Unusual selectivity of unprotected aziridines in palladium-catalyzed allylic amination enables facile preparation of branched aziridines

Functionalized branched aziridines can be prepared in high yields and with high levels of regioselectivity using unprotected aziridines as nitrogen sources in palladium-catalyzed allylic amination. High levels of enantioselectivity can be achieved with BINAP on palladium. This methodology allows for strategic placement of an aziridine-containing fragment within a complex molecule environment for further elaboration.     

Modified nucleosides in human serum.

Methylated purines and pyrimidines derived from the degradation of transfer ribonucleic acid have been shown to be excreted in abnormal amounts in the urine of patients with cancer. Recent technology developed by Gehrke and Kuo has allowed the separation and quantification of modified nucleosides in serum using reversed-phase high-performance liquid chromatography with diode-array measurement. Serum levels of ten modified nucleosides were measured in 37 normal healthy adults to establish normal values and to correlate activity with age and sex. In addition, serum levels of patients with several malignancies were measured to determine activity in these diseases. Levels of modified nucleosides in normal individuals were consistently reproducible and showed no significant variation among males versus females or with age. Patients with malignant diseases showed consistent elevations and these were highest in patients with more advanced disease. The evidence of no significant differences in the mean levels of modified nucleosides in serum with age or sex in normal adults and elevations in patients with malignancies demonstrate the potential value of modified nucleosides as cancer biomarkers.     

The first structural characterization of a phosphonium amide: synthesis, isolation and molecular structure of (Ph3PEt)(NPh2)

The first isolation of a phosphonium amide, (Ph₃PEt)⁺(NPh₂)⁻, obtained via the deprotonation of a secondary amine (diphenylamine) with a phosphorus ylide (triphenylphosphonium ethylide), is described. An X-ray crystal structure provides the first observation of an essentially ion-separated Ph₂N⁻ anion in the solid state, though weak association with Ph₃PEt⁺ cations take plac through C---H…N hydrogen bonding. Cryoscopic and NMR spectroscopic data suggest that this association is not maintained in benzene solution.     

Strong intramolecular secondary si.N bonds in trifluorosilylhydrazines

The simple silylhydrazines F(3)SiN(Me)NMe(2) (1), F(2)Si(N(Me)NMe(2))(2) (2), and F(3)SiN(SiMe(3))NMe(2) (3) have been prepared by reaction of SiF(4) with LiN(Me)NMe(2) and LiN(SiMe(3))NMe(2), while F(3)SiN(SnMe(3))NMe(2) (4) was prepared from SiF(4) and (Me(3)Sn)(2)NNMe(2) (5). The compounds were characterized by gas-phase IR and multinuclear NMR spectroscopy ((1)H, (13)C, (14/15)N, (19)F, (29)Si, (119)Sn), as well as by mass spectrometry. The crystal structures of compounds 1-5 were determined by X-ray crystallography. The structures of free molecules 1 and 3 were determined by gas-phase electron diffraction. The structures of 1, 2, and 4 were also determined by ab initio calculations at the MP2/6-311+G** level of theory. These structural studies constitute the first experimental proof for the presence of strong Si.N beta-donor-acceptor bonds between the SiF(3) and geminal NMe(2) groups in silylhydrazines. The strength of these non-classical Si.N interactions is strongly dependent on the nature of the substituent at the alpha-nitrogen atom of the SiNN unit, and has the order 3>4>1. The valence angles at these extremely deformed alpha-nitrogen atoms, and the Si.N distances are (crystal/gas): 1 104.2(1)/106.5(4) degrees, 2.438(1)/2.510(6) A; 3 83.6(1)/84.9(4) degrees, 2.102(1)/2.135(9) A; 4 89.6(1) degrees, 2.204(2) A.     

Collective and single-molecule interactions of alpha5beta1 integrins

A novel biomimetic system was used to study collective and single-molecule interactions of the alpha5beta1 receptor-GRGDSP ligand system with an atomic force microscope (AFM). Bioartificial membranes, which display peptides that mimic the cell adhesion domain of the extracellular matrix protein fibronectin, are constructed from peptide-amphiphiles. The interaction measured with the immobilized alpha5beta1 integrins and GRGDSP peptide-amphiphiles is specifically related to the integrin-peptide binding. It is affected by divalent cations in a way that accurately mimics the adhesion function of the alpha5beta1 receptor. The recognition of the immobilized receptor was significantly increased for a surface that presented both the primary recognition site (GRGDSP) and the synergy site (PHSRN) compared to the adhesion measured with surfaces that displayed only the GRGDSP peptide. At the collective level, the separation process of the receptor-ligand pairs is a combination of multiple unbinding and stretching events that can accurately be described by the wormlike chain (WLC) model of polymer elasticity. In contrast, stretching was not observed at the single-molecule level. The dissociation of single alpha5beta1-GRGDSP pairs under loading rates of 1-305 nN/s revealed the presence of two activation energy barriers in the unbinding process. The high-strength regime above 59 nN/s maps the inner barrier at a distance of 0.09 nm along the direction of the force. Below 59 nN/s a low-strength regime appears with an outer barrier at 2.77 nm and a much slower transition rate that defines the dissociation rate (off-rate) in the absence of force (k(off) degrees = 0.015 s(-1)).     

