Recent developments in chiral anion mediated asymmetric chemistry

Chemical reactions and processes often involve cationic prostereogenic or racemic reagents, intermediates or products. To afford instead non-racemic or enantiopure compounds, an asymmetric ion pairing of the cations with chiral anionic counterions can be considered. This review presents recent examples of the synthesis and use of chiral anions for stereoselective purposes.     

A comparative study of microemulsion electrokinetic capillary chromatography and micellar electrokinetic capillary chromatography for the analysis of UV-absorbing compounds in human urine

Separations of human urine by microemulsion electrokinetic chromatography (MEEKC) and micellar electrokinetic capillary chromatography (MEKC) with respect to resolution, migration times and efficiencies were optimized and compared. The optimised MEEKC and MEKC methods were simple and fast, both of which are excellent characteristics for the complex separations required in clinical and biomedical studies. However, resolution in MEKC was significantly greater than in MEEKC although migration times were 30% faster for the optimised MEEKC method. In addition, a faster analysis method (short-end injection) specifically for routine screening purposes was also investigated. With both MEEKC and MEKC modes, this provided short separations (less than 4 min for urine) with no major compromise in resolution. In conclusion, we found that MEEKC offered no real advantage over MEKC for urine analysis.     

Grafting of [Mn(CH2tBu)2(tmeda)] on silica and comparison with its reaction with a silsesquioxane

The reaction of [Mn(CH2tBu)2(tmeda)] (1) and a silica partially dehydroxylated at 700 degrees C (SiO(2/700)) yields a single surface species [([triple bond]SiO)Mn(CH2tBu)(tmeda)] (2a; tmeda = tetramethylethylendiamine), while a mixture of 2a and [([triple bond]SiO)2Mn(tmeda)] (2b) is obtained by using SiO(2/200), SiO(2/300), or SiO(2/500) as evidenced by mass balance analysis, and IR and EPR spectroscopy. The reaction of 1 and (c-C5H9)7Si7O12SiOH (3), a soluble silanol that is a molecular model for a silica support, generates the bis-siloxy complex 4, [[(c-C5H9)7Si7O12SiO]2Mn(tmeda)2], in a quantitative yield; compound 4 was characterized by single-crystal X-ray diffraction. These reactions exemplify the limitation of considering molecular silanol derivatives as straightforward and reliable homogeneous models for silica, and address the need for thorough characterization of surface species by the use of surface-science techniques. These studies show the possibility of preparing coordinatively and geometrically unique surface species that would be difficult to prepare by solution chemistry methods; insights into the chemical and physical properties of these surface species are also gained.     

Straightforward Access to Enantioenriched cis-3-Fluoro-dihydroquinolin-4-ols Derivatives via Ru(II)-Catalyzed-Asymmetric Transfer Hydrogenation/Dynamic Kinetic Resolution

Herein we report a practical method for the asymmetric transfer hydrogenation/dynamic kinetic resolution of N-Boc 3-fluoro-dihydrotetrahydroquinolin-4-ones into the corresponding cis-fluoro alcohols in 70–96% yields, up to 99:1 diastereomeric ratio (dr) and up to >99% ee (enantiomeric excess) by using the ruthenium complex Ts-DENEB and a formic acid/triethylamine (1:1) mixture as the hydrogen donor under mild conditions.     

Rapid Assembly of Oligosaccharides: Total Synthesis of a Glycosylphosphatidylinositol Anchor of Trypanosoma brucei

Six building blocks, six reaction steps : The recently developed innovative methodology facilitated the convergent synthesis of the complex oligosaccharide core 1 (shown here with protecting groups) for the total synthesis of a glycosylphosphatidylinositol (GPI) anchor. The key factors are the tuning of the reactivity of the building blocks by using 1,2-diacetal protecting groups and the desymmetrization of glycerol and myo-inositol with a chiral bis(dihydropyran).     

Confinement of polystyrene at interfaces: Consequences on chain orientation

The knowledge of the structure and orientation of polymer chains adsorbed at an interface could be of major importance to predict the level of interfacial interactions and adhesion that depend strongly on the properties of the interface formed between the two materials (polymer and substrate) brought into contact. In this work, we were interested to study thin films of atactic polystyrene after adsorption (spin‐coating) on two chemically different substrates (inert and OH‐grafted gold substrates). The main aim is to analyze the resulting anisotropy due to the confinement in a quasi‐bidimensional geometry, as well as to investigate the incidence of the interfacial interactions, potentially established between the polymer and the surface, on the chain organization. Our infrared spectroscopy results allowed us to access the adsorption model of polystyrene chains and to highlight the relation between chain orientation and interfacial acid–base interactions. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 1268–1276, 2006     

Theory of chirped photonic crystals

In chirped photonic crystals, the structural parameters describing a unit cell are progressively varied from a unit cell to the nearest ones. Geometric and dielectric response functions can be affected by this modulation, but here we only investigate the effect of a long-range, slowly varying, modulation of the refractive index. The Bloch modes are modified by essentially being modulated by an envelope function which adapts to the long-range dielectric function perturbation. It is shown that this envelope function obeys a simple linear Schrödinger equation of classical (non-quantum) origin. Close to a band extremum, at a gap edge, the envelope functions can be interpreted as wave functions of particles, called “energy carriers”. These carriers have a mass and come as two species, referred to as “effective photons” (for positive band curvatures) or “photonic holes” (for negative band curvatures). The energy transfer through the chirped structure can be viewed as resulting from the migration of these particles under forces implied by the long-range dielectric function modulation.