Isomerization of 5-(2H-Azirin-2-yl)oxazoles: An Atom-Economic Approach to 4H-Pyrrolo[2,3-d]oxazoles

An atom economical method for the preparation of variously substituted 4H-pyrrolo[2,3-d]oxazoles was developed on the basis of thermal isomerization of 5-(2H-azirin-2-yl)oxazoles. The latter were prepared by Rh₂(oct)₄ catalyzed reaction of 2-(3-aryl/heteroaryl)-2-diazoacetyl-2H-azirines with a set of substituted acetonitriles, benzonitriles, acrylonitrile and fumaronitrile. According to DFT calculations the transformation of 5-(2H-azirin-2-yl)oxazole to 4H-pyrrolo[2,3-d]oxazole occurs through the nitrenoid-like transition state to give a 3aH-pyrrolo[2,3-d]oxazole intermediate, followed by 1,5-H-shift.     

Degenerate quasilinear pseudoparabolic equations with memory terms and variational inequalities

The existence of solutions of degenerate quasilinear pseudoparabolic equations, where the term _∂tu${\partial }_{t}u$ is replace by _∂tb(u)${\partial }_{t}b\left(u\right)$, with memory terms and quasilinear variational inequalities is shown. The existence of solutions of equations is proved under the assumption that the nonlinear function b$b$ is monotone and a gradient of a convex, continuously differentiable function. The uniqueness is proved for Lipschitz-continuous elliptic parts. The existence of solutions of quasilinear variational inequalities is proved under stronger assumptions, namely, the nonlinear function defining the elliptic part is assumed to be a gradient and the function b$b$ to be Lipschitz continuous.     

Arabidopsis camelliol C synthase evolved from enzymes that make pentacycles

We establish by heterologous expression that the Arabidopsis thaliana oxidosqualene cyclase At1g78955 (CAMS1) makes camelliol C (98%), achilleol A (2%), and beta-amyrin (0.2%). CAMS1 is the first characterized cyclase that generates predominantly a monocyclic triterpene alcohol. Phylogenetic analysis shows that CAMS1 evolved from enzymes that make pentacycles, thus revealing that its pentacyclic beta-amyrin byproduct is an evolutionary relic. Sequence alignments support prior suggestions that decreased steric bulk at a key active-site residue promotes monocycle formation.     

Stereochemistry of water addition in triterpene synthesis: the structure of arabidiol

An oxidosqualene cyclase from Arabidopsis thaliana makes arabidiol, a tricyclic triterpene reported with indeterminate side-chain stereochemistry. We established the full structure of arabidiol through chemical degradation, NOE experiments, and molecular modeling. By examining the mechanistic constraints that govern water addition in triterpene synthesis, we further show how the stereochemistry of hydroxylation can generally be deduced a priori, why deprotonation is more common than hydroxylation, and why cyclases that perform hydroxylation also generate olefinic byproducts.     

Nonlinear pseudoparabolic equations as singular limit of reaction–diffusion equations

In this article, a solution of a nonlinear pseudoparabolic equation is constructed as a singular limit of a sequence of solutions of quasilinear hyperbolic equations. If a system with cross diffusion, modelling the reaction and diffusion of two biological, chemical, or physical substances, is reduced then such an hyperbolic equation is obtained. For regular solutions even uniqueness can be shown, although the needed regularity can only be proved in two dimensions.     

Kinetics studies of rapid strain-promoted [3 + 2]-cycloadditions of nitrones with biaryl-aza-cyclooctynone

Strain-promoted cycloadditions of cyclic nitrones with biaryl-aza-cyclooctynone (BARAC) proceed with rate constants up to 47.3 M(-1) s(-1), this corresponds to a 47-fold rate enhancement relative to reaction of BARAC with benzyl azide and a 14-fold enhancement over previously reported strain promoted alkyne-nitrone cycloadditions (SPANC). Studies of the SPANC reaction using the linear free energy relationship defined by the Hammett equation demonstrated that the cycloaddition reaction has a rho value of 0.25 ± 0.04, indicating that reaction is not sensitive to substituents and thus should have broad applicability.     

Scintillation kinetics and thermoluminescence of SrI2:Eu2+ single crystals

Single crystal of strontium iodide doped with 1% europium (SrI₂:1% Eu²⁺) was grown by Vertical Gradient Freeze technique. UV excited emission spectra were studied as a function of temperature. Results indicate the thermal quenching of Eu²⁺ emission starts from ～400 K with a thermal activation energy of 0.39 eV. Gamma and UV excited decay measurements indicate that the scintillation decay time of SrI₂:Eu²⁺ is longer than the lifetime of Eu²⁺ luminescence center in the SrI₂ host. The thermoluminescence glow curve revealed a highly concentrated charge carrier trap at 50 K. Elimination of this trap is expected to enhance the energy migration of charge carriers and result in faster scintillation decay.     

Crystal growth and characterization of CsSr1–xEux I3 high light yield scintillators

In this work, we report on single crystal growth and characterizations of a new scintillation material: CsSr_1–xEu_x I₃ (0 ≤ x ≤ 1). Single crystals of CsSr_0.99Eu_0.01I₃, CsSr_0.92Eu_0.08I₃ and CsEuI₃ were grown via the Vertical Gradient Freeze method. The crystals exhibit good crystal quality, high light yield, and excellent energy resolution. Initial results show that the scintillation light yield of our CsSr_0.92Eu_0.08I₃ crystal is ～65,000 ph/MeV with 5.9% energy resolution at 662 keV. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)