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Sugars, Alkaloids, and Heteroaromatics: Exploring Heterocyclic Chemistry with Alkoxyallenes

admin — Tue, 12 May 2009 06:10:26 +0000

As master craftsmen, modern synthetic chemists are challenged to achieve remarkable feats of efficiency and elegance toward molecular targets. The nature of this pursuit necessitates the collection of synthetic repertoires that are tried and true. With methodologies and pathways increasingly scrutinized, the adept chemist must seek out propitious tools to incorporate into the arsenal. With this in mind, this Account highlights the versatility of alkoxyallenes as precursors to valuable heterocyclic building blocks for such efforts as natural product synthesis. Accessed by the etherification of either propargyl alcohols or propargylic halides, alkoxyallenes are obtained after base-catalyzed isomerizations of the propargylic ethers. A host of umpolung synthons are available through this scheme after metalation, generating C3 nucleophiles synthetically equivalent to vital anionic and zwitterionic synthons. Reactions with a diverse set of heteroatomic electrophiles yield carbohydrates, spiroketals, alkaloids, and heteroaromatics via [3 + 2] or [3 + 3] cyclizations. By employing lithiated alkoxyallenes into transformation routes, the natural product chemist can utilize this methodology as a viable resource in stereoselective synthesis. A survey of our own utilization of alkoxyallenes along synthetic pathways toward natural product targets reveals their suitability for generating advantageous precursors. A set of four stereoisomeric 2,6-dideoxyhexoses were stereoselectively obtained after an initial lithiated alkoxyallene and lactaldehyde cyclization, followed by the oxidative ring opening of the dihydrofurans. Through the addition of a lithiated alkoxyallene to a functionalized benzaldehyde, an essential spiroketal diastereomer was rapidly achieved in a few steps. We greatly benefitted from alkoxyallenes in the construction of complex nitrogen-containing synthetic targets, whether pyrrolidine alkaloids, substituted imidazole derivatives, or functionalized pyridines. A pinnacle example of their utility came from the coupling of alkoxyallenes to nitrones affording 1,2-oxazines, which served as a gateway to an array of novel polyfunctionalized compounds such as aminopolyols, hydroxylated pyrrolidines, or carbohydrate mimetics. Alkoxyallenes have proven themselves to be powerful C3 building blocks toward complex molecular targets, revealing novel pathways to a variety of desirable highly functionalized heterocycles. In our view, the full extent of their synthetic utility has yet to be truly realized.

Malte Brasholz?Hans-Ulrich Reissig?Reinhold Zimmer
Freie Universitt Berlin, Institut fr Chemie and Biochemie, Takustrasse 3, D-14195 Berlin, Germany

Servicing Modernity: White Women Shop Workers on the Rand and Changing Gendered Respectabilities

admin — Tue, 12 May 2009 06:09:53 +0000

The article discusses the gendered respectability, which was built into a labour process of shop work in South Africa. It offers a brief history of the interactions between white women and black service workers in the late 1960s and early 1970s. It is said that the National Union of Distributive Workers (NUDW) had protected the rights and reputability of white women workers. It is cited that the NUDW had raised public claims to employers and to their white customers, to defend for the legitimacy of women’s status as workers.

Kenny, Bridget1
[1]University of the Witwatersrand,

Structural Color Films with Lotus Effects, Superhydrophilicity, and Tunable Stop-Bands

admin — Tue, 12 May 2009 05:23:02 +0000

The structural blue color of a Morpho butterfly originates from the diffraction of light and interference effects due to the presence of the microstructures on the wing of the butterfly. Structural color on the surface of a damselfish reversibly changes between green and blue. Inspired by these creatures, we have been trying to prepare high-quality and functional structural color films. We describe our efforts in this Account. A useful technique to prepare such structural color films in colloidal solution is a �lifting� method, which allows us to quickly fabricate brilliant colloidal crystal films. The thicknesses of the films can be controlled by precisely adjusting the particle concentration and the lifting speed. Moreover, in order to prepare a complicated structure, we have used template methods. Indeed, we have successfully prepared the inverse structure of the wing of a Morpho butterfly with this technique. Initially, however, our structural color films had a whitish appearance due to the scattering of light by defects in the colloidal crystal film. Later, we were able to prepare a non-whitish structural color film by doping an appropriate dye in the colloidal particles to absorb the scattering light. In addition to the structural blue color, the wing of the Morpho butterfly has superhydrophobic properties. According to Wenzel�s equation, the hydrophobic and hydrophilic properties are enhanced when the roughness of the hydrophobic and hydrophilic surface is increased, respectively. Based on this mechanism, we have successfully prepared structural color films with superhydrophobic properties, as well as with superhydrophilic properties. Another important property that can be seen in nature is tunable structural color, such as the color change that can be seen on the surface of a damselfish. In order to mimic such color change, we have developed several tunable structural color films. In particular, we have successfully prepared phototunable photonic crystals using photoresponsive azobenzene derivatives. In order to apply these structural color films, we developed a technique for patterning them by taking advantage of the wettability of the substrate surface. These materials can be used in the future for self-cleaning pigments and tunable photonic crystals.

