Practical and efficient enantioselective synthesis of alpha-amino acids in aqueous media

Enantiomerically pure natural and unnatural alpha-amino acids have been synthesized from a chiral methyleneoxazolidinone by means of a highly diastereoselective 1,4-conjugate addition of alkyl iodides in aqueous media. The zinc-copper conjugate addition reaction exhibits high chemoselectivity, with the possibility of using functionalized iodides, to afford a single diastereomer in short reaction times and with good yields.     

On-line characterisation of apple polyphenols by liquid chromatography coupled with mass spectrometry and ultraviolet absorbance detection

Apple polyphenols were characterised by means of hyphenated techniques such as HPLC coupled to UV photodiode array detection (LC-DAD) and to mass spectrometry (LC-MS). LC-MS using atmospheric pressure ionisation (APCI) in the positive ion mode provided the molecular weight, the number of hydroxyl groups, the number of sugars and an idea about the substitution pattern of the polyphenols. LC-DAD with postcolumn addition of UV shift reagents afforded precise structural information about the position of the free hydroxyl groups in the polyphenolic nucleus. Five isorhamnetin glycosides, two hydroxyphloretin glycosides and quercetin were reported in apple peel for the first time. Postcolumn addition of UV shift reagents in LC-DAD analysis confirmed the presence of isorhamnetin glycosides and not the isomeric glycosides of rhamnetin. Moreover, isorhamnetin-3-O-rhamnoglucoside was identified unambiguously by comparison with a standard. These results are relevant not only from a chemotaxonomic point of view, but also in the control of authenticity of fruit derived products in order to detect fraudulent admixtures.     

Laser induced decohesion of coatings: probing by laser ultrasonics

The aim of the present study is to investigate a conventional laser-ultrasonics technique for the determination of intrinsic properties of oxide coatings and their adhesion strength on a metallic substrate. The good agreement between experiments and computations in an epicenter configuration allows determining the longitudinal wave velocity as well as the Young's modulus of the oxide coatings versus the porosity. For a critical value of the laser energy, a breakdown at the coating-substrate interface is generated by the laser irradiation. The critical tensile stress field developed at the coating/substrate interface, which leads to the interfacial fracture, can be easily calculated. The value of the practical adhesion which is defined is found to be in accordance with those obtained by classic contact techniques (tensile adhesion test, indentation, bending test). Finally, this work demonstrates that this quantitative, contactless test fits well to simultaneously characterise the oxide coatings and evaluate the coating-substrate adhesion.     

Reaction between N-alkylhydroxylamines and chiral enoate esters: more experimental evidence for a cycloaddition-like process, a rationale based on DFT theoretical calculations, and stereoselective synthesis of new enantiopure beta-amino acids

The reactions between N-benzyl- and N-methylhydroxylamine and chiral enoate esters, derived from D-glyceraldehyde and (-)-verbenone, respectively, have been investigated. Theoretical calculations show that the most favorable mechanism involves the concerted cycloaddition of the hydroxylamine to the substrate. This result is in good agreement with the stereospecificity observed when the trisubstituted olefins are used. The open-chain adducts have been isolated when the processes are carried out at low temperatures and for short reaction times. These compounds evolve to the corresponding isoxazolidinones on standing at room temperature or under acid catalysis. The high pi-facial diastereoselection has been rationalized on the basis of steric effects induced by the dioxolane ring for D-glyceraldehyde derivatives or by the cyclobutane gem-dimethyl substitution for esters prepared from (-)-verbenone. As an application of these reactions, new beta-amino acids have been synthesized in a highly efficient and stereocontrolled manner.     

