Zinc(II)-templated synthesis of a [2]-catenane consisting of a 2,2',6',2' '-terpyridine-incorporating cycle and a 1,10-phenanthroline-containing ring

A nonsymmetrical [2]-catenane has been synthesized, with a 5-coordinated metal center (Zn(2+)) as template. One of the two rings contains a terdentate ligand (2,2',6',2' '-terpyridine) and the other one incorporates a bidentate chelate (1,10-phenanthroline). The first ring was prepared separately and, subsequently, Zn(2+) was used as the gathering and threading element to pass the stringlike component through the ring. This open-chain species bears two terminal olefins, which were reacted with Grubbs first-generation catalyst (ring-closing metathesis) to afford the desired catenane. Hydrogenation of the double bond and removal of the zinc(II) template afforded the final free [2]-catenane in 40% yield from the terdentate ligand-containing cycle and the diolefinic compound. Complexation studies on this new pentacoordinating catenane were carried out with Fe(II) or Cu(II). The most interesting observation is that the 5-coordinated complexes obtained are strongly stabilized. Their electrochemical reduction occurs at negative potentials.     

Nitration in the imidazo[1,2-a]pyrazine series. Experimental and computational results

Nitration was carried out on a series of imidazo[1,2-a]pyrazine derivatives. The reactivities of diversely substituted derivatives and of all positions of substitution were analysed and experimental results compared with ¹³-nmr data and semi empirical calculations (AMI). Although the unsubstituted heterocycle is highly resistant to nitration, electron-donating groups such as alkoxy or alkylamino on position 8 enhance the reactivity of the imidazo[1,2-a]pyrazine derivatives towards electrophilic substitution and, more specifically, nitration. The ¹³-nmr experiments, electronic distributions and Molecular Electrostatic Potential isodensity surfaces calculated on the neutral forms are in good agreement with experimental results indicating position 3 is the most reactive position towards nitration.     

High-resolution X-ray structure of an acyl-enzyme species for the class D OXA-10 beta-lactamase

Beta-lactamases are resistance enzymes for beta-lactam antibiotics. These enzymes hydrolyze the beta-lactam moieties of these antibiotics, rendering them inactive. Of the four classes of known beta-lactamases, the enzymes of class D are the least understood. We report herein the high-resolution (1.9 A) crystal structure of the class D OXA-10 beta-lactamase inhibited by a penicillanate derivative. The structure provides evidence that the carboxylated Lys-70 (a carbamate) is intimately involved in the mechanism of the enzyme.     

Synthesis of some spin-labeled bithiazoles,Useful probes for studying bleomycin-DNA binding

The antibiotic antitumor drug Bleomycin is known to bind to DNA by intercalation of its bithiazole chromophore into the DNA base-pairs. Four spin-labeled bithiazoles, bearing a 4-(2,2,6,6-tetramethyl-1-piperidinooxy) free radical, have been synthesized. These derivatives where the nitroxide group is linked to the end of spacers of various lengths, can be used as probes for the study, by esr spectroscopy, of the intercalation of the heterocyclic moiety of Bleomycin.     

Probing the shape and stereochemistry of molecular orbitals in locally flexible aromatic chains: a penning ionization electron spectroscopy and Green's function study of the electronic structure of biphenyl

