On Excited States of Alkyl Bridged [14] Annulenes

The linear dichroic absorption spectrum of 1,3,6,8-trans-15, 16-hexamethyl-dihydropyrene has been measured in stretched polyethylene at 77K, and CNDO-CI calculations with inclusion of singly and doubly excited configurations have been carried out on a series of alkyl bridged [14] annulenes with pyrene- and anthracene-shaped perimeters. Transitions to e_3g → e_4u type ¹B states are well described, and the results indicate that additional low-energy excited states originate from e_3g→ e_5g and e_2u → e_4u type configurations interacting strongly with doubly excited configurations of the e_3g, e_3g → e_4u, e_4u type. The second excited singlet state responsible for the weak transition observed between the ¹L bands may be assigned to one of these additional states, but it is probably of complex nature, similar to the ‘phantom’ state in linear polyenes.     

On the exceptional reactivity of the norbornene double bond

The anomalous reactivity of the bicyclo [2.2.1] heptene double bond is interpreted in terms of hyperconjugative effects, leading to an exceptionally low endo out-of-plane deformation potential.     

Synthesis and PET oxidative cyclization of silyl enol ethers: build-up of quasi-steroidal carbocycles

PET oxidative cyclization of silyl enol ethers carrying suitable side chains with olefinic double bonds results in the stereoselective formation of carbocycles. Two model compounds for investigating the influence of silyl enol ether ring size are synthesized. Furthermore the synthesis of a quasi-steroidal carbocycle with an unnatural configuration is presented.     

Report on three aliphatic dimethylarsinoyl compounds as common minor constituents in marine samples. An investigation using high-performance liquid chromatography/inductively coupled plasma mass spectrometry and electrospray ionisation tandem mass spectrometry

Three water-soluble aliphatic arsenicals, dimethylarsinoyl acetate (DMAA), dimethylarsinoyl ethanol (DMAE), and dimethylarsinoyl propionate (DMAP), were identified in marine biological samples. Sample extracts in methanol/water (1 + 1) were analysed by cation-exchange high-performance liquid chromatography/inductively coupled plasma mass spectrometry (HPLC/ICPMS). Eluate fractions from the HPLC/ICPMS analyses containing the compounds in question were collected and subjected to analysis by electrospray ionisation tandem mass spectrometry (ESI-MS/MS), which provided supportive evidence for the structures of the three compounds. The concentrations of the three arsenicals were determined in 37 marine organisms comprising algae, crustaceans, bivalves, fish and mammals by HPLC/ICPMS. The three arsenicals DMAA, DMAE and DMAP, which occurred at microg kg(-1) concentrations, were detected in 25, 23 and 17 of the 37 samples analysed, respectively. The limits of detection were 2-3 microg kg(-1) dry mass. The data illustrate that the three compounds are common minor constituents in marine samples. This is the first report on DMAE and DMAP as naturally occurring species in marine samples. The presence of DMAA and DMAE supports a proposed biosynthesis of arsenobetaine (AB) from dimethylarsinoylribosides. Alternative proposals, which explain the presence of the compounds in marine samples, are addressed briefly in the paper.     

Catalyzed Asymmetric Arylation Reactions

Addition and substitution reactions with carbon nucleophiles are fundamental processes in organic synthesis, and the development of general catalytic asymmetric variants thereof is still a major challenge today. In contrast to enantioselective alkyl transfer reactions, the corresponding arylations have not yet reached a high level of maturity. The existing protocols are either of no general applicability or are limited in terms of selectivity. This article summarizes established routes for catalytic asymmetric aryl transfer together with the latest developments in this area. The scope and limitations of this reaction are discussed.     

On the 6-exo-trig ring closure of substituted 5-hexen-1-oxyl radicals

Substituted 5-hexen-1-oxyl radicals have been generated from N-(5-hexen-1-oxy)-5-(p-methoxyphenyl)-4-methylthiazole-2(3H)-thiones under tin-free conditions and have been successfully applied as reactive intermediates in a mechanistic study on the formation of bromomethyl-substituted tetrahydropyrans via 6-exo-trig selective cyclizations.     

Raman and cathodoluminescence spectroscopic investigations on Permian fossil wood from Chemnitz--a contribution to the study of the permineralisation process

Samples of different three-dimensionally preserved fossil plants (Medullosa sp., Dadoxylon sp., Calamodendron striatum, Psaronius sp.) from the Lower Permian petrified forest of Chemnitz were examined with regard to their chemical composition and structural order. Raman spectroscopy and cathodoluminescence microscopy are shown to be powerful tools for such investigations. Silicified wood from Chemnitz-Hilbersdorf generally shows yellow cathodoluminescence (CL) of the cell walls and only weak yellow-brownish CL of the cell lumina. By time-resolved cathodoluminescence spectroscopy, a secondary mineralisation of hydrothermal origin was recognized. The latter is shown by short-lived blue CL at the cell walls extinguishing the yellow signal. Therefore, after the primary silicification step a secondary mineralisation step initiated by hydrothermal processes, seems to have taken place at probably slightly higher temperatures. The resulting silica matrix consists of phanerocrystalline and microcrystalline alpha-quartz as well as microcrystalline moganite, both partially associated with iron oxides. Dadoxylon sp. is a prominent example for parallel permineralisation by alpha-quartz and fluorspar, which is outstanding for the Chemnitz Petrified Forest. CL on this samples shows parallel silicification and fluoritisation, followed by infiltration of iron oxides. Permineralised samples show very low percentage of original organic remains. The seed fern Medullosa, for example, shows dispersed carbon, which is mainly restricted to the centres of the typical star-shaped vascular bundles. Raman spectroscopy revealed that these carbonaceous particles are of an anthracite structure. For experimental confirmation coal samples of different rank, especially anthracite from different geological times and localities, were studied by means of Raman spectroscopy. The remaining pith of the vascular bundles is white-coloured and consists of alpha-quartz and moganite, whereas surrounding tracheides exhibit white and reddish coloured parts. The reddish parts, mainly found in the rays, additionally contain alpha-Fe2O3 and Fe3O4 among the SiO2 polymorphs of alpha-quartz and moganite. Sometimes iron oxides could have dominated permineralisation processes as the peak intensities of distinct parts of the samples suggest.     

Direct identification of all oncogenic mutants in KRAS exon 1 by cycling temperature capillary electrophoresis

Over the past few decades, advances in genetics and molecular biology have revolutionized our understanding of cancer initiation and progression. Molecular progression models outlining genetic events have been developed for many solid tumors, including colon cancer. Previous reports in the literature have shown a relationship between different KRAS mutations and prognosis and response to medical treatment in colon cancer patients. Furthermore, the presence of a mutated KRAS has been correlated with different clinicopathological variables including age and gender of patients and tumor location. To our knowledge, few institutions screen for KRAS mutations on regular basis in colon cancer patients despite such evidence that knowledge of KRAS exon 1 status is informative. Here, we report on a mutation analysis method adapted to a 96-capillary electrophoresis instrument that allows identification of all 12 oncogenic mutations in KRAS exon 1 under denaturing conditions. To determine the optimal parameters, a series of DNA constructs generated by site-directed mutagenesis was analyzed and the migration times of all mutant peaks were measured. A classification tree was then made based on the differences in migration time between the mutants and an internal standard. A randomized series of 500 samples constructed with mutagenesis as well as 60 blind samples from sporadic colon carcinomas was analyzed to test the method. No wild-type samples were scored as mutants and all mutants were correctly identified. Post polymerase chain reaction (PCR) analysis time of 96 samples was performed within 40 min.