Über den Einfluss der pK-Werte von Anilinen auf die Bildung und Folgereaktionen von substituierten Butadienen aus Cumalinsäure-methylester

The Influence of Aniline pK Values on the Formation and Reactivity of Substituted Butadienes from Methyl Coumalate The product of the reaction between 2 equiv. of methyl coumalate ( 1 ) and 1 equiv. of a substituted aromatic amine depends on the pK value of the latter. Aromatic amines with pK values between 1.05 and 2.8 produce bicyclic lactones 4 , whereas those with higher pK values also give 2-azabicyclo[3.3.1]nona-3,7-diene-9-carboxylic acids 9 . The latter, products of the intramolecular Diels-Alder reaction 8 → 9 , may in certain cases even prevail.     

The Synthesis and Liquid-Crystal Transition Temperatures of Some Fluoro-Substituted Benzonitriles

Various laterally fluoro-substituted benzonitriles have been prepared containing a trans-4-(n-alkyl)cyclohexane ring linked to the 4-position of the benzonitriles either through a methyleneoxy (? CH₂O–) or an ethylene (? CH₂CH₂-) bridge. The bridging group links the benzonitrile and cyclohexane rings either directly or through an additional 1,4-bonded cyclohexane or benzene ring. The synthesis and liquid-crystal transition temperatures of these new compounds are described. In several cases the nematic-isotropic transition temperatures of F-substituted benzonitriles are found to be higher than those of the non-laterally substituted analogues.     

Arc-smooth generalized universal covering spaces

Let Y be a path-connected subset of a CAT(0) space Z, allowing for a map to a 1-dimensional separable metric space X, such that the nontrivial point preimages of f form a null sequence of convex subsets of Z. Such Y need not be homotopy equivalent to a 1-dimensional space.

We prove that Y admits a generalized universal covering space, which we equip with an arc-smooth structure by consistently and continuously selecting one tight representative from each path homotopy class of Y. It follows that all homotopy groups of Y vanish in dimensions greater than 1.     

Production of energetic electrons in the process of photodetachment of F-

An imaging technique is used to record an energy and angle resolved spectrum of electrons produced by photodetachment of F- in a strong infrared laser pulse. The spectrum involves contributions from more than 23 excess photon detachment channels. Its higher energy part extends beyond the classical cutoff value, and it appears as a pronounced plateau localized within a small angle along the laser polarization axis. A Keldysh-like theory is able to qualitatively reproduce the spectrum without taking into account the rescattering mechanism. The role of the parity of the initial bound state is discussed.     

Ribose 2'-F labeling: a simple tool for the characterization of RNA secondary structure equilibria by 19F NMR spectroscopy

We present a facile 19F NMR spectroscopic approach for the characterization of bistable RNA. The concept is based on the site-specific labeling of RNA with single 2'-deoxy-2'-fluoro (2'-F) nucleosides. Depending on the position of the 2'-F label (either within a double helix or a single-stranded region), significantly different 19F chemical shifts were observed and assigned to the particular alternative folds. This has been exemplified for a series of bistable RNAs. The approach is promising for the detailed investigation of complex RNA folding pathways.     

Zerfällungskörper radizieller Körpererweiterungen

Ohne Zusammenfassung     

13C n.m.r. spectra of N,N-dialkylamides

Carbon-13 n.m.r. spectra of formic, acetic, propionic and butyric acid amides with N,N-di-n-alkyl substituents have been completely assigned with the aid of extensive double resonance experiments. The data obtained were used to study long range steric effects on chemical and solvent shifts.