Synergistic Strategies in Aminocatalysis

Synergistic catalysis offers the unique possibility of simultaneous activation of both the nucleophile and the electrophile in a reaction. A requirement for this strategy is the stability of the active species towards the reaction conditions and the two concerted catalytic cycles. Since the beginning of the century, aminocatalysis has been established as a platform for the stereoselective activation of carbonyl compounds through HOMO-raising or LUMO-lowering. The burgeoning era of aminocatalysis has been driven by a deep understanding of these activation and stereoinduction modes, thanks to the introduction of versatile and privileged chiral amines. The aim of this review is to cover recent developments in synergistic strategies involving aminocatalysis in combination with organo-, metal-, photo-, and electro-catalysis, focusing on the evolution of privileged aminocatalysts architectures.     

Molecular Structure of a Chromatographic Chiral Selector Based on a Terguride Derivative

The structure of (+)1-(3-allylpropyl)-(5R,8S,10R)-N,N-diethyl-N-[6-methylergolin-8-yl]urea, C₂₂H₃₃N₄O (allyl-terguride), has been determined as part of a study on the chiral recognition mechanism of ergot alkaloids when they are used as the chiral stationary phase for the separation of racemic mixtures in liquid chromatographic methods. At the pH of the solution used for the crystallization, the molecules of allyl-terguride are protonated at N(6). All bond distances and angles are in the expected ranges. In the asymmetric unit one hydroxide ion is present. Hydrogen bonds join molecules of allyl-terguride in pairs along the b axis, connecting O(2) of the hydroxide ion to O(1) of one molecule and to N(2) and N(6) of another.     

Capillary Electrochromatography on Methacrylate Based Monolithic Columns: Evaluation of Column Performance and Separation of Polyphenols

Fused-silica capillary columns (100 μm I.D.) englobing a porous monolithic stationary phase were prepared by in situ copolymerization of 2-ethylhexyl methacrylate, ethylene glycol dimethacrylate and 2-acrylamido-2-methylpropanesulfonic acid (AMPS) in the presence of a porogenic mixture containing 1-propanol, 1,4 butanediol and water. The influence of the monomers ratio and the porogen solvent composition as well as the content of AMPS in the polymerization mixture on column total porosity and efficiency was investigated to attain minimum HETP values for the reversed-phase capillary electrochromatography separation of bioflavonoids. For the most promising column, the van Deemter plots, in both μ-HPLC and CEC, were also evaluated. In CEC the reduced plate height was found almost constant (1.6–2.0) within the range of linear mobile phase velocity between 0.2–2.0 mm s⁻¹. The chemical and mechanical stabilities of the monolithic column over a wide range of buffer pH (2-10) and time were satisfactory. Furthermore, the effects of mobile phase parameters, such as buffer concentration and organic modifier content, on the electroosmotic flow were studied systematically. CEC separations of standard mixtures of polyphenols, including flavonols, flavanones and flavanones-7-O-glycosides, were accomplished in less than 8 min. The CEC separation of the major flavanone glycoside constituents in the extract from a freshly squeezed grapefruit juice was also reported.     

High-performance liquid chromatography study of the enantiomer separation of chrysanthemic acid and its analogous compounds on a terguride-based stationary phase

The direct enantioseparation of chrysanthemic acid [2,2-dimethyl-3-(2-methylpropenyl)-cyclopropanecarboxylic acid] and its halogen-substituted analogues was systematically studied by HPLC using a terguride-based chiral stationary phase in combination with a UV diode array and chiroptical detectors. Isomers with (1R) configuration always eluted before those with (IS) configuration. The elution sequence of cis- and trans-isomers was strongly affected by mobile phase pH, whereas the enantioselectivity remained the same. Conditions for the separation of all the enantiomers were also examined. This method was used for monitor the hydrolytic degradation products of Cyfluthrin (Baythroid) in soil under laboratory conditions.     

Sensitive profiling of biogenic amines in human urine by capillary electrophoresis with field amplified sample injection

In order to monitor biogenic amines in human urine, a method based on field‐amplified sample injection combined with capillary electrophoresis and direct UV absorption detection was developed. Dopamine, tyramine, tryptamine, serotonin and epinephrine were effectively separated and identified in human urine samples, and detection limits were 0.072, 0.010, 0.027, 0.010 and 0.120 µmol/L, respectively. Detection limits comparable to laser‐induced fluorescence detection or solid phase extraction combined with capillary electrophoresis were achieved. Parameters affecting electrophoretic system detection sensitivity were investigated. Optimal separation conditions were obtained using as background electrolyte a pH 6.5 mixture of 2‐(morpholino)ethanesulfonic acid 20 mmol/L and 30 mmol/L phosphate buffer, containing 0.05% hydroxypropylcellulose and 10% v/v methanol. Injections of the sample solution were performed by applying a voltage of 12 kV for 50 s. Recovery and accuracy ranged between 89.4 and 94.9%, and 89 and 112%, respectively. The method was successfully applied on actual urine samples (from a healthy volunteer): target bioamine content was consistent with endogenous levels reported in the literature. The proposed method is simple, fast and inexpensive and can be conveniently employed in work‐related stress studies. The affordability and noninvasive sampling of the method allow epidemiological studies on large number of exposed persons to be performed. Copyright © 2013 John Wiley & Sons, Ltd.     

CEC enantioseparations on chiral monolithic columns: a study of the stereoselective degradation of (R/S)-dichlorprop [2-(2,4-dichlorophenoxy)propionic acid] in soil

For the study of the stereoselective degradation of the herbicide 2-aryloxipropionic acid dichlorprop (DCPP) in soil, a porous monolithic chiral column (100 microm id) was prepared by in situ copolymerization of glycidyl methacrylate, methyl methacrylate and ethylene glycol dimethacrylate in the presence of formamide and 1-propanol as the porogen solvents. Subsequently, the epoxide groups at the surface of the monolith were reacted with (+)-1-(4-aminobutyl)-(5R,8S,10R)-terguride as the chiral selector. Optimum conditions for the herbicide resolution by CEC were found using mobile phases consisting of acetic acid/triethylamine mixtures in ACN-methanol (9:1 v/v). Under these conditions fully separation of DCPP enantiomers in the presence of clofibric acid (internal standard) was achieved in about 5 min. Experiments on the incubation of rac-DCPP in soil at room temperature showed the herbicide undergone during 23 incubation days to a degradation to levels 相似文献   

A multiplex molecular assay for the detection of uniparental disomy for human chromosome 7

Uniparental disomy (UPD) describes the inheritance of both homologues of a pair of chromosomes from only one parent. This condition is often caused by nondisjunction events during meiosis. UPD has been reported as a rare cause of the autosomal recessive disorder and aberrant expression of imprinted genes that are expressed from only one parental allele, either maternal or paternal. Maternal and/or paternal UPD for chromosome 7 is the most frequently observed UPD after UPD15. Here we developed and validated, for the first time, an effective, CE‐based method for a rapid and economic detection based on two‐fluorescent STR multiplexes.