New homochiralortho-palladated matrix bearing a bulky substituent at the carbon stereocenter

A new homochiral dimericortho-palladated complex bearing a bulkytert-butyl substituent at the carbon stereocenter was synthesized from optically activeN,N-dimethyl-α-tert-butylbenzylamine. Regioselective activation of only the aromatic C−H bond was shown to occur during the cyclometallation process proceeding under very mild conditions due to steric effects. Spectral characteristics of mononuclear derivatives of the new dimeric complex indicate that the five-membered palladacycle exists predominantly in one of two possible chiral conformations with the axial position of thetert-butyl substituent. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 7, pp. 1375–1384, July, 1997.     

Optical Properties of Model Compounds of Polyaniline

The UV and visible spectra of six model compounds were studied. These compounds can be considered as models of polyaniline in the reduced, cation radical, partially oxidized and oxidized forms. After treatment of a mixture of equal molar quantities of the reduced form (DPPD) and the oxidized form (QDIM) with acid, the following reaction was observed: DPPD(I)+QDIM(III)+2H⁺=2DPPD⁺(II) After similar treatment of the partially oxidized form (V) with acid, the radical cation salt formed. The UV and visible spectra of polyaniline in the reduced form, oxidized form and conductive form are similar to the spectra of DPPD, QDIM and DPPD⁺ or radical cation salt of V respectively. We propose that the polyaniline synthesized by chemical oxidation of aniline consists of oxidized and reduced repeat units. Upon protonation a redox reaction (or electron rearrangement) occurs and forms delocalized radical cations (polarons) in the polymer chain which are highly conductive.     

Recent development in HPLC enantioseparation — a selected review

Summary Recent developments in the field of separating optical isomers by HPLC are discussed in this brief review. In particular it is focussed on various techniques for direct enantioseparation, the interpretation of possible enantioselective separation mechanisms, and the advantages and limitations of the diverse methods. This goal is supported by representative and selected examples of chiral separations.     

Mechanism of silicon film deposition in the RF plasma reduction of silicon tetrachloride

Plasma-chemical reduction of SiCl₄ in mixtures with H₂ and Ar has been studied by optical emission spectroscopy (OES) and laser interferometry techniques. It has been found that the Ar:H₂ ratio strongly affects the plasma composition as well as the deposition (r _D) and etch (r _E) rates of Si: H, Cl films and that the electron impact dissociation is the most important channel for the production of SiCl_x species, which are the precursors of the film growth. Chemisorption of SiCl_x and the reactive surface reaction SiCl_x+H–SiCl_(x–1)0+HCl are important steps in the deposition process. The suggested deposition model givesr _D [SiCl_x][H], in agreement with the experimental data. Etching of Si: H, Cl films occurs at high Ar: H₂ ratio when Cl atoms in the gas phase become appreciable and increases with increasing Cl concentration. The etch rate is controlled by the Cl atom chemisorption step.     

Synthesis and optical resolution of a series of inherently chiral calix[4]crowns with cone and partial cone conformations

A series of inherently chiral calix[4]arenes with cone and partial cone conformations and with crown ether moieties of variable size have been readily synthesized. By taking advantage of the carboxy appendage on the lower rim, these were condensed with the chiral auxiliary (S)-BINOL to form diastereomers which, in most cases, could be separated by preparative TLC, or more desirably, by column chromatography on silica gel (diastereomeric excess >99 % based on HPLC analysis). Seven enantiopure antipodes of inherently chiral calix[4]crowns were obtained after hydrolysis. It has been found that both the size of the crown moiety and alkylation of the last phenolic hydroxy group (accompanied with or without a change in the conformation) affect the separation of the diastereomers.     

Charge transfer complexes and salts of ionenes

A review of the synthesis and properties of ionenes is presented. Particular attention is paid to the electrical and related properties of conductive teteracyano-p-quinodimethane (TCNQ) complex salts of ionenes containing ammonium or sulfonium groups in the main chain and to their chemical stability. The recent work on preparation of such ionenes with inorganic and organic counterions which show interesting linear and nonlinear optical properties is also discussed. It is shown that in the solid state ionenes act as a kind of a matrix fixing, in a chemically controlled way, positions and distribution of different kinds of counterions. Because the reaction of counterion exchange is easy in the case of ionenes it provides a convenient way to prepare a broad class of polyelectrolytes with many different properties.     

