Tétraazamacrocycles lipophiles: Extraction d'ions métalliques par une résine imprégnée

Lipophilic Tetraazamacrocyles: Extraction of Metal Ions by Impregnated Resin Extraction of Co²⁺, Ni²⁺, Cu²⁺, Zn²⁺, Cd²⁺, Pb²⁺ and Ag⁺ by lipophilic tetraazamacrocycles dispersed on a solid matrix (Amberlite XAD7) is reported. Extraction efficiency is sensitive to the metal ion identity and the cavity size of the macrocycle. The influence of kinetic factors upon extraction efficiency is discussed.     

Réactions de cyclisation de pentynyl-2 pyrimidones-4

Acetylenic amindines HC≈?C·(CH₂)_n·C(?NH)NH₂ give, by condensation with β-ketoesters, pyrimidones substituted on carbon 2 by an acetylenic chain. Two types of evolution are observed when compounds are heated without any catalyst. The minor route is a cyclization by attack on a triple bond by the amidic nitrogen atom. The main reaction is a cycloaddition involving the non activated triple bond and an azadienic system, leading to non isolated tricyclic intermediates which retrocyclize to stable bicyclic compounds.     

Crystal structure and hydrogen bonding system in cellulose I(alpha) from synchrotron X-ray and neutron fiber diffraction

The crystal and molecular structure, together with the hydrogen-bonding system in cellulose I(alpha), has been determined using atomic-resolution synchrotron and neutron diffraction data recorded from oriented fibrous samples prepared by aligning cellulose microcrystals from the cell wall of the freshwater alga Glaucocystis nostochinearum. The X-ray data were used to determine the C and O atom positions. The resulting structure is a one-chain triclinic unit cell with all glucosyl linkages and hydroxymethyl groups (tg) identical. However, adjacent sugar rings alternate in conformation giving the chain a cellobiosyl repeat. The chains organize in sheets packed in a "parallel-up" fashion. The positions of hydrogen atoms involved in hydrogen-bonding were determined from a Fourier-difference analysis using neutron diffraction data collected from hydrogenated and deuterated samples. The differences between the structure and hydrogen-bonding reported here for cellulose I(alpha) and previously for cellulose I(beta) provide potential explanations for the solid-state conversion of I(alpha) --> I(beta) and for the occurrence of two crystal phases in naturally occurring cellulose.     

Synthesis and reactivity of pyrrolo[1,2-α]quinoxalines. Crystal structure and AM1 calculation

2-Methylquinoxaline reacts with ethyl bromopyruvate giving 2-substituted pyrrolo[1,2-α]quinoxalines. The yield of the condensation depends on the functionalization of starting materials, and optimization is obtained with 2-dimethylamino-3-methylquinoxaline ( 1c ). Reactivity of the resulting pyrrolo[1,2-a]-quinoxalines was investigated and supported by a theoretical approach (AM1 calculation performed with the MOPAC 6.0 software). X-ray analysis of 5 which crystallizes in the monoclinic system, space group P2₁/n, with a = 9.095(1), b = 8.972(1), c = 17.749(3) A, β = 96.56(1)°, is also reported.     

Synthesis and structure of the first diamagnetic trinuclear cobalt carbonyl sulfur complexes,SCo3(CO)7(μ-R1ĆNR2)

Reaction of primary thioamides with dicobalt octacarbonyl affords in good yield the first diamagnetic monomeric trinuclear cobalt carbonyl sulfur complexes SCO₃(CO)₇(μ-R¹?NR²), which contain a bridging bidentate imino ligand. The structure of the product with R¹  Me, R²  C₆H₁₁ has been determined by X-ray diffraction, and shown to include a very short cobaltcobalt bond distance and a long cobaltsulfur bond distance when compared to other cobaltsulfur clusters.     

