Structural and stereochemical revision of isocyanide and isothiocyanate amphilectenes from the Caribbean marine sponge Cribochalina sp.

The absolute stereochemistry of amphilectene metabolites from Cribochalina sp. has been revised by a detailed NMR spectroscopic study of the Mosher ester derivatives of a related alcohol. The relative stereochemistry of the previously described amphilectenes has been reinvestigated and reassigned on the basis of the X-ray structural analysis carried out on one of them. The structure of a new amphilectene metabolite, which is an isothiocyanato analogue is also presented.     

Olefin metathesis: what have we learned about homogeneous and heterogeneous catalysts from surface organometallic chemistry?

Since its early days, olefin metathesis has been in the focus of scientific discussions and technology development. While heterogeneous olefin metathesis catalysts based on supported group 6 metal oxides have been used for decades in the petrochemical industry, detailed mechanistic studies and the development of molecular organometallic chemistry have led to the development of robust and widely used homogeneous catalysts based on well-defined alkylidenes that have found applications for the synthesis of fine and bulk chemicals and are also used in the polymer industry. The development of the chemistry of high-oxidation group 5–7 alkylidenes and the use of surface organometallic chemistry (SOMC) principles unlocked the preparation of so-called well-defined supported olefin metathesis catalysts. The high activity and stability (often superior to their molecular analogues) and molecular-level characterisation of these systems, that were first reported in 2001, opened the possibility for the first direct structure–activity relationships for supported metathesis catalysts. This review describes first the history of SOMC in the field of olefin metathesis, and then focuses on what has happened since 2007, the date of our last comprehensive reviews in this field.

Surface organometallic chemistry bridges the gap between homogeneous and heterogeneous olefin metathesis catalysts.     

Synthesis and evaluation of sphinganine analogues of KRN7000 and OCH

[structures: see text] The phytosphingosine-containing alpha-galactosylceramides (alpha-GalCers), KRN7000 and OCH, have been shown to activate NKT cells via interaction with CD1d, a member of the CD1 family of antigen presenting proteins. Evidence from KRN7000 stimulation of NKT cells suggests that alpha-GalCers may have applications in the treatment or prevention of a range of viral, bacterial, and autoimmune conditions. Moreover, OCH, a truncated analogue of KRN7000, appears to induce a T(H)2 bias, which could have implications for the treatment of autoimmune and inflammatory conditions. We have prepared the direct sphinganine-containing analogues of KRN7000 and OCH, 1 and 2, and found them to be comparable in activity to the parent compounds in inducing the release of IL-2, IL-4, and IFNgamma. In addition, compound 2 leads to a cytokine bias similar to that seen with OCH. This is significant because sphinganines are more easily accessed than phytosphingosines, which should facilitate SAR studies.     

Measurement of the Michel parameter rho in muon decay

The TWIST Collaboration has measured the Michel parameter rho in normal muon decay, mu(+)--> e(+)nu(e)nu (mu). In the standard model, rho = 3/4. Deviations from this value imply mixing of left- and right-handed muon and electron couplings. We find rho=0.750 80+/-0.000 32(stat) +/- 0.000 97(syst) +/- 0.000 23, where the last uncertainty represents the dependence of rho on the Michel parameter eta. This result sets new limits on the W(L)-W(R) mixing angle in left-right symmetric models.     

Spectral characterization of two sided ideals inB(H)

In this paper we prove that an elementA ∈ B(H) belongs to a proper two sided ideal inB(H) if and only if {fx26-1} In particular, ifH is separable, this property completely characterizes compact operators onH. This research was supported by Air Force Office of Scientific Research, grant AFOSR 69-1667.     

Complete charge pooling is prevented in viologen-based dendrimers by self-protection

We have investigated the electrochemical behavior, and chemical and photosensitized reduction of two dendrimers based on a 1,3,5-trisubstituted benzenoid core, which contain 9 and 21 4,4'-bipyridinium (usually called viologen) units, respectively, in their branches and are terminated with tetraarylmethane groups. For comparison purposes, the behavior of reference compounds that contain a single viologen unit have also been investigated. We have found that only part of the viologen units can be reduced in the dendrimer species. For the larger dendrimer, the number of reducible viologens (out of the 21 present) is 14 in electrochemical experiments (in MeCN), 9 on reduction with bis(benzene)chromium (in MeCN), and 13 by photoinduced electron transfer with 9-methylanthracene as a photosensitizer and triethanolamine as a sacrificial reductant in CH2Cl2. The reduced viologen units undergo partial dimerization. The photochemical experiments have shown that only monomeric, one-electron-reduced viologen units are formed at the beginning of the irradiation, followed by dimer formation, until a photostationary state is reached that contains 40 % nonreduced, 33 % monomeric reduced, and 27 % reduced units associated in the dimeric form. The results suggest that, upon reduction of a fraction of the viologen units, the dendrimer structure shrinks, with the result that the bulky terminal groups protect other viologen units from being reduced.     

Driven reconnection about a magnetic X-line with strong guide component

A two-dimensional, two-fluid model is used to investigate driven magnetic reconnection in collisionless or semicollisional plasmas. The reconnection is driven by externally induced plasma flows in a background magnetic configuration that has a hyperbolic null component in the reconnection plane and a strong component, the so-called guide component, perpendicular to that plane. A dynamic solution is obtained in which the reconnection proceeds in two phases: an initial one whose characteristic rate is a fraction of the Alfvén frequency, and a later one whose rate is determined by the electron collision frequency.