Cluster Build-up Reactions using Di-gold Cationic Fragments: Synthesis and Structural Characterization of [Os7(CO)19(Au2dppm)]

The reaction of the di-gold cation [Au₂(dppx)]²⁺ with the heptanuclear cluster dianion [Os₇(CO)₂₀]^2– affords the mixed metal cluster [Os₇(CO)₂₀{Au₂(dppx)}] (x=m (1), e (2), b (3)). On standing, in solution, this complex undergoes decarbonylation to give the cluster [Os₇(CO)₁₉{Au₂(dppx)}] (x=m (4), e (5), b (6)). The complexes have been characterised spectroscopically, and an X-ray structure determination of the dppm derivative shows that it contains a metal core based on an Os₇ edge-bridged bicapped tetrahedron with the two ₃-Au atoms capping adjacent triangular Os₃ faces of the central tetrahedron. In an analogous reaction, the carbido anion [Os₇(H)C(CO)₁₉]^– affords the neutral cluster [Os₇C(CO)₁₉{Au₂(dppm)}] (7) when treated with [Au₂(dppm)]²⁺ in the presence of base.     

Preparation of 5-substituted 3-aminofuran-2-carboxylate esters

[reaction: see text] An efficient method for the preparation of 3-aminofuran-2-carboxylate esters has been developed. This method is based on the reaction of an alpha-cyanoketone with ethyl glyoxylate under Mitsunobu conditions to produce a vinyl ether in good yield. Subsequent treatment of the vinyl ether with sodium hydride afforded the 3-aminofuran. It was also found that a one-pot procedure using the Mitsunobu reaction followed by cyclization afforded the 3-aminofuran in comparable yield. Currently, this method is limited to the synthesis of 5-alkyl-, 5-aryl-, and 4,5-fused bicyclic furans.     

Molecular dynamics simulations of a nucleoside analogue of 1,4-dihydro-4-oxoquinoline-3-carboxylic acid synthesized as a potential antiviral agent: Conformational studies in vacuum and in water

Conformational analysis of nucleosides may have direct applications to the structure–activity relationship (SAR) studies and in the design of new drug candidates. Although conformational analysis may be accessed in many different ways, in this work it was performed using molecular dynamics (MD) simulation in order to study the dynamic behavior of a nucleoside derivative of 1,4-dihydro-4-oxoquinoline-3-carboxylic acid, synthesized by our group as a potential antiviral agent. The MD simulation was carried out during 10 ns in vacuum and in a box of water at two different temperatures (i.e., 300 and 600 K) using the AMBER force field. The in vacuum MD simulation results are in agreement with the crystallographic structure and with the DFT calculations of the nucleoside, revealing the anti conformer as the more stable one. The simulation in water, however, shows that both conformers may exist at 300 K, the temperature of the in vivo and in vitro assays, revealing that both the syn and anti conformers should be considered in a MD simulation study of the inhibitor–enzyme complex. Simulations are also in agreement with the NOE experiment, which shows that the anti conformer is the preferential one in DMSO-d6 solution at 298 K.     

Determining molecular structures and conformations directly from electron diffraction using a genetic algorithm.

A global optimization strategy, based upon application of a genetic algorithm (GA), is demonstrated as an approach for determining the structures of molecules possessing significant conformational flexibility directly from gas-phase electron diffraction data. In contrast to the common approach to molecular structure determination, based on trial-and-error assessment of structures available from quantum chemical calculations, the GA approach described here does not require expensive quantum mechanical calculations or manual searching of the potential energy surface of the sample molecule, relying instead upon simple comparison between the experimental and calculated diffraction pattern derived from a proposed trial molecular structure. Structures as complex as all-trans retinal and p-coumaric acid, both important chromophores in photosensing processes, may be determined by this approach. In the examples presented here, we find that the GA approach can determine the correct conformation of a flexible molecule described by 11 independent torsion angles. We also demonstrate applications to samples comprising a mixture of two distinct molecular conformations. With these results we conclude that applications of this approach are very promising in elucidating the structures of large molecules directly from electron diffraction data.     

