Ingwer-Verf?lschung

Ohne Zusammenfassung     

On the use of the modified Julia olefination for bryostatin synthesis

Modified Julia olefination reactions using aldehyde 27 and the benzothiazol-2-yl sulfones 13 and 39 provide efficient syntheses of alkenes 28 and 42, which are advanced intermediates for syntheses of bryostatins.     

Solid-state Raman laser generating <1 ns, multi-kilohertz pulses at 1096 nm

A new Raman oscillator is described based on a diode end-pumped Nd:YAG laser and an intra-cavity rubidium titanyl phosphate (RTP) crystal that serves as the active Q-switch. This compact and robust laser generates <1 ns, kilohertz pulses at 1096 nm with a 1064 nm pump and has potential for 1480 nm eye-safe operation using a 1340 nm pump. Based on rate equation modelling a Raman gain of ≈1.7–2.2 cm/GW at 1064 nm is estimated for RTP.     

Observation of magnetic-field-enhanced excitation and ionization in the plume of KrF-laser-ablated magnesium

The influence of a magnetic field on the plume produced by KrF-laser ablation of magnesium in vacuum has been investigated using time-integrated photography, streak photography, spectroscopy and charge probes. Line emission spectra in the 200–600 nm interval and effective stream velocities for the plume obtained from the spatiotemporal emission from specific neutral and ion lines are reported. Time of flight velocities are also deduced from measurements using simple charge-collector probes. Changes in the plume structure and dynamics, and enhanced emission and ionization are observed in the presence of the field. A qualitative explanation of the results is given in terms of a magnetohydrodynamic model.     

Recent Applications of Infrared and Raman Spectroscopy in Art Forensics: A Brief Overview

Abstract

A brief overview on the recent applications of infrared and Raman spectroscopy in the field of art forensics is presented.     

Analysis of red and yellow ochre samples from Clearwell Caves and Catalhöyük by vibrational spectroscopy and other techniques

Ochre samples excavated from the neolithic site at Catalh?yük, Turkey have been compared with "native" ochres from Clearwell Caves, UK using infrared spectroscopy backed up by Raman spectroscopy, scanning electron microscopy (with energy-dispersive X-rays (EDX) analysis), powder X-ray diffraction, diffuse reflection UV-Vis and atomic absorption spectroscopies. For the Clearwell Caves ochres, which range in colour from yellow-orange to red-brown, it is shown that the colour is related to the nature of the chromophore present and not to any differences in particle size. The darker red ochres contain predominantly haematite while the yellow ochre contains only goethite. The ochres from Catalh?yük contain only about one-twentieth of the levels of iron found in the Clearwell Caves ochres. The iron oxide pigment (haematite in all cases studied here) has been mixed with a soft lime plaster which also contains calcite and silicate (clay) minerals.     

Metal–ligand cooperativity in the soluble hydrogenase-1 from Pyrococcus furiosus

Metal–ligand cooperativity is an essential feature of bioinorganic catalysis. The design principles of such cooperativity in metalloenzymes are underexplored, but are critical to understand for developing efficient catalysts designed with earth abundant metals for small molecule activation. The simple substrate requirements of reversible proton reduction by the [NiFe]-hydrogenases make them a model bioinorganic system. A highly conserved arginine residue (R355) directly above the exogenous ligand binding position of the [NiFe]-catalytic core is known to be essential for optimal function because mutation to a lysine results in lower catalytic rates. To expand on our studies of soluble hydrogenase-1 from Pyrococcus furiosus (Pf SH1), we investigated the role of R355 by site-directed-mutagenesis to a lysine (R355K) using infrared and electron paramagnetic resonance spectroscopic probes sensitive to active site redox and protonation events. It was found the mutation resulted in an altered ligand binding environment at the [NiFe] centre. A key observation was destabilization of the Ni_a³⁺–C state, which contains a bridging hydride. Instead, the tautomeric Ni_a⁺–L states were observed. Overall, the results provided insight into complex metal–ligand cooperativity between the active site and protein scaffold that modulates the bridging hydride stability and the proton inventory, which should prove valuable to design principles for efficient bioinspired catalysts.

Metal–ligand cooperativity is an essential feature of bioinorganic catalysis.     

Duplex stability of DNA.DNA and DNA.RNA duplexes containing 3'-S-phosphorothiolate linkages

3'-S-Phosphorothiolate (3'-SP) linkages have been incorporated into the DNA strand of both a DNA.RNA duplex and a DNA.DNA duplex. Thermal melting (T(m)) studies established that this modification significantly stabilises the DNA.RNA duplex with an average increase in T(m) of about 1.4 degrees C per modification. For two or three modifications, the increase in T(m) was larger for an alternating, as compared to the contiguous, arrangement. For more than three modifications their arrangement had no effect on T(m). In contrast to the DNA.RNA duplex, the 3'-S-phosphorothiolate linkage destabilised the DNA.DNA duplex, irrespective of the arrangement of the 3'-SP linkages. The effect of ionic strength on duplex stability was similar for both the phosphorothiolate-substituted and the unmodified RNA.DNA duplexes. The results are discussed in terms of the influence that the sulfur atom has on the conformation of the furanose ring and comparisons are also drawn between the current study and those previously conducted with other modifications that have a similar conformational effect.     

Total Synthesis,Stereochemical Assignment,and Biological Activity of Chamuvarinin and Structural Analogues

A highly stereocontrolled synthesis of (+)‐chamuvarinin has been completed in 1.5 % overall yield over 20 steps. The key fragment coupling reactions were the addition of alkyne 8 to aldehyde 7 (under Felkin–Anh control), followed by the two step activation/cyclization to close the C20–C23 2,5‐cis‐substituted tetrahydrofuran ring and a Julia–Kocienski olefination at C8–C9 to introduce the terminal butenolide. The inherent flexibility of our coupling strategy led to a streamlined synthesis with 17 steps in the longest sequence (2.2 % overall yield), in which the key bond couplings are reversed. In addition, a series of structural analogues of chamuvarinin have been prepared and screened for activity against HeLa cancer cell lines and both the bloodstream and insect forms of Trypanosoma brucei, the parasitic agent responsible for African sleeping sickness.