A New Chemo‐Enzymatic Route to Side‐Chain Liquid‐Crystalline Polymers: The Synthesis and Polymerization of 6‐(4‐Methoxybiphenyl‐4′‐oxy)hexyl Vinyl Hexanedioate

A novel synthetic method combining chemo and enzymatic synthesis strategies was employed to prepare a vinyl acetate type monomer, 6‐(4‐methoxybiphenyl‐4′‐oxy)hexyl vinyl hexanedioate (VA‐LC). Homo‐ and copolymers of VA‐LC with maleic anhydride (MAn) were prepared by conventional free radical polymerization using 2,2′‐azobisisobutyronitrile (AIBN) and 1,1′‐azobis (cyclohexane carbonitrile) (AHCN) as an initiator at 95 and 60 °C, respectively. The thermal properties of the generated polymeric material were investigated by differential scanning calorimetry (DSC), and the optical texture was inspected by polarizing optical microscopy (POM). While the monomer VA‐LC does not exhibit liquid‐crystalline properties, poly(VA‐LC), and the alternating copolymer of VA‐LC with maleic anhydride both displayed such properties.

     

Conformation of Tax-response elements in the human T-cell leukemia virus type I promoter

Background: HTLV I Tax is believed to activate viral gene expression by binding bZIP proteins (such as CRIB) and increasing their affinities for proviral THE target sites. Each 21 by THE target site contains an imperfect copy of the intrinsically bent CRE target site (the TRE core) surrounded by highly conserved flanking sequences. These flanking sequences are essential for maximal increases in DNA affinity and transactivation, but they are not, apparently, contacted by protein. Here we employ non-denaturing gel electrophoresis to evaluate TRE conformation in the presence and absence of bZIP proteins, and to explore the role of DNA conformation in viral transactivation.Results: Our results show that the TRE-1 flanking sequences modulate the structure and modestly increase the affinity of a CREB bZIP peptide for the TRE-1 core recognition sequence. These flanking sequences are also essential for a maximal increase in stability of the CREB-DNA complex in the presence of Tax.Conclusions: The CRE-like TRE core and the TRE flanking sequences are both essential for formation of stable CRIB-TRE-1 and Tax-CREB-TRE-1 complexes. These two DNA segments may have co-evolved into a unique structure capable of recognizing Tax and a bZIP protein.     

Trifluoromethyl- and difluoromethyl-β-lactams as useful building blocks for the synthesis of fluorinated amino acids, dipeptides, and fluoro-taxoids

Enantiopure 1-acyl-3-hydroxyl-4-CF₂H-azetidin-2-ones and 1-acyl-3-hydroxy-4-CF₃-azetidin-2-ones serve as versatile intermediates for the syntheses of CF₂- and CF₃-containing α-hydroxy-β-amino acids, dipeptides, and taxoid anticancer agents. Both enantiomers of 3-hydroxy-4-CF₂H-β-lactams can be obtained in high yields through the diethylaminosulfur trifluoride (DAST) reaction of the corresponding enantiopure 4-formyl-β-lactam that is prepared through [2+2] cycloaddition of acetoxyketene to a 3-methyl-2-butenaldimine, followed by enzymatic optical resolution and ozonolysis. (+)-3-Hydroxy-4-CF₃-β-lactams and (−)-3-hydroxy-4-CF₃-β-lactams can also be readily obtained in enantiopure form through [2+2] cycloaddition of a CF₃-imine with a ketene, followed by enzymatic optical resolution. Practical processes for the preparations of these enantiopure 3-hydroxy-4-R_f-β-lactams as well as their synthetic applications are described.     

Optimization of Ultrasonic-Assisted Extraction of Active Components and Antioxidant Activity from Polygala tenuifolia: A Comparative Study of the Response Surface Methodology and Least Squares Support Vector Machine

Dried roots of Polygala tenuifolia (YuanZhi in Chinese) are widely used in Chinese herbal medicine. These components in YuanZhi have significant anti-oxidation properties owing to high levels of 3,6’-disinapoylsucrose (DISS) and Polygalaxanthone III (PolyIII). In order to efficiently extract natural medicines, response surface methodology (RSM) and least squares support vector machine (LSSVM) were used for the modeling and optimization of ultrasound-assisted extraction of DISS and PolyIII together to determine the antioxidant activity of the extracts obtained from YuanZhi. For the optimal combination of the comprehensive yield of DISS and PolyIII (Y), the Box-Behnken design (BBD) was used to improve extraction time (X₁), extraction temperature (X₂), liquid–solid ratio (X₃), and ethanol concentration (X₄). The optimal process parameters were determined to be as follows: extraction time, 93 min; liquid–solid ratio, 40 mL/g; extraction temperature, 48 °C; and ethanol concentration, 67%. With these conditions, the predictive optimal combination comprehensive evaluation value is 13.0217. It was clear that the LS-SVM model had higher accuracy in predictive and optimization capabilities, with higher antioxidant activity and lower relative deviations values, than did RSM. Hence, the LS-SVM model proved to be more effective for the analysis and improvement of the extraction process.