Multiphoton ionization vibrational state selection of H2O, D2O and HDO

We present a study of single color (2 + 1) resonance-enhanced multiphoton ionization (REMPI) of H₂O, D₂O, and HDO via several Rydberg states lying in the energy range from 80 000 to 86 000 cm⁻¹. Photoelectron spectra (PES) show that the corresponding cations can be vibrationally state-selected for most vibrational states. The exception is the bend of H₂O⁺ and HDO⁺, where mixing in the REMPI intermediate level results in weak ion intensity and only 50% state purity.     

Synthesis and reactions of 4-isopropylidene-1-aryl-3-methyl-2-pyrazolin-5-ones

The reactions of 4-isopropylidene-1-aryl-3-methyl-2-pyrazolin-5-ones 4a-d were investigated under a variety of conditions. In the presence of thiols or piperidine, 4a-d failed to yield conjugate addition products, presumably due to the steric bulk provided by the two methyl substituents of the isopropylidene side chain. Reaction of 4a-d with hydrazine derivatives gave the 1-aryl-3-methyl-2-pyrazolin-5-ones 3a-d and isopropyl-hydrazones. Treatment of 4a with potassium cyanide yielded a stable conjugate addition product which exists as a mixture of tautomers in different solvents. Also, oxidation of 4a with hydrogen peroxide gave a spiroepoxide 22 , while m-chloroperbenzoic acid oxidation afforded both the spiroepoxide 22 , and a small quantity of a hydroxyspiroepoxide 23.     

Novel resorcinarene-based pH-triggered gelator

Iminodiacetate resorc[4]arene is shown to produce gels that are pH-reversible. The gels formed are clear and stable to inversion. (The gels are not stable above a certain temperature, which varies with concentration.) When the pH of an iminodiacetate resorc[4]arene solution (concentration > 7.6 mM) is lowered to below 2.5, which is near one of the Ka values of the molecule, the dissolved molecules aggregate and cause gelation of water. 1H NMR showed that at the pH of gelation a change occurs in the chemical environment of the iminodiacetate group. Scanning electron microscopy showed that the gel is composed of long strands that interweave and create a molecular mesh.     

EVIDENCE AGAINST THE PRODUCTION OF SUPEROXIDE BY PHOTOIRRADIATION OF HEMATOPORPHYRIN DERIVATIVE

Abstract Experiments were performed to ascertain whether superoxide anion (O₂⁻) was produced by the photodynamic activation of hematoporphyrin derivative (HPD). Three different systems were utilized to detect formation of O₂⁻, oxidation of epinephrine to adrenochrome, reduction of cytochrome c and reduction of nitro blue tetrazolium (NBT). The effects on these detectors under identical conditions for HPD + h ν were compared to those obtained with two O₂⁻ generating systems, riboflavin + by and xanthine-xanthine oxidase, and to a singlet oxygen generating system, photoradiation of methylene blue. The results indicated that HPD + hv differed from the two O₂⁻ generating systems in failing to reduce cytochrome c or NET, and that HPD + h ν was similar to the behavior of methylene blue + h ν . In addition, HPD + h ν but not the O₂⁻ generating systems could inhibit mitochondrial cytochrome c oxidase activity. We conclude that the photodynamic activation of HPD does not produce O₂⁻ as a major oxygen radical and that the effects of HPD + h ν on mitochondrial cytochrome c oxidase are not caused by O₂⁻.     

Stereoselective synthesis of optically active bicyclic beta-lactam carboxylic acids that target pilus biogenesis in pathogenic bacteria

Optically active bicyclic beta-lactams were synthesized, starting from 2-H-delta 2-thiazolines and Meldrum's acid derivatives. Several methods to accomplish an ester hydrolysis without damaging the beta-lactam framework were investigated. A rapid CsOH saponification of the beta-lactam methyl esters was developed and protonation of the Cs-carboxylates by Amberlite (IR-120 H+) afforded a series of bicyclic beta-lactam carboxylic acids. Moreover, a convenient method for the synthesis of 2-H-delta 2-thiazolinecarboxylic acid methyl ester 2 was developed. Bicyclic beta-lactam carboxylic acids 7a-g and aldehydes 4a-d were screened for their affinity to the bacterial periplasmic chaperone PapD using a surface plasmon resonance technique. beta-Lactams substituted with large acyl substitutents showed better binding to the chaperone than the native C-terminal peptide PapG 8, demonstrating that bicyclic beta-lactams constitute a new class of potential bacterial chaperone inhibitors.