The question of concerted or consecutive bond formation arises in all types of cycloaddition reactions. Stereochemical investigations and studies of regiospecificity as well as intensive kinetic investigations with regard to substituent and solvent effects, and the dependence of the reaction on temperature and pressure permit in many cases mechanistic conclusions concerning the Diels-Alder reaction. Efforts towards a theoretical interpretation, ab initio and semiempirical calculations, the application of frontier molecular orbital theory (FMO) as well as thermochemical measurements permit a description of the energy hypersurface of these [4 + 21-c~cloadditions. An attempt is made here to draw a line of distinction between the mechanistic alternatives-one-step reaction versus two-step reaction-considering all experimental and theoretical criteria [4 + 2]-cy~loadditions[~~, named after their discoverers, were later joined by [2 + 11-, [2 + 21-, [3 + 21-cycloadditions (1,3-dipolar cycloadditions), and those leading to more extended ring systems. Diels and Alder were already able to identify essential characteristics of the [4 + 21-cycloaddition:1 . The addition reactions take a stereospecific course resulting in a cis-addition. 2. In the case of cyclic dienes the endo principle is often followed. 3. Unsymmetrically substituted reactants combine regiospecifically to give the cycloadduct. 4. With maleic acid anhydride, valued in those days as the most reactive test dienophile, electron releasing groups in the diene accelerate the reaction, while electron withdrawing groups in the diene retard it. Kinetic measurem e n t~~~ quantified the so-called "Alder-rule", formulated by Alder.A preparatively useful extension of the cycloaddition was provided by the discovery of its catalysis by Lewis-acids ['], which, with retention of stereospecificity, accelerates the rate of addition by several powers of ten. In the presence of Lewis-acids, endo-products and the normally favored regioisomers were formed even to a higher extent.Concepts of the mechanisms of the Diels-Alder reaction were most strongly influenced by the Woodward-Hoffmann rule^^'"-'^'. If in those days the experimental results were con-N atoms implies 3 N -6 degrees of freedom of vibration on a hypersurface of the same dimension-all methods of interpretation are limited from the outset, on the one hand by the selection of simple model reactions and, on the other, by the approximations which are necessary in the case of selected theoretical procedures. Also solvent effects are generally not considered, because theoretical models are normally developed for isolated molecules in in the gas phase. The fact that these limitations can lead to a false interpretation of experimental results, has been demonstrated on a number of occasions in recent years, particularly with regard to vapor phaseacidity and basicity experiments[431.Conclusions drawn from such investigations must always be examined carefully in the light of these approximations. Herndon[2'1 gives a survey of t...

A very small, predominantly cytosolic pool of iron ions plays the central role in the cellular iron metabolism. It links the cellular iron uptake with the insertion of the metal in iron storage proteins and other essential iron-containing molecules. Furthermore, this transit ('labile') pool is essentially involved in the pathogenesis of a number of diseases. Due to its high physiological and pathophysiological significance, numerous methods for its characterization have been developed during the last five decades. Most of these methods, however, influence the size and nature of the transit iron pool artificially, as they are not applicable to viable biological material. Recently, fluorescence spectroscopic methods for measurements within viable cells have become available. Although these methods avoid the artifacts of previous methods, studies using fluorescent iron indicators revealed that the 'intracellular transit iron pool', which is methodically assessed as 'chelatable iron', is substantially defined by the method and/or the iron-chelating indicator applied for its detection, since the iron ions are bound to a large number of different ligands in different metabolic compartments. A more comprehensive characterization of the nature and the role of the thus not uniform cellular transit iron pool therefore requires parallel employment of different indicator molecules, which clearly differ in their intracellular distribution and their physico-chemical characteristics.

A reactivity model for concerted cycloaddition reactions is presented which allows a systematization of substituent effects. The treatment is based on the frontier electron theory of Fukui. The consideration of the energy separations of HOMOs (Highest Occupied Molecular Orbitals) and LUMOs (Lowest

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A new pyrene-based fluorescent probe for the determination of critical micelle concentrations (CMC) is described. The title compound 1 is obtained in five steps, starting from pyrene. Fluorescence spectroscopic properties of 1 are studied in homogeneous organic solvents and aqueous micellar solutions. In a wide range of organic solvents, probe 1 exhibits a characteristic monomer emission of the pyrene fluorophore, with three distinct peak maxima at 382, 404, and 425 nm. The spectra change dramatically in aqueous solution, where no monomer emission of the pyrene fluorophore is detected. Instead, only strong excimer fluorescence with a broad, red-shifted emission band at lambda(max) = 465 nm is observed. In micellar aqueous solution, a superposition of the monomer and excimer emission is found. The appearance of the monomer emission in micellar solution can be explained on the basis of solubilization of 1 by the surfactant micelles. The ratio of the monomer to excimer fluorescence intensities of 1 is highly sensitive to changes in surfactant concentration. This renders 1 a versatile and sensitive probe molecule for studying the micellization of ionic and nonionic surfactants. For a representative selection of common surfactants, the critical micelle concentrations in aqueous solution are determined, showing excellent agreement with established literature data.