Chemoenzymatic Synthesis of All Four Diastereomers of 2,6‐Disubstituted Piperidines through Stereoselective Monoamination of 1,5‐Diketones

The regioselectivity of various enantiocomplementary ω‐transaminases was evaluated for the stereoselective monoamination of designated 1,5‐diketones; excellent conversions, enantio‐ and regioselectivities were observed. The resulting amino‐ketones underwent spontaneous intramolecular ring closure to afford Δ1‐piperideines, which served as precursors for the cis‐ and anti‐piperidine scaffold as demonstrated for the synthesis of the alkaloids dihydropinidine and epi‐dihydropinidine. Key to the success of accessing the trans‐piperidines was a Lewis acid mediated conformational change of the Δ1‐piperideines in the reduction step. Thus, all four diastereomers of 2,6‐disubstituted piperidines could successfully be prepared.     

P-C bond formation via P-H addition of a fluoroaryl phosphinic acid to ketones

The synthesis, structure and reactivity of the fluoroaryl phosphinic acid HF₄C₆-P(O)HOH is reported and compared to a sterically comparable yet non-fluorinated analog with similar size. The fluoroaryl phosphinic acid undergoes reversible P-H addition to the carbonyl functionality of ketones under formation of a P-C bond which is retained in the resulting α-hydroxy phosphinic acid. The latter shows an extended 2D hydrogen bonded network in the solid state.     

Self-assembly of tert-butyl silanetriol in solution and aggregation with tetrahydrofuran

tert-Butyl silanetriol behaves like a surfactant and self-assembles in aqueous and tetrahydrofuran (THF) solutions. Micelle formation was studied with pulsed field gradient stimulated spin echo (PFG-SSE) NMR, while multilayer lamellar vesicles can be concluded from small/wide angle X-ray scattering measurements in concentrated solutions at elevated temperature. The significant interaction between tert-butyl silanetriol and THF is further highlighted by a crystal structure featuring a 3:1 adduct of silanetriol and THF.     

Exploring the Anion–Cation Interaction in m‐Terphenyltetrafluorosilicates by Using Multinuclear NMR Spectroscopy,X‐ray Diffraction,and ICR‐FT‐MS

The synthesis of a series of m‐terphenyl‐substituted tetrafluorosilicates with different cations (Na⁺, K⁺, Rb⁺, Cs⁺, Ag⁺, Tl⁺) is described and the interactions between the anion and cation are investigated in the solid, solution, and gas states by using multinuclear NMR spectroscopy, X‐ray diffraction, and ion cyclotron resonance Fourier‐transform mass spectrometry (ICR‐FT‐MS). In solution, heteronuclear NMR spectroscopy parameters show only limited sensitivity to the nature of the cation, which furthermore can be affected by solvent effects. More pronounced effects are observed in the structural data obtained from X‐ray diffraction studies, which are in good agreement with experimental gas‐phase data from ESIMS. ESIMS also reveals the existence of dimeric species of the type [M(DmpSiF₄)₂]^? (Dmp=2,6‐dimesitylphenyl), the stability of which was determined by normalized collision energy experiments.     

Conformational analysis of caprolactam,cycloheptene and caprolactone

The conformations of ε-caprolactam, cis-cycloheptene and ε-caprolactone have been investigated at the B3LYP and CCSD(T) levels of theory using the 6-311+G(d,p) basis set. Inversion of the most stable chair conformation was calculated to require a free energy of ΔG ^‡ = 10.5 kcal/mol for caprolactam, ΔG ^‡ = 5.0 kcal/mol for cycloheptene and ΔG ^‡ = 8.4 kcal/mol for caprolactone. These results are in good agreement with the available experimental data of 10.3 and 5.0 kcal/mol for caprolactam and cycloheptene, respectively. Analysis of a classical force field expression fitted to the quantum mechanical energy surface suggests that the caprolactam ring is more strained than cycloheptene owing to unfavourable bending interactions.     

