Syntheses of Macrocyclic Amides from L-Amino Acid Esters by RCM

IntroductionThe discovery of stable and defined carbenecomplexes of molybdenum and ruthenium as efficientprecatalysts for olefin metathesis has made this transfor-mation one of the most important C—C bond formingreactions[1—11]. Specifically, ring-closi…     

Cohesive energy densities of polyethers: Poly(hexamethylene oxide)

The solubility parameter of poly(hexamethylene oxide) has been estimated by measuring solution viscosities and by turbidimetric titrations in a series of solvents. From both experimental methods, a value of $\text{δ}{}_{\mathrm{<mtext>p</mtext>}}\text{= 33.9}\text{J}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{cm}{}^{\mathrm{<mtext>?3</mtext><mtext>2</mtext>}}$ ($\text{8.1}\text{cal}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{cm}{}^{\mathrm{<mtext>?3</mtext><mtext>2</mtext>}}$) was obtained, whence the cohesive energy density is 274.5 J cm^?3 (65.6 cal cm^?3). These experimental values are compared with those calculated by empirical methods.     

New aspects of the electroadsorption of ethyl xanthate on copper electrodes

The interaction of the ethyl xanthate (EX) anion with a copper electrode in a borate buffer solution, pH 9.2, has been investigated by cyclic voltammetry (CV), electrochemical quartz crystal microbalance (EQCM), electrochemical impedance spectroscopy (EIS), and measurements of contact angle (CA) under controlled potential. The results obtained allow establishing that, in the potential range from -0.80 and -0.60 V, two parallel reactions were characterized. These reactions were the ethyl xanthate electroadsorption and the hydrogen evolution reaction (HER). This last reaction has not been described by previous authors. Besides, the EIS measurements show that the mechanism of the HER on copper electrodes is not affected by the presence of a ethyl xanthate species. The EQCM study shows that in the electrodesorption process the departure of each ethyl xanthate species from the copper electrode is accompanied with the simultaneous entry of four to five water molecules. This fact is in accordance with the number of copper atoms involved in the adsorption of one ethyl xanthate species.     

Synthesis and catalytic activity of a fluorous-tagged TEMPO radical

A fluorous-tagged TEMPO radical has been prepared and its catalytic activity in the chemoselective oxidation of alcohols to carbonyl compounds has been investigated. The new fluorous radical proved to be an efficient, selective and easily recoverable catalyst, which can be conveniently used in standard organic solvents and then isolated and recycled by fluorous liquid-liquid extraction.     

Domino reactions of 4,5-dicyanopyridazine with dihydroheterocycles: synthetic and mechanistic features

The title pyridazine 1 was found to react with both 2,3-dihydrofuran (2) and 3,4-dihydro-2H-pyran (9) to give the tetracyclic skeletons 5-8 and the phthalonitrile 12 through the intermediates 4 and 10, respectively. A more complex mechanism was ascertained for the reaction of 1 with the pyrroline 14 which, under suitable conditions, afforded the bicyclic derivative 19 as the predominant product; selective elaborations of this species into the 5,6-dicyanoindoles 22 and 23 are reported.     

Characterisation of different clones of Picea abies (L.) Karst using head-space sampling of cortical tissues combined with enantioselective capillary gas chromatography for the separation of chiral and non-chiral monoterpenes

Head-space sampling (HS) has been combined with enantioselective gas chromatography (GC) for the analysis of chiral and non-chiral monoterpenes present in the cortical tissues of five different Norway spruce clones. (1S)-(-)-alpha-Pinene, (1S,5S)-(-)sabinene, (1S)-(-)-beta-pinene, and (4S)-(-)limonene dominated over (1R)-(+)-alpha-pinene, (1R,5R)-(+)-sabinene, (1R)-(+)-beta-pinene, and (4R)-(+)-limonene. Results showed a large variation in the enantiomeric composition of cortical tissues between different clones. The development of HS-GC greatly increased the speed of precise analyses of chiral monoterpenes in small samples and therefore offer excellent opportunities in studies on the ecophysiological and chemotaxomic roles of these chiral components.     

Substituent-controlled photoisomerization in retinal chromophore models: Fluorinated and methoxy-substituted protonated Schiff bases

