Asymmetric synthesis of highly functionalized γ-lactams through an organocatalytic aza-Michael–Michael reaction cascade using fumaric acid amide esters as multi-reactive substrates

We developed a novel method for the asymmetric synthesis of highly functionalized γ-lactams through an organocatalytic aza-Michael–Michael reaction cascade using fumaric acid amide esters as multi-reactive substrates. Using chiral primary or secondary amine organocatalysts, we obtained two types of γ-lactams with three contiguous chiral centers in moderate to good yield with excellent enantioselectivity and diastereoselectivity.     

Uniaxial elongational viscosity of PS/a small amount of UHMW-PS blends

A series of polystyrene (PS) and a small amount of ultra high molecular weight (UHMW) PS blends have been prepared by using tetrahydrofuran (THF). Matrix PS has an M_w of 423,000 (M_w/M_n= 2.36) and UHMW-PS has either an M_w of 3,220,000 (M_w/M_n= 1.05) or 15,400,000 (M_w/M_n=1.30) in the range of concentration from 0 wt% to 1.5 wt%. The influence of a small amount of UHMW on dynamic viscoelasticity was investigated. At the frequency lower than 0.001 rad/s, the enhancement of G′ was observed by the incorporation of a small amount of UHMW. And the degree of enhancement was in the order of M_w of UHMW and its concentration. The measurement of uniaxial elongational viscosity for the blends was performed and the effects of UHMW on strain-hardening properties were analyzed at equal strain-rate conditions. The concentration of UHMW where the strain-hardening becomes substantially stronger was determined. To get more insight into the cause of enhancement of strain-hardening at a certain concentration, the damping function from step-shear stress relaxation was measured. The influence of a small amount of UHMW on the damping function was found to be small. It was interpreted, from time- and strain-dependency points, that the enhancement of strain-hardening by a small amount of UHMW was governed by the long relaxation time. Received: 6 September 2000 Accepted: 11 January 2001     

Temperature-sensitive photoluminescence of CdSe quantum dot clusters

The formation of narrow size dispersed and nanometer size aggregates (clusters) of cadmium selenide (CdSe) quantum dots (QDs) and their temperature-sensitive photoluminescence (PL) spectral properties close to room temperature (298 K) are discussed. CdSe QDs formed stable clusters with an average diameter of approximately 27 nm in the absence of coordinating solvents. Using transmission electron microscopy (TEM) imaging, we identified the association of individual QDs with 2-5 nm diameters into clusters of uniform size. A suspension of these clusters in different solvents exhibited reversible PL intensity changes and PL spectral shifts which were correlated with temperature. Although the PL intensity of CdSe QDs encapsulated in host matrixes and the solid state showed a response to temperature under cryogenic conditions, the current work identified for the first time QD clusters showing temperature-sensitive PL intensity variations and spectral shifts at moderate temperatures above room temperature. Temperature-sensitive reversible PL changes of clusters are discussed with respect to reversible thermal trapping of electrons at inter-QD interfaces and dipole-dipole interactions in clusters. Reversible luminescence intensity variations and spectral shifts of QD clusters show the potential for developing sensors based on QD nanoscale assemblies.     

Total Synthesis of Ciguatoxin

Something fishy : Ciguatoxin (see structure) is one of the principal toxins involved in ciguatera poisoning and the target of a total synthesis involving the coupling of three segments. The key transformations in this synthesis feature acetylene–dicobalthexacarbonyl complexation.

