Simplification of the 1H NMR spectra of enantiomers dissolved in chiral liquid crystals, combining variable angle sample spinning and selective refocusing experiments

This work presents a technique to simplify overcrowded proton spectra in chiral liquid crystal solvents using rotation of the sample near the magic angle, VASS, combined with homonuclear selective refocusing 2D NMR experiments, SERF. This methodology provides a powerful tool to visualise enantiomers out of unresolved proton spectra. A modified SERF sequence is presented where the resulting 2D spectrum can be phased to increase the resolution. Accurate enantiomeric excesses are determined that are not possible to measure on static samples. Two examples are presented.     

Minimal number of chromatographic test parameters for the characterisation of reversed-phase liquid chromatographic stationary phases

This paper focuses on the classification or differentiation of RP-HPLC columns based on measured chromatographic properties. A chemometric study has been conducted on a published data set consisting of 85 RP-HPLC columns and on a data set consisting of 47 self-tested columns. Principal component analysis enables determination of the number of parameters necessary for a rational differentiation. The results show that reducing the number of parameters for such differentiation still allows classification of the columns just as a higher number did. It is shown that three test parameters produce a classification similar to that obtained with five parameters.     

Lipase-catalyzed ester formation in organic solvents an easy preparative resolution α-substituted cyclohexanols

The commercial preparation “lipase My” catalyses ester formation from an alcohol and a fatty acid in organic solvents. Preparative resolution of α-substituted cyclohexanols, including menthol, was achieved under these conditions.     

Permeation liquid membrane for trace metal speciation in natural waters. Transport of liposoluble Cu(II) complexes

In this paper, the transport of Cu(II) in the presence of lipophilic Cu(II) organic complexes through permeation liquid membranes (PLMs) have been investigated. In natural waters, small organic compounds, which form liposoluble neutral complexes with Cu(II), are potentially toxic and bioavailable. Hence, to understand the role of liposoluble Cu(II) complexes in natural waters, four organic ligands: phthalic acid, bipyridyl, pyrocatechol and hydroxyquinoline, which form uncharged or lipophilic Cu(II) complexes, were tested. The results showed that the transport of lipophilic Cu(II) complexes through PLM depends on the lipophilicity of the complex. Applications of PLMs in natural waters are presented.     

Synthesis and elaboration of functionalised carbohydrate-derived spiroketals

The scope of a stereoselective three-step approach for the synthesis of sugar derived spiroketals is presented. The methodology consists of Grignard addition of vinyl- or allylmagnesium bromide to a carbohydrate lactone, followed by K-10 clay mediated glycosidation with a terminal alkenol and subsequent ring-closing metathesis of the resulting diene. The generality of this procedure is demonstrated by the synthesis of various pyranose- and furanose-derived spiroketals, as well as more advanced tricyclic spiroketal derivatives. It is shown that functionalisation of the double bond in the resulting spiroketals leads to fused polycyclic ethers.     

Studies toward labeling cytisine with [11C]phosgene: rapid synthesis of a delta-lactam involving a new chemoselective lithiation-annulation method

With the aim of the radiolabeling of cytisine, a potent agonist of nicotinic receptors, with [(11)C]phosgene, the rapid synthesis of a lactam model of our target has been studied. The key step of the delta-lactam formation is a new chemoselective lithiation-annulation method, under high dilution, of a suitable piperidinylcarbamoyl chloride. This precursor was obtained from (2-hydroxyethyl)piperidine in a linear synthetic sequence involving a Corey-Fuchs olefination of the corresponding aldehyde, followed by a selective reduction, using a diimide equivalent, of an iodoalkyne into a (Z)-iodopropene piperidine. This alkene served as main precursor to study the cyclization according to several procedures using phosgene as the required carbonylating reagent.     

