A tandem metal carbene cyclization-cycloaddition approach to the pseudolaric acids

An approach toward the synthesis of the antifungal and cytotoxic pseudolaric acids based on the tandem metal carbene cyclization-cycloaddition reaction is described. Using this strategy, the advanced intermediate 3a bearing three of the four stereocenters of the target molecules has been synthesized. The substrate-controlled diastereoselectivity of the tandem carbene cyclization-cycloaddition was preferential for the undesired diastereomer, but reagent control through the use of Hashimoto's chiral rhodium catalyst Rh(2)(S-BPTV)(4) reversed the selectivity in favor of 3a. Ring opening of the oxabicyclic nucleus to give a hydroxycycloheptene has been demonstrated in a model study.     

A convenient,one-step synthesis of optically active tertiary aminonaphthol and its applications in the highly enantioselective alkenylations of aldehydes

Optically active tertiary aminonaphthol 1 was obtained by a new, convenient procedure and was found to catalyze the enantioselective alkenylation of various aldehydes with high ee values, which provides a practical method for the synthesis of chiral (E)- allyl alcohols.     

New nanotube synthesis strategy--application of sodium nanotubes formed inside anodic aluminium oxide as a reactive template

Formation of Na nanotubes inside the channels of anodic aluminium oxide (AAO) membranes has been achieved by decomposing NaH thermally on AAO. The as-produced material, Na@AAO, is applied as a reactive template to prepare other tubular materials. Reacting Na@AAO with gaseous C6Cl6 generates carbon nanotubes (ca. 250 nm, wall thickness of 20 nm, tube length of 60 microm) inside the AAO channels. Highly aligned bundles of nearly amorphous carbon nanotubes are isolated after AAO is removed.     

pK(a) values and geometries of secondary and tertiary amines complexed to boronic acids-implications for sensor design

[structure in text] The pK(a) values and the geometries of secondary and tertiary amines adjacent to boronic acids were determined using potentiometric and (11)B NMR titrations. The studies showed that the secondary ammonium ion has a pK(a) similar to that of the tertiary ammonium species, which leads to the formation of tetrahedral boron centers at pH values above approximately 5.5. Therefore, secondary amines as well as tertiary amines, when placed proximal to boron centers, can be used to create tetrahedral boronic acids at neutral pH for diol complexation.     

Surface profile measurement using laser-scanning angle deviation microscopy

Laser-scanning angular deviation microscopy based on the small angle measurement using surface plasmon resonance (SPR) phase detection technique is proposed. The phase shift coming from a SPR sensor is measured by a common-path heterodyne interferometry. This phase deviation is proportional to the beam converging or diverging angle, due to the specimen departing from the focal plane of objective lens. Using the phase deviations, one can calculate the surface profile by use of numerical method. The specimen could be scanned in real-time and the axial-resolution could be better than 1 nm. PACS 07.79-v; 68.37.-d; 73.20.Mf; 42.30.Wb; 42.62.Eh     

Complete suppression of metastable phase and significant enhancement of magnetic properties of B-rich PrFeB nanocomposites prepared by devitrifying amorphous ribbons

The effect of refractory element addition on phase transformation, crystallization behavior and magnetic properties of Pr_8.5Fe_81.5B₁₀ (addition-free) and Pr_8.5Fe_81.5M₂B₁₀ (M=V, Cr, Nb, Zr, Ti) ribbons has been investigated. The annealed addition-free ribbon as well as the samples with V or Cr additions are mainly composed of the metastable Pr₂Fe₂₃B₃ phase, whereas annealed ribbons with Nb, Zr or Ti additions primarily consist of Pr₂Fe₁₄B and a minor amount of Fe₃B/boride. The complete suppression of the metastable Pr₂Fe₂₃B₃ phase due to Nb, Zr or Ti additions leads to a significant enhancement of the magnetic properties. For example, the remanence, the coercivity and the energy product are remarkably increased from 2.5 kG, 0.4 kOe and 0.2 MG Oe for the addition-free material to 9.2 kG, 4.7 kOe and 7.6 MG Oe for the specimens with Nb addition. The successful elimination of the metastable Pr₂Fe₂₃B₃ phase is believed to profit from two factors: (a) Nb, Zr or Ti atoms substitute the Pr site, comparatively increase the Pr content, and thus inhibit the nucleation of Pr-lean Pr₂Fe₂₃B₃ phases, and (b) the formation of Nb, Zr, or Ti borides consumes some part of B, which hinders the generation of the B-rich Pr₂Fe₂₃B₃ phase.     

Locating the two-median of a tree network with continuous link demands

Typical formulations of thep-median problem on a network assume discrete nodal demands. However, for many problems, demands are better represented by continuous functions along the links, in addition to nodal demands. For such problems, optimal server locations need not occur at nodes, so that algorithms of the kind developed for the discrete demand case can not be used. In this paper we show how the 2-median of a tree network with continuous link demands can be found using an algorithm based on sequential location and allocation. We show that the algorithm will converge to a local minimum and then present a procedure for finding the global minimum solution.     

Rate enhancement for nitroxide stable free radical polymerization of n‐butyl acrylate by dodecylbenzenesulfonic acid

Dodecylbenzenesulfonic acid, DDBSA, was chosen as a new rate‐accelerating additive for 2,2,6,6,‐teramethyl‐1‐piperidinyloxy (TEMPO)‐mediated stable free radical polymerization of n‐butyl acrylate (n‐BA) monomers with 2,2′‐azobisisobutyronitrile (AIBN). It was found that the number‐average molecular weight of polymers could reach about ten thousand with a narrow polydispersity index (PDI) of 1.4 in a few minutes, which was faster than other systems reported previously. But, at higher conversion, the molecular weight distribution of polymers became broad, and a bimodal distribution occurred. The macro‐initiators isolated from the former polymers with narrow PDI could be extended by polymerization with monomers by the addition of DDBSA. Furthermore, a proposed kinetic model demonstrated that the decay of the concentration of DDBSA would reduce the living polymer concentration and retard the growth of the polymers, which could be further propagated by the supplement of DDBSA. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 42–49, 2005