Inexpensive isolation of beta-D-glucopyranosidase from alpha-L-arabinofuranosidase,alpha-L-rhamnopyranosidase,and o-acetylesterase

β-d-Glucopyranosidase (βG, EC 3.2.1.21) has been isolated from some collateral activities, α-l-arabinofuranosidase (Ara, EC3.2.1.55), α-l-rhamnopyranosidase (Rha, EC 3.2.1.40), and o-acetylesterase (Est, EC 3.1.1.53), using a commercial enzyme preparation and a simple method economically sustainable for the food industry. The procedure comprises precipitation of extraneous substances by adding ethanol and CaCl₂, ultrafiltration, and adsorption, first on bentonite and then on chitosan. The results obtained were the complete isolation of βG from the above-mentioned activities, a drastic reduction in extraneous compounds, such as brown substances and polysaccharides, and a slight increase in purification.     

Determination of locust bean gum and guar gum by polymerase chain reaction and restriction fragment length polymorphism analysis

A polymerase chain reaction (PCR) was developed to differentiate the thickening agents locust bean gum (LBG) and the cheaper guar gum in finished food products. Universal primers for amplification of the intergenic spacer region between trnL 3' (UAA) exon and trnF (GAA) gene in the chloroplast (cp) genome and subsequent restriction analysis were applied to differentiate guar gum and LBG. The presence of <5% (w/w) guar gum powder added to LBG powder was detectable. Based on data obtained from sequencing this intergenic spacer region, a second PCR method for the specific detection of guar gum DNA was also developed. This assay detected guar gum powder in LBG in amounts as low as 1% (w/w). Both methods successfully detected guar gum and/or LBG in ice cream stabilizers and in foodstuffs, such as dairy products, ice cream, dry seasoning mixes, a finished roasting sauce, and a fruit jelly product, but not in products with highly degraded DNA, such as tomato ketchup and sterilized chocolate cream. Both methods detected guar gum and LBG in ice cream and fresh cheese at levels <0.1%.     

Electron attachment processes in gas mixtures containing haloethanes

Electron attachment processes in the mixture of haloethanes with carbon dioxide and nitrogen have been investigated. The rate constants for two-body processes have been measured. A link between rate constants and electron polarizabilities was demonstrated.     

Colouring Steiner systems with specified block colour patterns

We consider colourings of Steiner systems S(2,3,v) and S(2,4,v) in which blocks have prescribed colour patterns, as a refinement of the classical weak colourings. The main question studied is, given an integer k, does there exist a colouring of given type using exactly k colours? For several types of colourings, a complete answer to this question is obtained while for other types, partial results are presented. We also discuss the question of the existence of uncolourable systems.     

Synthesis and isolation of iodocarbazoles. Direct iodination of carbazoles by N‐iodosuccinimide and N‐iodosuccinimide‐silica gel system

Carbazole ( 1 ) undergoes electrophilic aromatic substitution with various iodinating reagents. Although, 3‐iodocarbazole ( 1b ) and 3,6‐diiodocarbazole ( 1d ) obtained by iodination of carbazole were isolated and characterized sometime ago, 1‐iodocarbazole ( 1a ), 1,6‐diiodocarbazole ( 1c ) and 1,3,6‐triiodocarbazole ( 1e ) had never been isolated from the reaction mixture. The preparation and subsequent isolation and characterization of 1a, 1b, 1c, 1d and 1e are reported (mp, t_r, R_f, ¹H‐nmr, ¹³C‐nmr and ms). As iodinating reagents, NaIO₄/I₂ and NaIO₄/KI mixtures in (i) ethanol doped with catalytical amount of sulfuric acid and in (ii) acetic acid, and N‐odosuccinimide and N‐iodosuccinimide‐silica gel in dichloromethane and in chloroform have been used and their uses have been compared. The iodination reaction of different carbazole derivatives such as 2‐acetoxycarbazole ( 2 ), 3‐bromocarbazole (3) and 3‐nitrocarbazole ( 4 ) was also studied and the corresponding iododerivatives, 2a, 2b, 2c, 3a, 3b, 4a and 4b , are described for the first time. Semiempirical PM3 calculations have been performed in order to predict reactivity of carbazole ( 1 ), substituted carbazoles (2‐4) and iodocarbazoles ( 1a‐1e, 2a‐2c, 3a‐3b, 4a and 4b ) (Scheme 1). Theoretical and experimental results are discussed briefly.