Spectroscopic and electrochemical characterization of an aqua ligand exchange and oxidatively induced deprotonation in diiron complexes

Reaction of the unsymmetrical phenol ligand 2-((bis(2-pyridylmethyl)amino)methyl)-6-(((2-pyridylmethyl)benzylamino)methyl)-4-methylphenol (HL-Bn) or its 2,6-dichlorobenzyl analogue (HL-BnCl(2)) with Fe(H(2)O)(6)(ClO(4))(2) in the presence of disodium m-phenylenedipropionate (Na(2)(mpdp)) followed by exposure to atmosphere affords the diiron(II,III) complexes [Fe(2)(L-Bn)(mpdp)(H(2)O)](ClO(4))(2) and [Fe(2)(L-BnCl(2))(mpdp)(CH(3)OH)](ClO(4))(2), respectively. The latter complex has been characterized by X-ray crystallography. It crystallizes in the monoclinic system, space group P2(1)/n, with a = 13.3095(14) A, b = 20.1073(19) A, c = 19.4997(19) A, alpha = 90 degrees, beta = 94.471(2) degrees, gamma = 90 degrees, V = 5202.6(9) A(3), and Z = 4. The structure of the compound is very similar to that of [Fe(2)(L-Bn)(mpdp)(H(2)O)](BPh(4))(2) determined earlier, except for the replacement of a water by a methanol on the ferrous site. Magnetic measurements of [Fe(2)(L-Bn)(mpdp)(H(2)O)](BPh(4))(2) reveal that the two high-spin Fe ions are moderately antiferromagnetically coupled (J = -3.2(2) cm(-)(1)). Upon dissolution in acetonitrile the terminal ligand on the ferrous site is replaced by a solvent molecule. The acetonitrile-water exchange has been investigated by various spectroscopic techniques (UV-visible, NMR, M?ssbauer) and electrochemistry. The substitution of acetonitrile by water is clearly evidenced by M?ssbauer spectroscopy by a reduction of the quadrupole splitting value from 3.14 to 2.41 mm/s. In addition, it causes a 210 mV downshift of the oxidation potential of the ferrous site and a similar reduction of the stability domain of the mixed-valence state. Exhaustive electrolysis of a solution of [Fe(2)(L-Bn)(mpdp)(H(2)O)](2+) shows that the aqua diferric species is not stable and undergoes a chemical reaction which can be partly reversed by reduction to the mixed-valent state. This and other electrochemical observations suggest that upon oxidation of the diiron center to the diferric state the aqua ligand is deprotonated to a hydroxo. This hypothesis is supported by M?ssbauer spectroscopy. Indeed, this species possesses a large quadrupole splitting value (DeltaE(Q) >or= 1.0 mm.s(-)(1)) similar to that of analogous complexes with a terminal phenolate ligand. This study illustrates the drastic effects of aqua ligand exchange and deprotonation on the electronic structure and redox potentials of diiron centers.     

A new synthesis of 1,3-dihydrobenzo[1,2]oxasiloles by a novel rearrangement of a pentavalent silicon intermediate

A new synthesis of benzo[1,2]oxasiloles is described, wherein an unprecedented intramolecular allylic transposition takes place probably involving a pentavalent silicon intermediate.     

A theoretical investigation of the enantioselective hydrogenation mechanism of α-ketoesters

The enantioselective hydrogenation of α-ketoesters to α-hydroxyesters over Pt/Al₂O₃ catalysts modified by cinchona alkaloids is an interesting model reaction for the investigation of heterogeneous catalysis capable of producing optically active products. The aim of the present theoretical study is to rationalize the interaction between protonated cinchona alkaloids (modifiers) and methyl pyruvate (substrate) by investigating the possible weak complexes formed by these two species. For this purpose we use molecular mechanics and the AM1 semiempirical method. The optimization leads to two stable forms of the complexes, where the substrate is bound to the modifier via hydrogen bonding between the oxygen of the α-carbonyl of pyruvate and the quinuclidine nitrogen of the alkaloid. In such complexes the methyl pyruvate is transformed into a half-hydrogenated species which can be adsorbed on the platinum surface and, after hydrogenation, leads to methyl lactate product. The results show that adsorption of the complex leading to (R)-methyl lactate is more favorable than that of the corresponding system yielding (S)-methyl lactate, which may be the key for the enantio-differentiation.     

Molecular organization of DHP membrane fragments

The phase transition of dihexadecyl phosphate (DHP) bilayered disks has been studied using EPR spectroscopy. In the acid form of DHP, a phase transition temperature exists, that we have monitored through the spin-spin interaction between the nitroxide molecules at high concentration (8%) in DHP bilayers. This spin-spin interaction is due to the gathering of solutes in a fluid defect of the membrane: the border. The fluorescence quenching of two probes by the nitroxide stearic acids in DHP bilayers has been studied by stationary and time-resolved fluorescence measurements. The quenching process is mainly static. Both magnetic and fluorescent probes are localized in the periphery of the bilayered disks. An erratum to this article is available at .