     

Friedel-Crafts-Type Acylation and Amidation Reactions in Strong Brønsted Acid: Taming Superelectrophiles

In this review, we discuss Friedel-Crafts-type aromatic amidation and acylation reactions, not exhaustively, but mainly based on our research results. The electrophilic species involved are isocyanate cation and acylium cation, respectively, and both have a common ⁺C=O structure, which can be generated from carboxylic acid functionalities in a strong Brønsted acid. Carbamates substituted with methyl salicylate can be easily ionized to the isocyanate cation upon (di)protonation of the salicylate. Carboxylic acids can be used directly as a source of acylium cations. However, aminocarboxylic acids are inert in acidic media because two positively charged sites, ammonium and acylium cation, will be generated, resulting in energetically unfavorable charge-charge repulsion. Nevertheless, the aromatic acylation of aminocarboxylic acids can be achieved by using tailored phosphoric acid esters as Lewis bases to abrogate the charge-charge repulsion. Both examples tame the superelectrophilic character.     

The aminomethylation of electron-rich aromatics with an N-silyl-N,O-acetal catalyzed by a metal triflate-TMSCl system: facile synthesis of aromatic primary amines, 1-aryl-trichloroethylamines

The copper(II) triflate- and hafnium(IV) triflate-catalyzed aminomethylation of indole (2) with an N-silyl-N,O-acetal 1 containing a trichloromethyl group provides the primary amine derivative (3a) in modest yield. When 1 equiv of trimethylchlorosilane (TMSCl) was added to the reaction mixture, the reaction proceeded smoothly, and the yield of 3a was dramatically improved (>90%). The use of this catalytic system permitted the introduction of an aminomethyl group onto indoles 2a-h bearing a variety of functional groups, which appears to deactivate the Lewis acid, in 52-92% yields. Hf(OTf)4-doped TMSCl catalyzed the successful aminomethylation of various electron-rich aromatic compounds 4a-j to produce 1-aryl-trichloroethylamine derivatives 5a-j.     

Synthesis of (1-azabicyclo[3.3.0]octanyl)methyl-substituted aromatic heterocycles and their muscarinic activity.

In our development of drugs effective against Alzheimer's disease, we have researched a series of aromatic compounds having a characteristic cyclic amine, 1-azabicyclo[3.3.0]octane ring. In this report, we describe synthesis of a series of aromatic heterocycles with the 1-azabicyclo[3.3.0]octane ring and their pharmacological evaluation. 3-Amino-5-(1-azabicyclo[3.3.0]octan-5-yl)methyl-1,2,4-oxadiazole (2b) showed the highest M1 selectivity.     

Synthesis of extremely simplified compounds possessing the key pharmacophore units of taxol, phenylisoserine and oxetane moieties.

Straight chain compounds having a phenylisoserine unit and an oxetane ring at the alpha- and omega- position, respectively as extremely simplified analogues of taxol were prepared. None of these compounds showed promising tubulin inhibitory activity.     

Total Synthesis of Ciguatoxin

Eines der Haupttoxine bei Fischvergiftungen, das Ciguatoxin (siehe Struktur), war Ziel einer Totalsynthese. Der Syntheseweg beruht auf der Kupplung dreier Segmente, und die Schlüsselumwandlungen umfassen eine Komplexbildung zwischen Acetylen und Dicobalthexacarbonyl.

     

Uranium determination by133Xe in mineral separates from igneous rocks comparative study with fission track and thermal release characters of133Xe

Samples of an andesite (Asama-yama, Japan) and a basalt (Kilauea, Hawaii) were finely fractioned by density and the U distributions among the separates were determined by homogenized fission track method. Groundmass was found to be enriched with U; one half of the andesite U and almost all of the basalt U are accounted for by the groundmass U. Cerium and other REE are correlated with U. In the andesite separates Na is accompanied by these elements, but Fe and Co are rather anticorrelated. Uranium concentrations in the bulk samples and some of the separates were then compared with those determined by¹³³Xe extracted by heating at 1600°C. Fissiogenic¹³³Xe seems to become labile in groundmass, especially of the andesite, during reactor irradiations for 5 h or more. Stepwise heating experiments suggested that¹³³Xe in U-rich groundmass tends to escape at low temperatures and the remaining¹³³Xe is mostly retained in highly refractory sites in phenocryst rather poor in U.