Modular Total Synthesis of Rhizopodin: A Highly Potent G‐Actin Dimerizing Macrolide

A highly convergent total synthesis of the potent polyketide macrolide rhizopodin has been achieved in 29 steps by employing a concise strategy that exploits the molecule′s C₂ symmetry. Notable features of this convergent approach include a rapid assembly of the macrocycle through a site‐directed sequential cross‐coupling strategy and the bidirectional attachment of the side chains by means of Horner–Wadsworth–Emmons (HWE) coupling reactions. During the course of this endeavor, scalable routes for synthesis of three main building blocks of similar complexity were developed that allowed for their stereocontrolled construction. This modular route will be amenable to the development of syntheses of other analogues of rhizopodin.     

Monitoring polymeric compounding processes inline with THz time-domain spectroscopy

We demonstrated the potential of terahertz spectroscopy for contactless and nondestructive inline control of polymeric compounding processes. Using a fiber-coupled terahertz spectrometer, we investigated molten polymers and polymeric compounds. On one hand, we studied the influence of temperature and pressure on the terahertz properties of molten polypropylene. On the other hand, various polypropylene–CaCO₃ and PA6–glass-fiber compounds were examined. We showed that the volumetric additive content can be determined from the terahertz refractive index of the compound. Real-time measurements demonstrated the applicability of terahertz technology as a valuable tool for the inline control of production processes.     

Collinear asymptotic dynamics for massive particles

The dynamics of massive particles in the collinear high momentum regime is investigated. Methods hitherto exploiting large time asymptotic Hamiltonians in the Dirac picture for the treatment of infrared divergences are adapted to the collinear asymptotic dynamics. The essential role of time ordering of the dressing operators is brought out.     

Reversible chain transfer between organoyttrium cations and aluminum: synthesis of aluminum-terminated polyethylene with extremely narrow molecular-weight distribution

Aminopyridinato-ligand-stabilized organoyttrium cations are accessible in very good yield through alkane elimination from trialkyl yttrium complexes with sterically demanding aminopyridines, followed by abstraction of one of the two alkyl functions using ammonium borates. At 80 degrees C and in the presence of small amounts of aluminum alkyl compounds, very high ethylene polymerization activities are observed if very bulky aminopyridinato ligands are used. During these polymerizations a reversible polyethylene chain transfer is observed between the organoyttrium cations and aluminum alkyls. The chain-transfer catalyst system described here is able to produce relatively long-chain (up to 4000 g mol-1) Al-terminated polyethylene with a molecular-weight distribution<1.1. In the synthesis of higher molecular PE a slight increase in polydispersity with increasing chain length (15,600 g mol-1, approximately 1.4) is observed owing to reduced reversibility caused by higher viscosity and precipitation of polymer chains (temperature of 80-100 degrees C).     

The feasibility of the TRIC experiment at COSY

For the Time Reversal Invariance experiment at COSY (TRIC) that had been proposed at the Forschungszentrum Jülich a novel technique for measuring total cross sections in internal experiments was developed and the achievable precision of this measuring technique was tested. The experimental idea constitutes a true P-even T-odd null test of TRI. In order to compare to the theoretical upper bound from edm measurements (Haxton and Höring, Nucl Phys A560:469, 1993), the TRI violating observable A _y,xz in proton-deuteron forward scattering has to be measured with an accuracy of 10^???6. An internal TRI experiment can determine the total cross section via the optical theorem by measuring the forward scattering amplitude. In contrast to external experiments, the only calibrated device that is needed is a precise beam current transformer (BCT). The feasibility of an internal TRI experiment was tested at COSY in a measurement of A _y,y in p-p scattering, since these values are known. The analysis of the data showed the presence of unexpected large 1/f noise contributions, which eventually reduced the achievable accuracy by a factor 3. The origin of the 1/f noise was traced and found to be the Barkhausen noise from the ferrite-core of the BCT. A global weighted fitting technique based on a modified Wiener-Khinchin method was applied to suppress the influence of the 1/f noise.     

Decharging of complex plasmas: first kinetic observations

The first experiment on the decharging of a complex plasma in microgravity conditions was conducted. After switching off the rf power, in the afterglow plasma, ions and electrons rapidly recombine and leave a cloud of charged microparticles. Because of microgravity, the particles remain suspended in the experimental chamber for a sufficiently long time, allowing precise measurements of the rest particle charge. A simple theoretical model for the decharging is proposed which agrees quite well with the experiment results and predicts the rest charge at lower gas pressures.     

Improved limits on nu(e) emission from mu+ decay

We investigated mu(+) decays at rest produced at the ISIS beam stop target. Lepton flavor (LF) conservation has been tested by searching for nu(e) via the detection reaction p(nu(e),e(+))n. No nu(e) signal from LF violating mu(+) decays was identified. We extract upper limits of the branching ratio (BR) for the LF violating decay mu(+)-->e(+)+nu(e)+nu(-) compared to the standard model (SM) mu(+)-->e(+)+nu(e)+nu(mu) decay: BR<0.9(1.7) x 10(-3) (90% C.L.) depending on the spectral distribution of nu(e) characterized by the Michel parameter rho=0.75(0.0). These results improve earlier limits by one order of magnitude and restrict extensions of the SM in which nu(e) emission from mu(+) decay is allowed with considerable strength. The decay mu(+)-->e(+)+nu(e)+nu(mu) often proposed as a potential source for the nu(e) signal observed in the LSND experiment can be excluded.     

Studies on oscillations in the platinum catalyzed CO oxidation by means of an omegatron mass probe

A versatile all glass UHV apparatus for the study of catalytic reactions in a wide range of pressure is described. An omegatron in connection with different capillary leaks located close to the catalyst surface serves as a mass probe. As an example oscillating instabilities in the non-stationary CO/O₂ reaction in the mbar range are reported.