A novel simplified synthesis of acivicin

This report describes an efficient synthesis of the natural isomer of acivicin, which is the only one provided with a noteworthy biological activity. The present procedure allowed the synthesis of (+)-1 in just five steps with a 34% overall yield. Due to the easy separation of the two diastereomers and to the availability of the starting material at low cost, the present procedure can be scaled-up to gram quantities.     

Study of chain branching in natural rubber using size-exclusion chromatography coupled with a multi-angle light scattering detector (SEC-MALS)

The different rheological behaviour of natural rubber (NR) compared to industrial synthetic poly(cis-1,4-isoprene) (SR) has been attributed to the gel phase and long-chain branching. Previous studies on branching in NR were carried out using the fractionation technique by precipitation to obtain narrow molar mass distribution. In this study, chain branching of poly(cis-1,4-isoprene) in NR was characterised by size-exclusion chromatography coupled with an online multi-angle light scattering detector (SEC-MALS). The nanoaggregates adsorbed on the column packing interfered with branching characterisation for short and medium chains (M_w < 1000 kg/mol). Using a master curve of linear standard poly(cis-1,4-isoprenes), SEC-MALS revealed no or very little branching in the higher chains (1000 < M_w < 10,000 kg/mol) of natural rubber contrary to previous studies. This study showed that the soluble portion of NR samples was composed of almost linear poly(cis-1,4-isoprene) and nanoaggregates with rather compact structures.     

Furanoside phosphite–phosphoroamidite and diphosphoroamidite ligands for Cu-catalyzed asymmetric 1,4-addition reactions

A sugar-based phosphite–phosphoroamidite and diphosphoroamidite ligand library L1–L5a–g was tested in the asymmetric Cu-catalyzed 1,4-conjugate addition reactions of β-substituted and β,β′-disubstituted enones. Our results indicated that the selectivity was strongly dependent on the ligand parameters and on the substrate structure. Moderate-to-good enantioselectivities (ees up to 84%) were obtained in the 1,4-addition of several types of β-substituted cyclic and linear substrates. Of particular note is the high enantioselectivity (ees up to 90%) obtained for the more challenging β,β′-disubstituted 3-methyl-cyclohexenone.     

Classification of Subsystems for Local Nets¶with Trivial Superselection Structure

Let ℱ be a local net of von Neumann algebras in four spacetime dimensions satisfying certain natural structural assumptions. We prove that if ℱ has trivial superselection structure then every covariant, Haag-dual subsystem ℬ is of the form ℱ₁ ^G ⊗I for a suitable decomposition ℱ=ℱ₁⊗ℱ₂ and a compact group action. Then we discuss some application of our result, including free field models and certain theories with at most countably many sectors. Received: 26 January 2000 / Accepted: 28 September 2000     

Smoothed hat functions in subdivision

Following the general idea of smoothing through convolution, we discuss schemes for smoothing hat functions on the three-directional and the four-directional mesh. The smoothing masks–which in the four-directional case are matrix masks–have the same support as the refinement mask for the hat functions. Several smoothing schemes are presented, and in some cases the smoothness properties of the smoothed functions are analysed. In particular, the results for the four-directional case follow new and original lines of research.     

High Q silica microbubble resonators fabricated by arc discharge

Microbubble resonators combine the unique properties of whispering gallery mode resonators with the intrinsic capability of integrated microfluidics. Here an improved fabrication method of microbubble resonators is presented, based on the heating of a slightly pressurized capillary by a rotating arc discharge. Rotation of the electrodes, moved out of a fiber splicer, ensures a homogeneous distribution of the heat all over the capillary surface. The demonstrated microbubble resonators have Q factors up to 6×10(7) at 1550?nm. Microbubbles were filled with water and aqueous solutions of ethanol in order to test the refractive index sensing capabilities of such resonators, which also show a good temporal stability. The limit of detection of our microbubble resonator sensor is 10(-6)?RIU.     

Cooperative recognition of Cu<Superscript>2+</Superscript> based on amino acids tethered benzothiadiazoyl-bistriazoles

Fluoroionophoric properties of benzothiadiazoyl-bistriazoyl amino acids derivatives toward metal ions were investigated by UV-Vis and fluorescence spectroscopy. Our results show that the watersoluble lysine derivative 2 exhibited a significant selectivity toward Cu²⁺ in “on-off” type response in buffer solution at pH 7.4.     

Compatibility Range in Polymer Mixtures. An approach using analogue calorimetry and group contribution procedures

The mixing enthalpies of blends of polymethylmethacrylate (PMMA) with poly(styrene-co-acrylonitrile) (SAN) were investigated by analogue calorimetry through the determination of the excess enthalpies of pseudobinary model mixtures corresponding to the addition of methyl-i-butyrate to a binary mixture of acetonitrile or propionitrile plus toluene or ethylbenzene. A group contribution procedure, based on UNIQUAC equation, was also devised and the polymeric mixing enthalpies were calculated from properly defined group contributions. Enthalpies for polymeric interactions were introduced into the Flory-Huggins equation and the miscibility window of PMMA-SAN mixtures was calculated. The results show a qualitative agreement with the experimental miscibility data and indicate that both the analogue calorimetry and the group contribution procedures yield correct results when acetonitrile, and not propionitrile, is chosen as the model for the polyacrylonitrile repeat unit of the copolymer. This revised version was published online in July 2006 with corrections to the Cover Date.