Efficient synthesis of cellulose furoates in 1-<Emphasis Type="Italic">N</Emphasis>-butyl-3-methylimidazolium chloride

The ionic liquid 1-N-butyl-3-methylimidazolium chloride ([C₄mim]⁺Cl⁻) was investigated as reaction media for the homogeneous acylation of cellulose with 2-furoyl chloride in the presence of pyridine. The preparation of cellulose furoate depending on the reaction conditions, the cellulose type and the pyridine content was studied. Cellulose furoates with a degree of substitution in the range from 0.46 to 3.0 were accessible, i.e., under mild conditions, with a low excess of reagent and in a short reaction time. The products were characterized by elemental analysis, perpropionylation, ¹H- and ¹³C NMR spectroscopy and FTIR spectroscopy. Thomas Heinze is the member of the European Polysaccharide Network of Excellence (EPNOE), www.epnoe.eu     

First organocatalyzed asymmetric Michael addition of aldehydes to vinyl sulfones

[reaction: see text] The first asymmetric direct Michael addition of aldehydes to vinyl sulfones catalyzed by N-iPr-2,2'-bipyrrolidine is described. 1,4-Adducts are obtained in good yields and enantioselectivities. The determination of absolute configuration allowed us to postulate a Si,Si transition state model, as shown previously for nitroolefins.     

On the representation of electrostatic fields around ab initio charge distributions

Summary We compare two methods (Mulliken charges and a distributed multipole analysis, DMA) of representing an ab initio charge distribution for calculating the electrostatic field and potential outside the molecule, using pyrimidine and the RNA base uracil as examples. This is done using a 3-D graphical display of the electrostatic fields, which, when used with real-time rotation, zooming and clipping, has many advantages for qualitatively assessing the electrostatic interactions of a molecule. The errors involved in using Mulliken point charges may be of similar magnitude to the total electrostatic field in regions which are important in recognition processes. The DMA representation automatically includes the anisotropic electrostatic effects of non-spherical features in the charge distribution of each atom, and yet the displayed electrostatic fields around the atoms which have lone-pair density do not show marked anisotropy.     

2]pseudorotaxane formation with N-benzylanilinium axles and 24-crown-8 ether wheels

[reaction: see text] As a hybrid of the N,N-dibenzylammonium and 1,2-bis(pyridinium)ethane axles, various N-benzylanilinium cations were investigated as suitable axles for the formation of [2]pseudorotaxanes with the 24-membered crown ethers 24C8 and DB24C8. The effect of electron-donating OCH(3) and electron-withdrawing CF(3) groups on both the anilinium and benzyl aromatic rings was studied. Formation constants and structural details were compared to the [2]pseudorotaxanes formed by the two aforementioned dibenzylammonium and 1,2-bis(pyridinium)ethane axles.     

Two new double Rydberg anions plus access to excited states of neutral Rydberg radicals via anion photoelectron spectroscopy

We have observed and characterized two new double Rydberg anions N6H19- and N7H22- through their anion photoelectron spectra. The vertical detachment energies of these anions were found to be 0.443 and 0.438 eV, respectively. In addition, for three of the seven double Rydberg anions now known, we measured photodetachment transitions not only to the ground electronic states of their corresponding neutral Rydberg radicals but also to their first electronically excited states. In each spectrum, the energy spacing between the resulting peaks provided the ground-to-first electronically excited-state transition energy for the double Rydberg anion's corresponding neutral Rydberg radical. For the radicals, N4H13, N5H16, and N6H19, the spacings were found to be 0.83, 0.70, and 0.67 eV, respectively. These values are in excellent agreement with ground-to-first excited-state transition energies measured in absorption for the same neutral Rydberg radicals by Fuke and co-workers [Eur. Phys. J. D 9, 309 (1999); J. Phys. Chem. A 106, 5242 (2002).] The duplication of this neutral Rydberg property by photodetachment of double Rydberg anions further confirms that double Rydberg anions are indeed the negative ions of their corresponding neutral Rydberg molecules and cluster-like systems.