The preparation of 2,6-diaminopyrazine, 2,6-diazidopyrazine and some of their derivatives

A much improved synthesis of the heretofore difficultly obtainable 2,6-diaminopyrazine (4) was afforded by the low-pressure catalytic hydrogenation (palladium on carbon) of 2,6-diazido-pyrazine (2) ; reaction of 2,6-dichloropyrazine (1) and sodium azide gave 2 in 84% yield. The outcome of the reduction was found to be solvent dependent: 1,2-dimethoxyethane containing aqueous ammonia gave 4 in 83% yield; 1,2-dimethoxyethane alone gave 5-aminotetrazolo[1,5-a]-pyrazine (3) in 26% yield. Additional alternative syntheses of 3 and 4 are described. A number of acyl and azo derivatives of 4 were prepared. Reactions of 2 with dimethyl acetylenedicarboxylate and ethyl acetate (base catalyzed) leading to vic-triazole derivatives are also described.     

Preparation of 3-methyl-4-vinyl-1,2,5-oxadiazole and 3-methyl-4-vinyl-1,2,5-thiadiazole

Short and high yielding preparations of 3-Methyl-4-vinyl-1,2,5-oxadiazole ( 6a ) and 3-Methyl-4-vinyl-1,2,5-thiadiazole ( 6b ) are described.     

Electroosmotic flow in channels with step changes in zeta potential and cross section

We consider the effects that step changes in zeta potential and cross section have on electroosmosis in long-and-narrow channels with arbitrary cross-sectional shapes. The Stokes equation of flow is solved analytically utilizing the thin Debye layer approximation to provide effective slip velocities on the channel walls. The effects of channel dimensions, surface potentials, applied pressure drop, and applied voltage are discussed. One anecdotal case, a two-region rectangular channel, is presented to illustrate the solution. The flow in each region is a combination of a uniform electroosmotic flow and a nonuniform pressure-driven flow. The electroosmotic pumping causes the pressure gradient in each region to adjust so that the flow rate is the same in each region and the overall applied pressure drop is met, resulting in convex velocity profiles in some regions and concave velocity profiles in other regions. By appropriate choice of the applied pressure drop, flat velocity profiles may be achieved in one or more regions.     

Aldehyde-functional thermoresponsive diblock copolymer worm gels exhibit strong mucoadhesion

A series of thermoresponsive diblock copolymer worm gels is prepared via reversible addition–fragmentation chain transfer (RAFT) aqueous dispersion polymerization of 2-hydroxypropyl methacrylate using a water-soluble methacrylic precursor bearing pendent cis-diol groups. Selective oxidation using an aqueous solution of sodium periodate affords the corresponding aldehyde-functional worm gels. The aldehyde groups are located within the steric stabilizer chains and the aldehyde content can be adjusted by varying the periodate/cis-diol molar ratio. These aldehyde-functional worm gels are evaluated in terms of their mucoadhesion performance with the aid of a fluorescence microscopy-based assay. Using porcine urinary bladder mucosa as a model substrate, we demonstrate that these worm gels offer a comparable degree of mucoadhesion to that afforded by chitosan, which is widely regarded to be a ‘gold standard’ positive control in this context. The optimum degree of aldehyde functionality is approximately 30%: lower degrees of functionalization lead to weaker mucoadhesion, whereas higher values compromise the desirable thermoresponsive behavior of these worm gels.

Optimizing the aldehyde content of thermoresponsive diblock copolymer worm gels via periodate oxidation leads to mucoadhesion performance comparable to that of chitosan (a gold standard positive control) in a fluorescence assay using porcine mucosa.     

Effective,thermal one-carbon + two-carbon cycloaddition of cyclopropenone ketals with electron-deficient olefins: Cyclopropane formation.

Treatment of olefins bearing one electron-withdrawing substituent with cyclopropenone ketals under mild, thermal conditions provide cyclopropane products.     

The function of Shp2 tyrosine phosphatase in the dispersal of acetylcholine receptor clusters

Background

Tone languages such as Thai and Mandarin Chinese use differences in fundamental frequency (F₀, pitch) to distinguish lexical meaning. Previous behavioral studies have shown that native speakers of a non-tone language have difficulty discriminating among tone contrasts and are sensitive to different F₀ dimensions than speakers of a tone language. The aim of the present ERP study was to investigate the effect of language background and training on the non-attentive processing of lexical tones. EEG was recorded from 12 adult native speakers of Mandarin Chinese, 12 native speakers of American English, and 11 Thai speakers while they were watching a movie and were presented with multiple tokens of low-falling, mid-level and high-rising Thai lexical tones. High-rising or low-falling tokens were presented as deviants among mid-level standard tokens, and vice versa. EEG data and data from a behavioral discrimination task were collected before and after a two-day perceptual categorization training task.

Results

Behavioral discrimination improved after training in both the Chinese and the English groups. Low-falling tone deviants versus standards elicited a mismatch negativity (MMN) in all language groups. Before, but not after training, the English speakers showed a larger MMN compared to the Chinese, even though English speakers performed worst in the behavioral tasks. The MMN was followed by a late negativity, which became smaller with improved discrimination. The High-rising deviants versus standards elicited a late negativity, which was left-lateralized only in the English and Chinese groups.