We report on the results of an exhaustive study of the interplay between the valence electronic structure, the topology and reactivity of orbitals, and the molecular structure of biphenyl by means of Penning ionization electron spectroscopy in the gas phase upon collision with metastable He*(2(3)S) atoms. The measurements are compared with one-particle Green's function calculations of one-electron and shake-up valence ionization spectra employing the third-order algebraic diagrammatic construction scheme [ADC(3)]. Penning ionization intensities are also analyzed by means of the exterior electron-density model and comparison with photoelectron spectra: in contrast with the lines originating from sigma orbitals, ionization lines belonging to the pi-band system have large Penning ionization cross sections due to their greater extent outside the molecular van der Waals surface. The involved chemi-ionization processes are further experimentally investigated using collision-energy-resolved Penning ionization electron spectroscopy. The cross sections of pi-ionization bands exhibit a markedly negative collision-energy dependence and indicate that the interaction potential that prevails between the molecule and the He*(2(3)S) atom is strongly attractive in the pi-orbital region. On the other hand, the partial ionization cross sections pertaining to sigma-ionization channels are characterized by more limited collision-energy dependencies, as a consequence of rather repulsive interactions within the sigma-orbital region. A comparison of ADC(3) simulations with the Penning ionization electron spectra and UV photoelectron spectra measured by Kubota et al. [Chem. Phys. Lett. 1980, 74, 409] on thin films of biphenyl deposited at 170 and 109 K on copper demonstrates that biphenyl molecules lying at the surface of polycrystalline layers adopt predominantly a planar configuration, whereas within an amorphous sample most molecules have twisted structures similar to those prevailing in the gas phase.     

The Ni-mediated cyclocarbonylation of allyl halides and alkynes made catalytic. Evidence supporting the involvement of pseudoradical Ni(i) species in the mechanism

We report here on a highly efficient catalytic method to synthesize intermolecularly the cyclopentane skeleton from starting products as simple as allyl halides, alkynes, and carbon monoxide under very mild reaction conditions by means of a substoichiometric amount of iron, acetone, and a catalytic amount of Ni(II) iodide.     

Sticking probability on zeolites

The sticking coefficient, i.e., the probability that, on hitting the surface of a nanoporous particle (zeolite), a molecule shall be able to enter the intracrystalline space, is a key quantity for the application of such materials in heterogeneous catalysis and molecular sieving. On the basis of pulsed field gradient NMR diffusion measurements and molecular dynamics simulations, typical values of this probability are found to be close to one. They exceed previous estimates on the basis of IR uptake measurements by many orders of magnitude.     

New chiral pi-conjugated polymers based on a (1R,2R)-diiminocyclohexane chiral unit with weak interchain pi stacking

New chiral pi-conjugated polymers consisting of alternating conjugated segments and (1R,2R)-diiminocyclohexane units with C2 symmetry were prepared by a palladium-catalyzed coupling reaction, they exhibited very high specific optical rotations ([alpha] up to -3000 degrees) and strong Cotton effects ([theta] 10(6) deg cm2 mol-1).     

Synthèse et caractérisation d'une série de bis-9,9′(thio-9-acridinyl)-α,ω-alcanes

Some new bis-acridine derivatives have been prepared under phase transfer catalysis conditions. These are α,ω-bis-(9-thioacridinyl)alkanes with or without heteroatoms such as N or O, included into the bridge. In the latter case, general procedure has to be slightly modified due to a side-reaction leading to thiobenzyl derivatives, which increases in the conditions proposed in the first place. The compounds so prepared were characterized by their melting points, ¹H and ¹³C nmr data. Preliminary results referring to activity against P-388 lymphocitic leukemia are presented.     

Rubidium-Pentafluoromanganat(III) und dessen Monohydrat,strukturelle Daten und Topotaxiebeziehungen

Rubidiumpentafluoromanganate (III) and its monohydrate, crystal data and topotaxy The unit cells and most probable space groups of the new compounds Rb₂MnF₅ · H₂O and Rb₂MnF₅ have been determined from single crystal X-ray diffraction data and the powder diffraction patterns indexed. . By analogy to the stoichiometrically similar phases high temperature K₂MnF₅ · H₂O, the unit cell of which has also been determined and its powder diffraction pattern indexed, and (NH₄)₂MnF₅, model structures for the rubidium salts are proposed. Both structures consist of infinite chains of corner sharing coordination octahedra [MnF₆], between which the rubidium ions and in the case of the hydrate additional water molecules are located. X-ray diffraction and scanning electron microscopical investigations of the dehydration of the hydrate clearly point to a chain controlled topotactic reaction mechnism. This supports the proposed model structures and confirms the possibility of predicting topotactic reaction mechanisms.