Controlling influences in bisspiroketal formation: synthesis of the ABC ring system of azaspiracid

Substitution effects on the stereochemical outcome of bisspiroketalization on the C₁-C₁₇ carbon backbone of azaspiracid is presented. A possible explanation is offered to explain the observed stereochemical outcome.     

Total synthesis of the proposed structures of the DNA methyl transferase inhibitors peyssonenynes, and structural revision of peyssonenyne B

The purported structures of the peyssonenynes A and B isolated from Peyssonnelia caulifera, and considered to be geometric isomers at the acetoxyenediyne moiety, have been synthesized. The E and Z geometries of the synthetic compounds were secured by the magnitude of the (3)J(H9-C7) values measured using the EXSIDE band-variant of the gradient HSQC pulse sequence and by the chemical shifts of C(6). Comparison of the NMR data of the synthetic and natural products revealed that only those of the Z isomers matched, which correspond to peyssonenyne A. Using HPLC analysis it was found that peyssonenyne B must correspond to the sn-2 positional isomer of the Z sn-1/3 counterpart. The four synthetic sn-1/3 diastereomers are roughly equipotent as DNMT1 inhibitors when evaluated on a radioactive methyl transfer enzymatic assay after immunoprecipitation from K562 human leukemia cells with anti-DNMT1 antibody.     

Selenium-77 NMR chemical shifts of five-membered selenium and sulphur heterocycles with CSe,CS,and CO groups

This paper reports the ⁷⁷Se NMR chemical shifts of 1,3-dithiole-, 1,3-thiaselenole- and 1,3-diselenole-2-ones, -thiones and -selones, of the corresponding saturated compounds 1,3-diselenolane-2-one, -thione and -selone, and the 1,3-thiaselenolium tetrafluoroborates, either unsubstituted or substituted by morpholino, ethylthio or ethylseleno groups in the 2-position. The ⁷⁷Se chemical shift values of the ring selenium and the C?Se groups are compared with the ¹³C chemical shift values of neighbouring carbon atoms. The relationships between the ⁷⁷Se chemical shifts of the C?Se groups and the wavelengths of their n→* absorption in the UV-visible spectrum are discussed with respect to the significance of the δE term in the contribution of the paramagnetic screening and the electron density distribution.     

Optical imaging of pattern formation: reflection anisotropy microscopy applied to globally coupled oscillatory CO-oxidation

A reflection anisotropy microscope (RAM) creates the contrast in its images from a change in the polarization orientation of reflected light from a surface. This may stem from local variations of the reconstruction of a surface, being initiated by changes in an adsorption layer. The advantages and disadvantages of a recently improved RAM versus other optical imaging techniques are discussed. We demonstrate the unique features of RAM and present the first experimental findings of so called 2pi phase kinks in the globally coupled CO-oxidation on Pt(110).     

TeV γ-ray astronomy

The TeV γ-ray sky displays at this time 9 sources comprising 6 supernova remnants (SNR) and 3 active galactic nuclei (AGN). They have been discovered with AIR ERENKOV TELESCOPES since 1989, and more sources are sure to be found. They are related to violent astrophysical events and promise to be used as unique tools for cosmological questions like the universal infrared photon field and the intergalactic magnetic field strength.     

Local structural order in carbonic acid polymorphs: Raman and FT‐IR spectroscopy

Two different polymorphs of carbonic acid, α‐ and β‐H₂CO₃, were identified and characterized using infrared spectroscopy (FT‐IR) previously. Our attempts to determine the crystal structures of these two polymorphs using powder and thin‐film X‐ray diffraction techniques have failed so far. Here, we report the Raman spectrum of the α‐polymorph, compare it with its FT‐IR spectrum and present band assignments in line with our work on the β‐polymorph [Angew. Chem. Int. Ed. 48 (2009) 2690–2694]. The Raman spectra also contain information in the wavenumber range ∼90–400 cm⁻¹, which was not accessible by FT‐IR spectroscopy in the previous work. While the α‐polymorph shows Raman and IR bands at similar positions over the whole accessible range, the rule of mutual exclusion is obeyed for the β‐polymorph. This suggests that there is a center of inversion in the basic building block of β‐H₂CO₃ whereas there is none in α‐H₂CO₃. Thus, as the basic motif in the crystal structure we suggest the cyclic carbonic acid dimer containing a center of inversion in case of β‐H₂CO₃ and a catemer chain or a sheet‐like structure based on carbonic acid dimers not containing a center of inversion in case of α‐H₂CO₃. This hypothesis is strengthened when comparing Raman active lattice modes at < 400 cm⁻¹ with the calculated Raman spectra for different dimers. In particular, the intense band at 192 cm⁻¹ in β‐H₂CO₃ can be explained by the inter‐dimer stretching mode of the centrosymmetric RC(OHO)₂ CR entity with ROH. The same entity can be found in gas‐phase formic acid (RH) and in β‐oxalic acid (RCOOH) and produces an intense Raman active band at a very similar wavenumber. The absence of this band in α‐H₂CO₃ confirms that the difference to β‐H₂CO₃ is found in the local coordination environment and/or monomer conformation rather than on the long range. Copyright © 2011 John Wiley & Sons, Ltd.     

Imaging of dynamic processes on surfaces by light

This work focuses on imaging of dynamic processes on surfaces, using light to illuminate the area of interest. The methods discussed here are those in which the photoelectrons emitted from or the light reflected off the surface are measured. While the first approach is well-known since electron microscopy was invented and has been used in surface science applications for a decade, genuine optical microscope methods using polarized light were first developed in 1995 for imaging surface reactions.

The results discussed here are from different fields of surface research. These include the imaging of adsorption phenomena, surface diffusion and growth processes. The main emphasis will be on pattern formation of surface reactions under strictly controlled parameters. The most recent techniques expand the range of observable pressure conditions by many orders of magnitude, thus bridging the pressure gap in imaging surface reactions.