfactants, allowed a simple and fast spectrophotometric determination of total cadmium. Indeed, the molar absorptivities of the Cd(II)-dithizone (Dz) complex formed in the presence of the neutral Triton X-100 and cationic cetyltrimethylammonium bromide (CTAB) surfactants are almost twice the value observed in the standard method and the maxima of absorption are shifted by about 40 nm when compared with the standard method. Clearly, the use of neutral and cationic surfactants promotes a higher value of the molar absorptivities of the complex, resulting in an increase in the sensitivity of the method. Application of the method to the desorption of Cd(II) ions from clays is illustrated.     

Molybdenum determination in iron matrices by ICP-AES after separation and preconcentration using polyurethane foam

A procedure is proposed for the separation and determination of molybdenum in iron matrices by a batch process. It is based on the solid-phase extraction of the molybdenum(V) ion as thiocyanate complex on polyurethane (PU) foam. The extraction parameters were optimized. Using 0.20 mol L-1 hydrochloric acid, a thiocyanate concentration of 0.10 mol L-1, 100 mg of polyurethane foam and shaking time of 10 min, molybdenum (5-400 micrograms) can be separated and preconcentrated from large amounts of iron (10 mg). Desorption was carried out instantaneously by conc. nitric acid or acetone. Distribution coefficients, sorption capacity of the PU foam and coefficients of variation were also evaluated. The effect of some ions on the separation procedure was assessed. Iron(III) should be reduced to iron(II). The proposed procedure was used to determine molybdenum in standard iron matrices such as steel and pure iron. The achieved results did not show significant differences with certified values.     

Aggregation effects on evaporation and the air/water interface of dodecyltrimethylammonium hydroxide solutions

 The system dodecyltrimethylammonium hydroxide (DTAOH)–water was studied by surface tension, ion-selective electrodes and evaporation in an electrobalance. Results confirmed earlier conclusions about a stepwise aggregation mechanism in DTAOH solutions. The aggregation process started at a total concentration C _T=(2.51±0.10)×10^-4 mol dm^-3) which probably corresponds to the formation of dimers. At C _T= (1.300±0.041)×10^-3 mol dm^-3 there was a change in the surface and evaporation behavior, corresponding to the formation of small, fully ionized aggregates which grew with increasing concentration. At C _T= (1.108±0.010)×10^-2 mol dm^-3 the formation of true micelles with hydroxide counterions in the Stern layer did not change significantly the evaporation and adsorption behavior. This means that between this concentration and C _T=(3.02±0.28)× M28.8n10^-2 mol dm^-3, the changes in structure were gradual. At the latter concentration there was a sudden change in the monolayer state at the air/water interface, with a strong surfactant desorption, and a major change in evaporation behavior. The changes are compatible with the formation of few, large aggregates reducing the total concentration of kinetically independent solute units, which in turn increased the activity of the solvent. This phenomenon is in agreement with literature information. The reduction in the evaporation rate of water was mainly due to the reduction of the water activity, caused by colligative effects. The reduction of the effective area available for evaporation had only a slight effect in water evaporation. Received: 9 January 1997 Accepted: 19 October 1997     

Modification of the luminescence properties of an Europium(III) Tris(β-diketonate) Complex by Inclusion in γ-cyclodextrin and 2,3,6-trimethyl-γ-cyclodextrin

A 1:1 inclusion compound between octakis(2,3,6-tri-O-methyl)-γ-cyclodextrin (TRIMEG) and the chelate complex Eu(NTA)₃·2H₂O (NTA=1-(2-naphthoyl)-3,3,3-trifluoroacetonate) was prepared and characterized by powder X-ray diffraction, thermogravimetric analysis and photoluminescence spectroscopy. The results were compared with those obtained for the corresponding native γ-CD adduct. Excitation and emission spectra were measured, and the lifetimes were determined for the Eu³⁺ first excited state (⁵D₀). The results indicate the presence of only one low-symmetry environment for the Eu³⁺ cations in the inclusion compounds. Encapsulation of the Europium complex in the two CDs increases the quantum efficiency of the ligand-to-metal energy transfer pathway, but the efficiency of the Eu³⁺ sensitization was significantly higher with TRIMEG as the host molecule. This may be related with the observation that the two hosts appear to have different influences on the Eu³⁺ coordination environments for the guest molecule.     

Antiedematogenic activity of two thiazolidine derivatives: N-tryptophyl-5-(3,5-di-tert-butyl-4-hydroxybenzylidene) rhodanine (GS26) and N-tryptophyl-5-(3,5-di-tert-butyl-4-hydroxybenzylidene)-2,4-thiazolidinedione (GS28)

The search for new anti-inflammatory drugs has been constant in several research centers. The use of the Bioisostery concept allows the elaboration of new bioactive compounds with different properties through the introduction of substitute groups in one or more positions of a main molecule with known biological activity. Preliminary works accomplished at our laboratory with 2,4-thiazolidinedione isosters demonstrated inhibitory activity on edema formation for N-tryptophyl-5-(3,5-di-tert-butyl-4-hydroxybenzylidene)-2,4-thiazolidinedione (GS28) and N-tryptophyl-5-(3,5-di-tert-butyl-4-hydroxybenzylidene) rhodanine (GS26). We verified the antiedematogenic and ulcerogenic activity of these two compounds in Wistar rats. The carrageenan induced paw edema suffered significant (p<0.05) inhibition (28.36% on average) for GS28 (100 mg/kg; v.o.) during the entire time of the experiment. GS26 (50 and 100 mg/kg; v.o.) significantly inhibited (p<0.05) the paw edema dextran induced (22.1 and 27.8%, for the respective doses) after 180 min. The compounds GS26 and GS28 did not show ulcerogenic activity on gastric mucous. The results suggest antiedematogenic action for both compounds without the appearance of gastric lesions.