Synthesis of azaspirocyclic compounds via organoiron complexes. Potential synthetic routes to histrionicotoxin and cephalotaxus alkaloids.

Reaction of cyclohexadienylium-Fe(CO)₃ complexes (7) and (8) with sodiomalono-nitrile gives the carbo-spirocyclic derivatives (9) and (10), whilst reaction with benzylamine and removal of iron leads to the azaspirocyclic enones (15) and (16), respectively.     

Axiomatisability and hardness for universal Horn classes of hypergraphs

We consider hypergraphs as symmetric relational structures. In this setting, we characterise finite axiomatisability for finitely generated universal Horn classes of loop-free hypergraphs. An Ehrenfeucht–Fraïssé game argument is employed to show that the results continue to hold when restricted to first order definability amongst finite structures. We are also able to show that every interval in the homomorphism order on hypergraphs contains a continuum of universal Horn classes and conclude the article by characterising the intractability of deciding membership in universal Horn classes generated by finite loop-free hypergraphs.     

Subluminal and superluminal propagation in Er<Superscript>3+</Superscript> doped fiber Bragg grating

The method to pump the FBG written into an Er³⁺-doped optical fiber is proposed to decrease or increase the group velocity of a probing pulse based on the fact that a pump-induced process changes the refractive index and dispersion associated with the ⁴I_15/2-⁴I_13/2 transition in Er³⁺-doped optical fiber. The system equations are derived. The group velocity modification is numerically demonstrated and discussed with the effects of an optical pump power, fiber Bragg grating length, doping concentration of Er³⁺ ions, and modulation amplitude of the grating.     

Divergent Late‐Stage (Hetero)aryl C−H Amination by the Pyridinium Radical Cation

(Hetero)arylamines constitute some of the most prevalent functional molecules, especially as pharmaceuticals. However, structurally complex aromatics currently cannot be converted into arylamines, so instead, each product isomer must be assembled through a multistep synthesis from simpler building blocks. Herein, we describe a late‐stage aryl C?H amination reaction for the synthesis of complex primary arylamines that other reactions cannot access directly. We show and rationalize through a mechanistic analysis the reasons for the wide substrate scope and the constitutional diversity of the reaction, which gives access to molecules that would not have been readily available otherwise.     

Computational and NMR Studies on the Complexation of Lithium Ion to 8-Crown-4

Lithium ion selective crown ethers have been the subject of much research for a multitude of applications. Current research is aimed at structurally rigidifying crown ethers, as restructuring of the crown ether ring upon ion binding is energetically unfavorable. In this work, the lithium ion binding ability of the relatively rigid 8-crown-4 was investigated both computationally by density functional theory calculations and experimentally by ¹H and ⁷Li NMR spectroscopy. Although both computational and experimental results showed 8-crown-4 to bind lithium ion, this binding was found to be weak compared to larger crown ethers. The computational analysis revealed that the complexation is driven by enthalpy rather than entropy, illustrating that rigidity is only of nominal importance. To elucidate the origin of the favorable interaction of lithium ion with crown ethers, activation strain analyses and energy decomposition analyses were performed pointing to the favorable interaction being mainly electrostatic in nature. 8-crown-4 presents the smallest crown ether reported to date capable of binding lithium ion, possessing two distinct conformations from which it is able to do so.     

Photoluminescence of ZnO nanowires dependent on O2 and Ar annealing

High-purity ZnO nanowires have been synthesized on Si substrates without the presence of a catalyst at 600 °C by a simple thermal vapor technique. Photoluminescence (PL) spectra of the annealed samples at 900 °C under oxygen and argon gases have been investigated. After O₂ or Ar annealing, the PL visible-emission intensity that is related to intrinsic defects (oxygen vacancies) is greatly reduced compared with as-grown ZnO nanowires because the oxygen-gas ions or oxygen interstitials diffuse into the oxygen vacancies during annealing process. The blue-band peak of the O₂- or Ar-annealed ZnO naonowires is also smaller than the green-band peak in the visible broadband because of the reduction of oxygen vacancies. Therefore, the main intrinsic defects (oxygen vacancies) of as-grown ZnO nanowires can be reduced by O₂ or Ar annealing, which is an important procedure for the development of advanced optoelectronic ZnO nanowire devices.     

Identification of functional information subgraphs in complex networks

We present a general information theoretic approach for identifying functional subgraphs in complex networks. We show that the uncertainty in a variable can be written as a sum of information quantities, where each term is generated by successively conditioning mutual informations on new measured variables in a way analogous to a discrete differential calculus. The analogy to a Taylor series suggests efficient optimization algorithms for determining the state of a target variable in terms of functional groups of other nodes. We apply this methodology to electrophysiological recordings of cortical neuronal networks grown in vitro. Each cell's firing is generally explained by the activity of a few neurons. We identify these neuronal subgraphs in terms of their redundant or synergetic character and reconstruct neuronal circuits that account for the state of target cells.     

Enhanced nondegenerate four-wave mixing owing to electromagnetically induced transparency in a spectral hole-burning crystal

We have demonstrated electromagnetically induced transparency (EIT) in an inhomogeneously broadened spectral hole-burning system of Pr(3+)-doped Y(2)SiO(5) at 6 K. We have also shown enhancement of four-wave mixing under conditions of reduced absorption. This demonstration opens the possibilities of pursuing EIT applications such as high-resolution optical image processing and optical data storage in solids.     

Stereoselective synthesis of (+)-polyoxamic acid based on the synthesis of chiral oxazine

A concise, stereocontrolled synthesis of (+)-polyoxamic acid was achieved. Starting from trans-oxazoline as a chiral building block, the key step involves diastereoselective oxazine formation catalyzed by palladium(0).