An unexpected Pummerer rearrangement in the synthetic route to ethyl (2′-hydroxy-4′,5′-methylenedioxyphenyl)acetate: An alternative approach to 2,3-dimethylthio benzofurans

The synthesis of ethyl (2′-hydroxy-4′,5′-methylendioxophenyl)acetate, a fragment of the antihyperglycemic natural coumarin subcoriacin, is reported. We found an expeditious route to the title compound in five steps. Final metal catalyzed acid ethanolysis of the vinylic 1,1-methylthio methylsulfoxide derivative afforded the required aryl acetic ester, but in the absence of metal catalyst, an unexpected Pummerer rearrangement produced the 2,3-dimethylthiofuran derivative as the major product. This last result provides an alternative entry to 2,3-dimethlythiobenzofurans.     

Towards a modular,versatile and portable sensor system for measurements in gaseous environments based on microcantilevers

Microfabricated silicon cantilever sensor arrays represent a powerful platform for sensing applications in physics, chemistry, material science, biology and medicine. The sensor response is mechanical bending due to absorption of molecules. In gaseous environment, polymer-coated microcantilevers are used as electronic nose for characterization of vapors, resulting in cantilever bending due to polymer swelling upon exposure. Medical applications involve fast characterization of exhaled patient's breath samples for detection of diseases, based on the presence of certain chemicals in breath. We present a portable, compact, modular microcantilever setup, which uses a micropump for aspiration and a bluetooth interface for remote data acquisition.     

Theoretical analysis of internal fluctuations and bistability in CO oxidation on nanoscale surfaces

The bistable CO oxidation on a nanoscale surface is characterized by a limited number of reacting molecules on the catalytic area. Internal fluctuations due to finite-size effects are studied by the master equation with a Langmuir-Hinshelwood mechanism for CO oxidation. Analytical solutions can be found in a reduced one-component model after the adiabatic elimination of one variable which in our case is the oxygen coverage. It is shown that near the critical point, with decreasing surface area, one cannot distinguish between two macroscopically stable stationary states. This is a consequence of the large fluctuations in the coverage which occur on a fast time scale. Under these conditions, the transition times between the macroscopic states also are no longer separated from the short-time scale of the coverage fluctuations as is the case for large surface areas and far away from the critical point. The corresponding stationary solutions of the probability distribution and the mean first passage times calculated in the reduced model are supported by numerics of the full two-component model.     

Experimental and theoretical electron density study of estrone

The electron density and the electrostatic potential (ESP) distributions of estrone have been determined using X-ray diffraction analysis and compared with theoretical calculations in the solid and gas phases. X-ray diffraction measurements are performed with a Rigaku Rapid rotating anode diffractometer at 20 K. The electron density in the estrone crystal has been described with the multipole model, which allowed extensive topological analysis and calculation of the ESP. From DFT calculations in the solid state a theoretical X-ray diffraction data set has been produced and treated in the same way as the experimental data. Two sets of single molecule DFT calculations were performed: (a) An electron density distribution was obtained via a single-point calculation with a large basis set at the experimental geometry and subsequently analyzed according to the quantum theory of atoms in molecules (AIM) to obtain the bond and most atomic properties, and (b) another electron density distribution was obtained with a smaller basis set, but at a geometry optimized using the same basis set for the analysis of atomic energies. An interesting locally stabilizing hydrogen-hydrogen bond path linking H(1) and H(11B) is found which represents the first characterization of such bonding in a steroid molecule. AIM delocalization indices were shown to be well correlated to the experimental electron density at the bond critical points through an exponential relationship. The aromaticity of ring A, chemical bonding, the O(1)...O(2) distance necessary for estrogenic activity, and the electrostatic potential features are also discussed.     

Grob fragmentation of norbornyl α-diketones: a route to α-ketoenols and aromatic compounds

An efficient acid-catalyzed Grob fragmentation of symmetrical and asymmetrically substituted norbornyl α-diketones to the corresponding six-membered α-ketoenols is reported. The regio- and stereochemical outcome of the Grob fragmentation of C2 mono- and disubstituted α-diketones was investigated. A single regioisomer resulting from a favorable half-chair intermediate was normally observed. A departure from the normal course was noticed for C2 disubstituted α-diketones possessing an exo-methyl and an endo-methoxycarbonyl derivative, giving the opposite regioisomers due to initial formation of the hemiketal. The bromo analogues of the C2 disubstituted α-diketones furnished an unusual byproduct, which appears to have been formed through highly reactive fused four-membered bicyclo[2.2.0]hexane intermediates. A plausible mechanistic proposal involving the gem-dihalo intermediate, which in one case was actually isolated as its BF(2)-complex, is outlined. The fragmentation protocol was applied to various norbornyl substrates including bis-α-diketone derivatives. The methodology was successfully utilized for the synthesis of substituted aromatic compounds.     