Friction of single polymers at surfaces

Atomic force microscope (AFM) single molecule force spectroscopy has been used to investigate the friction coefficient of individual polymers adsorbed onto a solid support. The polymer chains were covalently attached to an AFM tip and were allowed to adsorb on a mica surface. Different polymers (ssDNA, polyallylamine) were chosen to cover a range of friction coefficients. During the experiment, the AFM tip was retracted in- and off-plane which results, depending on the chosen conditions, in a desorption of the polymer from the surface, a sliding across the surface, or a combination of both. Thus, the obtained force-extension spectra reveal detailed information on the mobility of a polymer chain on a surface under experimentally accessible conditions. This study demonstrates that absorbed polymers with comparable desorption forces may exhibit drastically different in plane mobility.     

Electronic structure and optical properties of charged oligofluorenes studied by VIS/NIR spectroscopy and time-dependent density functional theory

The electronic structure and optical properties of charged oligofluorenes were studied experimentally and theoretically. Measurements of the optical absorption spectra of charged oligofluorenes in dilute solutions have been performed by using the pulse radiolysis technique. In addition, optical absorption spectra of radical cations and anions in a solid matrix were measured after gamma-irradiation at 77 K. The optical absorption spectra were measured in the range of 440-2100 nm (0.6-2.8 eV) and compared with results from time-dependent density functional theory (TDDFT) calculations. The calculated charge induced deformations and charge distribution do not indicate the occurrence of polaronic effects. The potential energy profiles for rotation around the inter-unit bond show that oligofluorenes are nonplanar in their neutral state, while they tend to more planar structures in their charged state. The optical absorption spectra of charged oligofluorenes are dependent on the angle between neighboring units. TDDFT absorption energies shift to lower values with increasing chain length, which suggests that the charge delocalizes along the oligomer chain.     

Microwave-assisted tissue processing for same-day EM-diagnosis of potential bioterrorism and clinical samples

The purpose of this study was to explore the turnaround times, section and image quality of a number of more "difficult" specimens destined for rapid diagnostic electron microscopy (EM) after microwave-assisted processing. The results were assessed and compared with those of conventionally processed samples. A variety of infectious agents, some with a potential for bioterrorism, and liver biopsies serving as an example for routine histopathology samples were studied. The samples represented virus-producing cell cultures (such as SARS-coronavirus, West Nile virus, Orthopox virus), bacteria suspensions (cultures of Escherichia coli and genetically knockout apathogenic Bacillus anthracis), suspensions of parasites (malaria Plasmodium falciparum, Leishmania major, Microsporidia cuniculi, Caenorhabditis elegans), and whole Drosophila melanogaster flies infected with microsporidia. Fresh liver samples and infected flies were fixed in Karnovsky-fixative by microwaving (20 min), all other samples were fixed in buffered glutaraldehyde or Karnovsky-fixative overnight or longer. Subsequently, all samples were divided to evaluate alternative processing protocols: one part of the sample was OsO4-postfixed, ethanol-dehydrated, Epon-infiltrated (overnight) in an automated tissue processor (LYNX, Leica), and polymerized at 60 degrees C for 48 h; in parallel the other part was microwave-assisted processed in the bench microwave device (REM, Milestone), including post-osmication and the resin block polymerization. The microwave-assisted processing protocol required at minimum 3 h 20 min: the respective epon resin blocks were uniformly polymerized allowing an easy sectioning of semi- and ultrathin sections. Sections collected on non-coated 200 mesh grids were stable in the electron beam and showed an excellent preservation of the ultrastructure and high contrast, thus allowing an easy, unequivocal and rapid assessment of specimens. Compared with conventional routine methods, microwave technology facilitates a significant reduction in sample processing time from days to hours without any loss in ultrastructural details. Microwave-assisted processing could, therefore, be a substantial benefit for the routine electron microscopic diagnostic workload. Due to its speed and robust performance it could be applied wherever a rapid electron microscopy diagnosis is required, e.g., if bioterrorism or emerging agents are suspected. Combining microwave technology with digital image acquisition, the 1-day diagnosis based on ultrathin section electron microscopy will become possible, with crucial or interesting findings being consulted or shared worldwide with experts using modern telemicroscopy tools via Internet.