The effect of substitution on the intrinsic (i.e. in vacuo) photoisomerization ability of retinal chromophore models has been explored using CASPT2//CASSCF minimum energy path computations to map the S₁ photoisomerization paths of two substituted minimal models of the retinal chromophore: the 2-cis-penta-2,4-dieniminium and the all-trans-epta-2,4,6-trieniminium cations, which have been modified using fluorine or methoxyl substituents as representative examples of electron-withdrawing and electron-releasing groups, respectively. A systematic analysis has been performed involving substitutions in all the possible positions along the chain. It is shown that the photochemical reactivity and photoisomerization efficiency of these systems may be tuned or indeed changed, although this effect strongly depends on the position of the substituent. In particular, we have shown that (i) most of the systems preserves qualitatively the reactivity of the parent (i.e. unsubstituted) compound; (ii) substitution at positions C4 or C6 leads to a different relaxed excited state structure of the chromophore and in general to a very flat photoisomerization path (or to a tiny S₁ energy barrier in some cases); (iii) the nature of the TICT state (i.e. the S₁ → S₀ decay funnel) may be turned from a conical intersection into an excited state minimum; (iv) for the C4 methoxy-substituted system the isomerization path as well as the S₁/S₀ decay funnel involve an unusual torsional angle. Thus, substitution turns out to be a good tool not only to tune the optical properties (i.e. the absorption and emission features) of the chromophore (as we have already shown in a previous work: I. Conti, F. Bernardi, G. Orlandi, M. Garavelli, Mol. Phys. 104 (2006) 915–924), but it may also play a crucial role in tuning and controlling photoisomerization selectivity and efficiency, affecting excited state lifetime and reaction rate. A rationale for these effects is presented, which provides a basis for understanding reactivity properties and the intrinsic photochemical behavior of substituted retinal chromophores.     

Die Millon'sche Reaktion des Harns und ihre Beeinflussung durch Zufuhr von Tyrosin

Zusammenfassung Es werden einige Versuche an Kaninchen und Menschen über die Millon'sche Reaktion des Harns mitgeteilt. Ihre Intensität steigt an, wenn Tyrosin zugeführt wird, und zwar erheblich mehr nach dl-Tyrosin als nach l-Tyrosin. Ein Teil der Millon-Substanzen ist in Äther löslich, der ebenfalls nach der razemisierten stärker zunimmt als der nach der natürlichen Aminosäure.     

Reverse micellar aggregates: effect on ketone reduction. 2. Surfactant role

Kinetics of the reduction of 3-chloroacetophenone (CAF) with sodium borohydride (NaBH(4)) were followed by UV-vis spectroscopy at 27.0 degrees C in different reverse micellar media, toluene/BHDC/water and toluene/AOT/water, and compared with results in an isooctane/AOT/water reverse micellar system. AOT is sodium 1,4-bis-2-ethylhexylsulfosuccinate, and BHDC is benzyl-n-hexadecyl dimethylammonium chloride. The kinetic profiles were investigated as a function of variables such as surfactant and NaBH(4) concentration and the amount of water dispersed in the reverse micelles, W(0) = [H(2)O]/[surfactant]. In all cases, the first-order rate constant, k(obs), increases with the concentration of surfactant as a consequence of incorporating the substrate into the interface of the reverse micelles where the reaction takes place. The reaction is faster at the cationic interface than at the anionic one probably because the negative ion BH(4)(-) is part of the cationic interface. The effect of the external solvent on the reaction shows that reduction is favored in the isooctane/AOT/water reverse micellar system than that with an aromatic solvent. This is probably due to BH(4)(-) being more in the water pool of the toluene/AOT/water reverse micellar system. The kinetic profile upon water addition depends largely on the type of interface. In the BHDC system, k(obs) increases with W(0) in the whole range studied while in AOT the kinetic profile has a maximum at W(0) approximately 5, probably reflecting the fact that BH(4)(-) is part of the cationic interface while, in the anionic one, there is a strong interaction between water and the polar headgroup of AOT below W(0) = 5 and, above that, BH(4)(-) is repelled from the interface once the water pool has formed. Application of a kinetic model based on the pseudophase formalism, which considers the distribution of the ketone between the continuous medium and the interface and assumes that reaction takes place only at the interface, has enabled us to estimate rate constants at the interface of the reverse micellar systems. At W(0) < 10, it was considered that NaBH(4) is wholly at the interface and, at W(0) >/= 10, where there are free water molecules, also the partitioning between the interface and the water pool was taken into account. The results were used to evaluate CAF and NaBH(4) distribution constants between the different pseudophases as well as the second-order reaction rate constant of the reduction reaction in the micellar interface.     

Complete-active-space self-consistent-field/Amber parameterization of the Lys296–retinal–Glu113 rhodopsin chromophore-counterion system

A special hybrid quantum mechanics/molecular mechanics forcefield is defined, parameterized and validated for studying the photoisomerization path of the retinal chromophore in the rhodopsin protein. It couples a multireference ab initio Hamiltonian (CASSCF and second-order multireference many-body perturbation theory using a CASSCF reference) to describe the chromophore while the rest of the protein is approximated with the Amber forcefield. The frontier has been carefully parameterized in order to reproduce full quantum mechanics torsional energy profiles, for both the ground state and the first excited state. It is also shown that replacing the chromophore counterion with point charges is a valid approximation. This result is interpreted in terms of a cancellation effect for which a possible explanation is given.