Ueber die Bestimmung des Gerbstoffs in Gerbstoffmaterialien

Ohne Zusammenfassung     

Chemiluminescent ion-molecule reactions in the system C+ + H2

The optical emission resulting from collisions between C⁺ ions and H₂ gas was measured in the energy range 2 to 20 eV_c.m.. The observed spectrum consists mainly of the CH⁺ A ¹Π → X ¹Σ⁺ band system; CH⁺ (A ^fΠ) is shown to be formed in the chemiluminescent reactio: C⁺(²P⁰) + H₂ → CH⁺(A ¹Π) + H(²S). The energy dependence of the emission cross section was measured. The occurrence of this reaction is discussed in terms of a electronic state correlation diagram for the system.     

Untersuchung der Sekundärstruktur von anodischen Deckschichten auf Aluminium durch Farbstoffaufnahmen

Zusammenfassung Der zeitliche Verlauf der Farbstoffaufnahme an den beiden aufeinanderliegenden Al₂O₃-Schichten bei anodisch oxydiertem Aluminium wird in beiden F?llen durch die „Anlaufparabel“ dargestellt. Die Gültigkeit dieser Gleichung best?tigt die Bildung eines Lackes zwischen dem benutzten Farbstoff und dem Aluminiumoxyd bei beiden Schichten. Die Geschwindigkeitskonstante für die erste Schicht desx ₁-Al₂O₃ ist der Stromdichte proportional, mit welcher die Oxydation vorgenommen wurde; für die zweite Schicht desx ₂-Al₂O₃ ist dasK″ unabh?ngig von der Stromdichte. In dem Diagramm desK″ gegen die Stromdichte zeigt sich ein Sprung in den Eigenschaften der beiden Schichten bei 8600 Coul/dm² (9,55 A/dm² · 15 · 60), d.h. an demselben Punkt, an dem er sich auch in der Arbeit (1) gezeigt hatte. Diese Lage des Sprunges best?tigt die Messungen und die Ergebnisse der beiden Arbeiten. Au?erdem wurde so von neuem bewiesen, da? an der Grundschicht anliegendesx ₁-Al₂O₃ por?s ist und infolgedessen die Geschwindigkeit der Farbstoffaufnahme von seiner Gesamtoberfl?che abh?ngt, d. h. von seiner Dicke (1) und deshalb von der Stromdichte. Im Gegensatz dazu ist die daraufliegende Schicht vonx ₂-Al₂O₃ weniger por?s; infolgedessen ist die Geschwindigkeit der Farbstoffaufnahme unabh?ngig von seiner Dicke (1) und von der Stromdichte, mit der die Oxydation durchgeführt worden ist (1). Die Gültigkeit der Arrheniusschen Gleichung für beide Schichten erlaubt auch die Bestimmung der Aktivierungsenergie, deren Wert (identisch für beide Schichten), auf Grund der Theorie der Fehlordnung und der Gültigkeit der Anlaufparabel, den Ablauf einer Platzwechselreaktion beweist. Durch die vorliegende Arbeit im Zusammenhang mit der Arbeit (1) wurde das Bild von der anodischen Oxydation des Aluminiums und von der Farbstoffaufnahme vervollst?ndigt, sowohl vom Standpunkt der Sekund?rstruktur des Oxyds (hier wurde zum erstenmal die Anwesenheit von zwei übereinanderliegenden Oxydschichten in der Zone der Hauptschicht mit deutlich verschiedenen Eigenschaften bewiesen), als auch vom Standpunkt der Reaktionskinetik aus (hier wurde der Reaktionsmechanismus aufgedeckt).     

A LASER FLASH PHOTOLYSIS STUDY OF PYRENE-1-ALDEHYDE. INTERSYSTEM CROSSING EFFICIENCY, PHOTOREACTIVITY AND TRIPLET STATE PROPERTIES IN VARIOUS SOLVENTS

Abstract— Triplet-and singlet-related photoprocesses of pyrene-1-aldehyde (PA) in various solvents have been investigated in detail using 337.1 and 355 nm laser flash photolysis in conjunction with time-correlated determination of fluorescence lifetimes (τ_F) and steady-state photochemical and absorption-emission spectral measurements. In benzene, the lowest triplet of PA (43 < E_T < 46 kcal/mol) has a lifetime of about 50 µs (τ_T) and displays the absorption maximum at 443 nm with a maximum extinction coefficient (ε_max) of 21000 M ^-1cm^-1; the corresponding ketyl radical has a sharp absorption maximum at 428 nm (ε_max≥ 25000 M ^-1cm^-1). The quantum yields (φ_T) of lowest triplet occupation are high in nonprotic solvents (0.6–0.8), decrease in protic solvents (alcohols) as the polarity of the latter is increased, and maintain a complementary relationship with the quantum yields (φ_F) of fluorescence. Quantum yields (φ_PC) of loss of PA due to photoreactions in some solvents have also been determined under conditions of steady irradiation at 366 nm; φ_PC is in the range 0.1–0.2 in electron-rich olefinic solvents such as cyclohexene and tetramethylethylene. These results concerning τ_F, τ_T, φ_F. φ_T and φ_PC as well as the effects of 1,2,4-trimethoxybenzene and 2,5-dimethyl-2,4-hexadiene as quenchers for fluorescence, triplet yield, and photochemistry are discussed in the light of possible state orders for PA in polar and nonpolar environments.