Unprecedented double benzylic rearrangement: regio- and stereospecific tandem 1,4-shift and Curtin rearrangement

An α-benzyloxyketone forming part of a strained cyclopentane carbon framework when treated with 10 equiv of anhydrous NaOH in absolute ethanol, for 2 h, affords in a 65% yield a new 2-benzyl-2-hydroxyketone, resulting from an unprecedented double benzylic rearrangement. This new rearrangement could be interpreted as an initial benzylic 1,4-shift between the O-enolate alkoxide of the ketone group and the oxygen atom of the benzyloxy ether, followed by a Curtin type benzylic 1,2-shift. Apart from the novelty and the synthetic application of this transformation it is worth noting the complete regio- and stereoselectivity observed. The structures of both substrate and product have been confirmed by X-ray diffraction studies. A tentative mechanism is herein proposed.     

Practical improvements in soil redox potential (Eh) measurement for characterisation of soil properties. Application for comparison of conventional and conservation agriculture cropping systems

The soil redox potential (Eh) can provide essential information to characterise soil conditions. In practice, however, numerous problems may arise regarding: (i) Eh determination in soils, especially aerobic soils, e.g. variations in the instrumentation and methodology for Eh measurement, high spatial and temporal Eh variability in soils, irreversibility of the redox reaction at the surface electrode, chemical disequilibrium; and (ii) measurement interpretation.     

Studying the Origin of the Antiferromagnetic to Spin‐Canting Transition in the β‐p‐NCC6F4CNSSN. Molecular Magnet

The crystal structure of the spin‐canted antiferromagnet β‐p‐NCC₆F₄CNSSN^. at 12 K (reported in this work) was found to adopt the same orthorhombic space group as that previously determined at 160 K. The change in the magnetic properties of these two crystal structures has been rigorously studied by applying a first‐principles bottom‐up procedure above and below the magnetic transition temperature (36 K). Calculations of the magnetic exchange pathways on the 160 K structure reveal only one significant exchange coupling (J(d1)=?33.8 cm^?1), which generates a three‐dimensional diamond‐like magnetic topology within the crystal. The computed magnetic susceptibility, χ(T), which was determined by using this magnetic topology, quantitatively reproduces the experimental features observed above 36 K. Owing to the anisotropic contraction of the crystal lattice, both the geometry of the intermolecular contacts at 12 K and the microscopic J_AB radical–radical magnetic interactions change: the J(d1) radical–radical interaction becomes even more antiferromagnetic (?43.2 cm^?1) and two additional ferromagnetic interactions appear (+7.6 and +7.3 cm^?1). Consequently, the magnetic topologies of the 12 and 160 K structures differ: the 12 K magnetic topology exhibits two ferromagnetic sublattices that are antiferromagnetically coupled. The χ(T) curve, computed below 36 K at the limit of zero magnetic field by using the 12 K magnetic topology, reproduces the shape of the residual magnetic susceptibility (having subtracted the contribution to the magnetization arising from spin canting). The evolution of these two ferromagnetic J_AB contributions explains the change in the slope of the residual magnetic susceptibility in the low‐temperature region.     

Fragmentation of the tryptophan cluster [Trp9-2H]2- induced by different activation methods

Electrospray ionization (ESI) of tryptophan gives rise to multiply charged, non‐covalent tryptophan cluster anions, [Trp_n–xH]^x?, in a linear ion trap mass spectrometer, as confirmed by high‐resolution experiments performed on a Fourier transform ion cyclotron resonance (FT‐ICR) mass spectrometer. The smallest multiply charged clusters that can be formed in the linear ion trap as a function of charge state are: x = 2, n = 7; x = 3, n = 16; x = 4, n = 31. The fragmentation of the dianionic cluster [Trp₉–2H]^2? was examined via low‐energy collision‐induced dissociation (CID), ultraviolet photodissociation (UVPD) at 266 nm and electron‐induced dissociation (EID) at electron energies ranging from >0 to 30 eV. CID proceeds mostly via charge separation and evaporation of neutral tryptophan. The smallest doubly charged cluster that can be formed via evaporation of neutral tryptophans is [Trp₇–2H]^2?, consistent with the observation of this cluster in the ESI mass spectrum. UVPD gives singly charged tryptophan clusters ranging from n = 2 to n = 9. The latter ion arises from ejection of an electron to give the radical anion cluster, [Trp₉–2H]^?.. The types of gas‐phase EID reactions observed are dependent on the energy of the electrons. Loss of neutral tryptophan is an important channel at lower energies, with the smallest doubly charged ion, [Trp₇–2H]^2?, being observed at 19.8 eV. Coulomb explosion starts to occur at 19.8 eV to form the singly charged cluster ions [Trp_x–H]^? (x = 1–8) via highly asymmetric fission. At 21.8 eV a small amount of [Trp₂–H–NH₃]^? is observed. Thus CID, UVPD and EID are complementary techniques for the study of the fragmentation reactions of cluster ions. Copyright © 2010 John Wiley & Sons, Ltd.     