Osamu Sato#8224?Shoichi Kubo#8225?Zhong-Ze Gu�

Studies on effect of pH on cross-linking of chitosan with sodium tripolyphosphate: A technical note

admin — Mon, 02 Feb 2009 13:11:06 +0000

Conclusions The ionotropic gelation method for formation of crosslinked chitosan particles can be easily modified from ionic cross-linking to deprotonation by adjusting the pH of TPP. Chitosan was cross-linked ionically with TPP at lower pH; by deprotonation mechanism at higher pH. The swelling behavior of cross-linked chitosan appeared to depend on the pH of TPP. The ionically cross-linked chitosan showed higher swelling ability. Thus the nature of crosslinked chitosan can be tailor made to obtain the desired properties in terms of cross-linking density, crystallinity,; hydrophilicity.

Devika R. Bhumkar1?Varsha B. Pokharkar1Email:vbpokharkar@yahoo.co.in
[1] Department of Pharmaceutics, Poona College of Pharmacy, Bharati Vidyapeeth Deemed University, Erandwane, 411038 Pune, Maharashtra State, India

Spherical agglomeration of mefenamic acid; nabumetone to improve micromeritics; solubility: A technical note

admin — Mon, 02 Feb 2009 13:09:11 +0000

Conclusions Spherical agglomeration techniques were developed for improving the flow; compressibility characteristics of microcrystalline mefenamic acid; nabumetone. The process involved agglomerating microcrystals using agglomerating solvents. Temperature; speed of agitation were optimized to obtain spherical agglomerates in a desired range, which was found to be essential to enhance compressibility. Incorporation of polymer HPMC during agglomeration significantly enhanced the dissolution rate of mefenamic acid while incorporation of solubilizing agent lecithin improved the solubility of nabumetone. Thus, spherical agglomeration is an important technique for improving direct compressibility of pharmaceutical powders; is especially useful when the drug dosage is high.

Chelakara L. Viswanathan1Email:chelakara_viswanathan@yahoo.com?Sushrut K. Kulkarni1?Dhanashri R. Kolwankar1
[1] Bombay College of Pharmacy, Kalina, Santacruz ;[East], 400 098 Mumbai, India

admin — Wed, 17 Sep 2008 06:27:58 +0000

Bulk ceramic materials based on TiC�TiB2�MexOy systems have been produced by means of pressure-assisted self-propagating high-temperature synthesis (SHS). The selection of the SHS systems was based on thermodynamic analyses in order to obtain regimes of synthesis with formation of liquid phase allowing a proper densification. The liquid phase formed in the products consisted of either melted oxides or TiC�TiB2 eutectic depending on the combustion temperature. The mechanisms of structure formation and the effect of grain growth inhibition due to the metal oxide particles have been evaluated. The microstructural investigations have confirmed the role of the metal oxides as promoter of the final products densification and of the grain refinement in the SHS ceramic materials. A core�shell model is proposed to explain the structure formation of the SHS composites through a chemical interaction involving two stages. The good agreement between calculated and experimental results confirmed the validity of the core�shell mechanism.

D. VallauriaEmail:dario.vallauri@polito.it?V.A. Shcherbakovb?A.V. Khitevb?F.A. Deorsolaa
[a]Department of Material Science; Chemical Engineering, Politecnico di Torino, Torino, Italy;[b]Institute of Structural Macrokinetics; Materials Science, Russian Academy of Science, Chernogolovka, Russia

Simulation on the phase behavior of binary blends contacted with external surfaces under two-step quench conditions

admin — Wed, 17 Sep 2008 04:06:33 +0000

The morphology evolution of binary blends subjected to a two-step quench in the presence of external surfaces, such as a static substrate or immobile fillers, was studied using the cell dynamical system (CDS) simulation method. When the binary blends were first annealed at a shallow quench for a long time followed by a second further deep quench into a two-phase region, transient lamellar structures were formed in the blends with both critical and off-critical compositions. The occurrence of the lamellar structure was found to be correlative with the compositional difference between the separated phases formed during the first shallow quench as well as the quench depth of the second quench. For blends contacted with a selective substrate or fillers, well-defined layer structures or target-like compositional waves which were paralleled to the external surfaces were formed during the two-step quench. In these systems, the bulk lamellar structure was affected by the concentration of components and fillers in the blend. The simulation by CDS demonstrated that the combination of various quench conditions and external surfaces play an important role in the pattern formation of binary blend, and provide a new approach in tailoring the morphologies of binary blends in addition to the other available methods.

