Total syntheses of (+/-)-cis-trikentrin A and (+/-)-cis-trikentrin B via electrocyclic ring closures of 2,3-divinylpyrrolines

[Structure: see text] A convergent and versatile strategy for the diastereoselective syntheses of (+/-)-cis-trikentrin A and B in 10 and 12 steps, respectively, from commercially available N-BOC-2-pyrrolidinone is described. The key step in each of the total syntheses is the construction of the central benzene ring via a facile 6pi-electrocyclic ring closure of an appropriately substituted 2,3-divinylpyrroline, in turn, readily available by a Stille coupling reaction.     

Photocrystallographic structure determination of a new geometric isomer of [Ru(NH3)4(H2O)(eta1-OSO)][MeC6H4SO3]2

The structure of a new metastable geometric isomer of [Ru(NH3)4(H2O)(SO2)][MeC6H4SO3]2 in which the SO2 group is coordinated through a single oxygen in an eta1-OSO bonding mode has been determined at 13 K; the new isomer was obtained as a 36% component of the structure within a single crystal upon irradiation using a tungsten lamp.     

Locally anisotropic toposes

This paper continues the investigation of isotropy theory for toposes. We develop the theory of isotropy quotients of toposes, culminating in a structure theorem for a class of toposes we call locally anisotropic. The theory has a natural interpretation for inverse semigroups, which clarifies some aspects of how inverse semigroups and toposes are related.     

Die Trennung des Kobalts von Mangan und Eisen durch Kaliumnitrit

Zusammenfassung Die Abscheidung des Kobalts als Kaliumkobaltinitrit ist ausser zur Trennung von Nickel nur noch zu der von Mangan zu empfehlen, bei Anwesenheit von Sulfaten nur bei nicht zu grossen Metallmengen. Die Trennung des Kobalts von Eisen gelingt nur bei geringen Mengen beider, und auch dann nur in Chloridlösung. — Die Bestimmung des Kobaltgehaltes in den erwähnten neukaledonischen Erdkobalten stellt einen besonders ungünstigen Fall dar, da der Gehalt an Eisen und Mangan den an Kobalt um ein Vielfaches übersteigt. — Der Vorschlag von Rose, nämlich die Reduktion des Ferrisalzes zu Ferrosalz vor der Kobaltfällung, ist zwecklos, da beim Zusatz des Kaliumnitrits durch die frei werdende salpetrige Säure das Ferrosalz wieder oxydiert wird.(Aus dem chemischen Laboratorium der Königlichen Bergakademie Freiberg)     

Preparation and X-ray structures of 3-[Bis(trifluoroacetoxy)iodo]benzoic acid and 3-[hydroxy(tosyloxy)iodo]benzoic acid: new recyclable hypervalent iodine reagents

Preparation, structural characterization, and reactivity of 3-[bis(trifluoroacetoxy)iodo]benzoic acid and 3-[hydroxy(tosyloxy)iodo]benzoic acid, new recyclable iodine(III) reagents derived from 3-iodosylbenzoic acid, are described. The reduced form of these reagents, 3-iodobenzoic acid, can be easily recovered from the reaction mixtures using ion-exchange resin or basic aqueous workup followed by acidification with HCl.     

Effect of particulate filler on cell size in membranes formed via liquid–liquid thermally induced phase separation

This paper discusses the effects of adding particulate filler to a system undergoing liquid–liquid thermally induced phase separation (L–L TIPS). While much is known about the growth of droplets in L–L TIPS, little is known about the effect particular fillers have on droplet growth and the final cell size in the resulting microporous membranes. In this work, zeolite particles are shown to have a significant effect on the final cell size of these microporous membranes, the extent of which depends on the particle loading and processing conditions used to form the membrane. Two polymer–diluent–zeolite systems are reported: isotactic polypropylene–diphenyl ether and poly(methyl methacrylate)–cyclohexanol, both with zeolite 4A particles.     

Oxidative Conversion of a Europium(II)‐Based T1 Agent into a Europium(III)‐Based paraCEST Agent that can be Detected In Vivo by Magnetic Resonance Imaging

The Eu^II complex of 1,4,7,10‐tetraazacyclododecane‐1,4,7,10‐tetraacetic acid (DOTA) tetra(glycinate) has a higher reduction potential than most Eu^II chelates reported to date. The reduced Eu^II form acts as an efficient water proton T₁ relaxation reagent, while the Eu^III form acts as a water‐based chemical exchange saturation transfer (CEST) agent. The complex has extremely fast water exchange rate. Oxidation to the corresponding Eu^III complex yields a well‐defined signal from the paraCEST agent. The time course of oxidation was studied in vitro and in vivo by T₁‐weighted and CEST imaging.     

Femtosecond dynamics of chemical reactions at surfaces

One of the major goals in physical chemistry is to obtain a microscopic understanding of chemical reactions. Recent developments in femtosecond laser techniques provide the opportunity to resolve the timescale of elementary steps of chemical reactions at surfaces. This is exemplified for the femtosecond laser-induced oxidation of CO on Ru(001). Among other adsorbate-specific probes vibrational sum-frequency generation spectroscopy offers the possibility to monitor adsorbates or reaction intermediates directly at the surface. Recently, we have employed this technique to investigate the dynamics of the CO-stretch vibration of CO adsorbed on Ru(001) after optical excitation leading to CO desorption. Received: 4 August 2000 / Accepted: 2 September 2000 / Published online: 12 October 2000     

Experiments on the Magnetism of Sc Implanted into Metals

The study of dilute Sc impurities in either heavier d-transition metals or in alkali metal hosts is difficult due to their limited solubility; however, the large host-impurity mismatch in these systems makes them particularly interesting in terms of local electronic structure. One way to overcome the solubility problem is implantation into the desired host; in particular, using recoil implantation following heavy-ion nuclear reactions, deep implantation into practically any host can be achieved. Here, we compare the implantation of ⁴³Sc into Cs, studied in situ by the in-beam TDPAD method [1], with the implantation of ^44m Sc into Fe, studied by low-temperature nuclear orientation (LTNO) and related techniques (NMRON, thermal cycling) [2].