Assay of urinary alpha-fluoro-beta-alanine by gas chromatography-mass spectrometry for the biological monitoring of occupational exposure to 5-fluorouracil in oncology nurses and pharmacy technicians

The validation of an analytical method for the measurement of the unnatural amino acid alpha-fluoro-beta-alanine (AFBA), the main metabolite of the antineoplastic drug 5-fluorouracil (5FU), in urine for the biological monitoring of the exposure of hospital workers to the drug when preparing the therapeutical doses and administering to cancer patients is described. The method employed a two-step extractive derivatization of the analyte from urine to the N-trifluoroacety-n-butyl ester derivative and detection by selected-ion monitoring gas chromatography-mass spectrometry of structurally specific fragments. The limit of detection was 20 ng/mL with quantification accuracy better than +/-20% and precision (CV%) better than +/-20% in the range 0.020-10 microg/mL. Norleucine was used as the internal standard and the sample-to-sample analysis time was less than 15 min. The validated method has been applied to the biological monitoring of some hospital workers potentially exposed to 5FU and to matched control subjects. On a total number of 65 analyzed urine samples from control and exposed subjects, only three, obtained from exposed subjects, were found to be positive, with values of 20, 30 and 1150 ng/mL, respectively.     

Isomeric distribution and catalyzed isomerization of cobalt(III) complexes with pentadentate macrocyclic ligands. importance of hydrogen bonding

We have investigated the isomeric distribution and rearrangement of complexes of the type [CoXLn]2+,3+ (where X = Cl-, OH-, H2O, and Ln represents a pentadentate 13-, 14-, and 15-membered tetra-aza or diaza-dithia (N4 or N2S2) macrocycle bearing a pendant primary amine). The preparative procedures for chloro complexes produced almost exclusively kinetically preferred cis isomers (where the pendant primary amine is cis to the chloro ligand) that can be separated by careful cation-exchange chromatography. For L13 and L14 the so-called cis-V isomer is isolated as the kinetic product, and for L15 the cis-VI form (an N-based diastereomer) is the preferred, while for the L14(S) complex both cis-V and trans-I forms are obtained. All these complexes rearrange to form stable trans isomers in which the pendent primary amine is trans to the monodentate aqua or hydroxo ligand, depending on pH and the workup procedure. In total 11 different complexes have been studied. From these, two different trans isomers of [CoClL14(S)]2+ have been characterized crystallographically for the first time in addition to a new structure of cis-V-[CoClL14(S)]2+; all were isolated as their chloride perchlorate salts. Two additional isomers have been identified and characterized by NMR as reaction intermediates. The remaining seven forms correspond to the complexes already known, produced in preparative procedures. The kinetic, thermal, and baric activation parameters for all the isomerization reactions have been determined and involve large activation enthalpies and positive volumes of activation. Activation entropies indicate a very important degree of hydrogen bonding in the reactivity of the complexes, confirmed by density functional theory studies on the stability of the different isomeric forms. The isomerization processes are not simple and even some unstable intermediates have been detected and characterized as part of the above-mentioned 11 forms of the complexes. A common reaction mechanism for the isomerization reactions has been proposed for all the complexes derived from the observed kinetic and solution behavior.     

Long-lived nuclear spin states in the solution NMR of four-spin systems

The existence of long-lived nuclear spin states in four-spin systems is explored by solution-state NMR experiments. Long-lived states are proved to exist in three different natural product molecules, each containing either a AA'BB' or a AA'XX' proton spin system. The measured state lifetimes are between four and eight times the spin-lattice relaxation time constants.     

A Passerini/Tsuji-Trost access to dienamide derivatives

The Passerini reaction of α, β-unsaturated aldehydes affords allylic acetates which may be involved in Tsuji-Trost elimations towards conjugated dienamides. The interest of these 2,4-dienamides has been demonstrated in several TiCl₄ triggered Diels-Alder reactions with N-phenylmaleimide.     

A Shack–Hartmann measuring head for the two‐dimensional characterization of X‐ray mirrors

The recent development of short‐wavelength optics (X/EUV, synchrotrons) requires improved metrology techniques in terms of accuracy and curvature dynamic range. In this article a stitching Shack–Hartmann head dedicated to be mounted on translation stages for the characterization of X‐ray mirrors is presented. The principle of the instrument is described and experimental results for an X‐ray toroidal mirror are presented. Submicroradian performances can be achieved and systematic comparison with a classical long‐trace profiler is presented. The accuracy and wide dynamic range of the Shack–Hartmann long‐trace‐profiler head allow two‐dimensional characterizations of surface figure and curvature with a submillimeter spatial resolution.     

Solid-state NMR characterization of a controlled-pore glass and of the effects of electron irradiation

Controlled-pore glasses (CPGs) are silica-based materials which provide an adequate model system for a better understanding of the radiation chemistry of glasses, especially under nanoscopic confinement. This paper presents a characterization of a nanoporous CPG before and after electron irradiation using multinuclear solid-state magnetic resonance (NMR). 1H MAS NMR has been used for studying the surface proton sites and it is observed that the irradiation leads to a dehydration of the material. Accordingly, concerning the silicon sites near the surface, the observed variation of the Q4, Q3 and Q2 species from 1H-29Si CPMAS spectra shows an increase of the surface polymerization under irradiation, implying in majority a Q2 to Q3/Q4 conversion mechanism. Similarly, 1H-17 O CPMAS measurements exhibit an increase of Si-O-Si groups at the expenses of Si-OH groups. In addition, modifications of the environment of the residual boron atoms are also put in evidence from 11B MAS and MQMAS NMR These data show that MAS NMR methods provide sensitive tools for the characterization of these porous glasses and of the tiny modifications occurring under electron irradiation.