Yajiang Huanga?Rui Wanga?Yajie Suna?Guangxian LiaEmail:nic8601@scu.edu.cn?Guangling Chena?Qi Yanga
[a]College of Polymer Science; Engineering, Sichuan University, Chengdu 610065, PR China;[b]State Key Laboratory of Polymer Material; Engineering, Sichuan University, Chengdu 610065, PR China

Self-consistent scale transition with imperfect interfaces: Application to nanocrystalline materials

admin — Wed, 17 Sep 2008 04:01:04 +0000

A hierarchal scale transition technique is introduced to model the effect of imperfect interfaces on the elastoviscoplastic response of composite materials. This novel framework is based on a two-step procedure. In the first step, an inclusion is embedded in a matrix phase and the interface between the two phases is imperfect. The embedded inclusion is homogenized via the use of a Mori�Tanaka scheme. In a second step the homogenized inclusion is introduced in a matrix phase representing the homogeneous equivalent material, and the macroscopic response of the material is obtained via the self-consistent approximation. The model is applied to the case of pure nanocrystalline copper and allows the activity of grain boundary sliding to be quantified.

L. CapolungoaEmail:laurentc@lanl.gov?S. Benkassemc?M. Cherkaouia?J. Qua
[a]UMI 2958 GT-CNRS, Metz 57000, France;[b]George W. Woodruff School of Mechanical Engineering, Atlanta, GA 30332-0405, USA;[c]LPMM-CNRS Universit� Paul Verlaine, Ile du Saulcy, 57045 Metz Cedex 1, France

Structure of nanoindentations in heavily n- and p-doped (0 0 1) GaAs

admin — Wed, 17 Sep 2008 03:27:50 +0000

We have studied the nanoindentation structures achieved at room temperature (RT) on (0 0 1) GaAs with either n or p doping. Elastic�plastic nanoindentations were made over a wide range of loads (between 0.2 and 50 mN) at RT with a Berkovich indenter using two different orientations. Transmission electron microscopy was used to observe systematically the nanoindentation structures (central zone and rosette arms) and to investigate changes in dislocation activity. The mechanical response of both types of samples is relatively similar in terms of hardness, critical shear stress or pop-in load amplitude. In contrast, the indentation rosette structure appears to be sensitive to both doping and indenter orientation. Perfect dislocations show long screw segments only in n-doped specimens, a finding that is attributed to mobility effects. Moreover, p-doped specimens show no partial dislocations while n-doped specimens show partial dislocations in both rosette arms.

E. Le Bourhisa Email:eric.le.bourhis@univ-poitiers.fr?G. Patriarcheb
[a]Universit� de Poitiers, Laboratoire de M�tallurgie Physique, UMR 6630 CNRS, SP2MI-T�l�port 2-Bd Marie et Pierre Curie, BP 30179, 86962 Futuroscope-Chasseneuil Cedex, France;[b]Laboratoire de Photonique et de Nanostructures, UPR 20 CNRS, Route de Nozay, 91460 Marcoussis, France

Strong single-crystalline Au films tested by a new synchrotron technique

admin — Wed, 17 Sep 2008 03:26:35 +0000

Although it is well known that thin films exhibit mechanical properties very different from those of their bulk counterparts, knowledge of the underlying mechanisms is incomplete. Single-crystalline films have a favorable microstructure for investigating the scaling behavior of mechanical properties. We present a novel experimental route for preparing single-crystalline Au films on a compliant polyimide substrate. For such single-crystals, we have developed a synchrotron-based tensile testing technique to measure the isothermal stress�strain curves and average peak widths. The analysis of Laue diffraction patterns as well as a parallel transmission electron microscopy study give new insight in the initial and evolving microstructure of the films. Complex novel deformation mechanisms are found, including a transition of the dominant deformation mechanism from full to partial dislocations in films thinner than 160 nm. The scaling behavior is described in view of the coexistence of different deformation mechanisms where the nucleation stress for single dislocations very likely governs the behavior.

Patric A. Grubera?Christian Solenthalerb?Eduard Arzta?Ralph Spolenakb Email:ralph.spolenak@mat.ethz.ch
[a]Universitt Stuttgart, Institute of Physical Metallurgy, Heisenbergstrasse 3, D-70569 Stuttgart, Germany;[b]Laboratory for Nanometallurgy, Department of Materials, ETH Zurich, Wolfgang-Pauli-Strasse 10, 8093 Zurich, Switzerland;[c]Max Planck Institute for Metals Research, Heisenbergstrasse 3, D-70569 Stuttgart, Germany