Voies d'accès à un analogue pyrannique de la puromycine

In a continuation of our studies with methyl 3-amino-3,4-dideoxypetopyranosides, two pyranic analogues of puromycin 3A and 3b have been synthesized by two routes starting from methyl 2,3-anhydro-4-deoxy-or D, L -erythro-pentopyranosides. In the first one, both amino group and erythro stereochemistry have been introduced before the introduction of the purine moiety; in the second one the introduction of the erythro stereochemistry and of the 3-butrigeb atom has, more advantageously, been performed in the same step using an intermediate (22) with a suitable conformation. The two diastereoisomerws 3A and 3B have been separated and, after assignment of absolute configuration and the heterocyclic studies, will permit and evaluation of the role plays played by the heterocyclic oxygen atom by comparison with previous studies on cyclohexyl puromycin analogues (Vince et al.).     

Organocatalyzed route to enantioenriched pipecolic esters: decarboxylation of an aminomalonate hemiester

Enantioenriched pipecolic esters were prepared in good yields in the decarboxylation, at room temperature, of N-protected piperidinohemimalonates catalyzed by cinchona alkaloids. Enantiomeric excesses as high as 72% were obtained when using 9-epi-cinchonine and the N-benzoyl substituted piperidinohemimalonate. A detailed study of the different reaction parameters revealed that the selectivity of this noncovalent organocatalyzed reaction is strongly dependent on the solvent, toluene or carbon tetrachloride being the best ones. The whole process based on the malonic acid synthesis was successfully tested on a 10 mmolar scale and established a practical alternative to the asymmetric protonation of lithium enolates.     

Ultrastructural aspects of the acetylation of cellulose

An ultrastructural study of the acetylation of cellulose was achieved by subjecting well characterized cellulose samples fromValonia cell wall and tunicin tests to homogeneous and heterogeneous acetylation. The study involved transmission electron microscopy observations on negatively stained microcrystals as well as diffraction contrast images of the cross sections of wall fragments at various stages of the reaction. These observations showed that the acetylation of crystalline cellulose proceeds by a reduction of the diameters of the crystals while their lengths are reduced to a lower extent. These results were corroborated by electron and X-ray diffraction experiments that showed that during the reaction there was a rapid decrease in the intensities of the equatorial diffraction spots of cellulose, whereas those located on the meridian or close to the meridian stayed constant. A model of acetylation of the cellulose crystal is presented. It is based on a non swelling reaction mechanism that affects only the cellulose chains located at the crystal surface. In the case of homogeneous acetylation, the partially acetylated molecules are sucked into the acetylating medium as soon as they are sufficiently soluble. In heterogeneous conditions the cellulose acetate remains insoluble and surrounds the crystalline core of unreacted cellulose.     

[2 + 2] Cycloaddition reactions of unsymmetrically substituted carbodiimides

N-Alkyl-N′-arylcarbodiimides add alkyl and aryl isocyanates to the N-alkyl substituted CN double bond to yield 4-arylimino-1,3-diazetidine-2-ones 3. In the case of bulky alkyl substituents the reaction still proceeds across the sterically hindered CN double bond. N-Aryl-N′-[(4-dimethylamino)phenyl]carbodiimides form [2 + 2] cycloadducts with aryl isocyanates preferentially across the CN double bond not attached to the electron-rich aryl groups. However, steric crowding on this CN double bond directs the reaction to the adjacent double bond of the heterocumulative system. The rate of the [2 + 2] cycloaddition reaction of N'-methyl-N′-phenylcarbodiimide with 4-nitrophenylisocyanate is about 2.5 times faster in acetonitrile than in benzene.     

Librational relaxation and IR line broadening of matrix isolated CO

The IR line profile corresponding to the fundamental vibrational transition of CO isolated in an Ar matrix has been studied as a function of the temperature. The spectrum consists of two superimposed bands, a sharp one and a broad one whose linewidths are respectively 0.45 and 2.7 cm^?1 at 10 K. The strong temperature dependence of the CO lineshape is mainly due to the large change in the intensity ratio of the two components. Such a behaviour is accounted for by relaxation of librational populations. The calculated contributions of the processes connecting the ground and first excited librational states to the IR linewidth are of the same order of magnitude as the values experimentally measured. However the experimental lineshape of the sharp component being rather gaussian than lorentzian, it is concluded that librational relaxation contributes significantly to the broadening of the infrared CO band but not exclusively.