Conclusion

Results showed that native speakers of English, Chinese and Thai recruited largely similar mechanisms when non-attentively processing Thai lexical tones. However, native Thai speakers differed from the Chinese and English speakers with respect to the processing of late F₀ contour differences (high-rising versus mid-level tones). In addition, native speakers of a non-tone language (English) were initially more sensitive to F₀ onset differences (low-falling versus mid-level contrast), which was suppressed as a result of training. This result converges with results from previous behavioral studies and supports the view that attentive as well as non-attentive processing of F₀ contrasts is affected by language background, but is malleable even in adult learners.     

Secondary ion yields produced by keV atomic and polyatomic ion impacts on a self-assembled monolayer surface

A suite of keV polyatomic or 'cluster' projectiles was used to bombard unoxidized and oxidized self-assembled monolayer surfaces. Negative secondary ion yields, collected at the limit of single ion impacts, were measured and compared for both molecular and fragment ions. In contrast to targets that are orders of magnitude thicker than the penetration range of the primary ions, secondary ion yields from polyatomic projectile impacts on self-assembled monolayers show little to no enhancement when compared with monatomic projectiles at the same velocity. This unusual trend is most likely due to the structural arrangement and bonding characteristics of the monolayer molecules with the Au(111). Copyright 1999 John Wiley & Sons, Ltd.     

Synthesis of 5-Iodo-1,4-disubstituted-1,2,3-triazoles Mediated by in Situ Generated Copper(I) Catalyst and Electrophilic Triiodide Ion

Mixing copper(II) perchlorate and sodium iodide solutions results in copper(I) species and the electrophilic triiodide ions, which collectively mediate the cycloaddition reaction of organic azide and terminal alkyne to afford 5-iodo-1,4-disubstituted-1,2,3-triazoles. One molar equivalent of an amine additive is required for achieving a full conversion. Excessive addition of the amine compromises the selectivity for 5-iodo-1,2,3-triazole by promoting the formation of 5-proto-1,2,3-triazole. Based on preliminary kinetic and structural evidence, a mechanistic model is formulated in which a 5-iodo-1,2,3-triazole is formed via iodination of a copper(I) triazolide intermediate by the electrophilic triiodide ions (and possibly triethyliodoammonium ions). The experimental evidence explains the higher reactivity of the in situ generated copper(I) species and triiodide ion in the formation of 5-iodo-1,2,3-triazoles than that of the pure forms of copper(I) iodide and iodine.     

In situ fabrication of macroporous polymer networks within microfluidic devices by living radical photopolymerization and leaching

Novel fabrication techniques and polymer systems are being explored to enable mass production of low cost microfluidic devices. In this contribution we discuss a new fabrication scheme for making microfluidic devices containing porous polymer components in situ. Contact lithography, a living radical photopolymer (LRPP) system and salt leaching were used to fabricate multilayer microfluidic devices rapidly with various channel geometries and covalently attached porous polymer plugs made of various photopolymerizable substrates. LRPP systems offer the advantages of covalent attachment of microfluidic device layers and facile surface modification via grafting. Several applications of the porous plugs are also explored, including a static mixer, a high surface area-to-volume reactor and a rapidly responding hydrogel valve. Quantitative and qualitative data show an increase in mixing of a fluorescein and a water stream for channels containing porous plugs relative to channels with no porous plugs. Confocal laser scanning microscopy images demonstrate the ability to graft a functional material onto porous plug surfaces. A reaction was carried out on the grafted pore surfaces, which resulted in fluorescent labelling of the grafted material throughout the pores of the plug. Homogenous fluorescence throughout the depth of the porous plug and along pore surfaces indicated that the porous plugs were surface modified by grafting and that reactions can be carried out on the pore surfaces. Finally, porous hydrogel valves were fabricated which swelled in response to contact with various pH solutions. Results indicate that a porous hydrogel valve will swell and close more rapidly than other valve geometries made with the same polymer formulation. The LRPP-salt leaching method provides a means for rapidly incorporating porous polymer components into microfluidic devices, which can be utilized for a variety of pertinent applications upon appropriate selection of porous plug materials and surface treatments.     

Fused s-triazino heterocycles. VIII. 1,3,4,6,9b-pentaazaphenalenes. Reactions of a methyl and bromomethyl side chain

Enhanced reactivity of the methyl group of 2-t-butyl-5-methyl-1,3,4,6,9b-pentaazaphenalene allowed acetic anhydride-catalyzed condensation reactions with several aromatic aldehydes, and base-catalyzed alkylation reactions with several alkyl halides to take place, albeit in low yields. Of the many nucleophiles tried, only salts of carboxylic acids, in the presence of 18-crown-6, were able to displace bromine from 2-(bromomethyl)-5-methyl-1,3,4,6,9b-pentaazaphenalene.