Comprehensive structural characterization of polyisoprene synthesized via cationic mechanism

The microstructure of polyisoprene synthesized with ^tBuCl/TiCl₄ initiating system is investigated using 1D and 2D (HSQC and HMBC) NMR spectroscopy. It is found that trans‐1,4‐units with regular (head‐to‐tail) and inverse (tail‐to‐tail) and (head‐to‐head) enchainments are predominant structures of unsaturated part of polymer chain, while 1,2‐ and 3,4‐units are presented in minor amounts. The new methodology for the quantitative calculation of the content of different structural units in polyisoprene chain including both types of inverse trans‐1,4‐addition (tail‐to‐tail and head‐to‐head) is proposed. It is shown that head groups consist of tert‐butyl group connected to trans‐1,4‐unit of polyisoprene chain. In addition, two types of chlorine‐containing end groups are found (trans‐4,1‐Cl and 4,3‐Cl), while conjugated double bonds at the chain end are totally absent. The methodology for the calculation of number‐average functionality by tert‐butyl head and chlorine end groups, respectively, is developed. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016 , 54, 2430–2442     

Asymmetric synthesis via aziridinium ions: exploring the stereospecificity of the ring opening of aziridinium ions and a formal synthesis of (−)-swainsonine

The use of aziridinium ions in two different projects is described. First, the stereospecificity of the ring opening of aziridinium ions with MeNH₂ as a route to chiral diamines has been explored. When the aziridinium ion contained a phenyl or para-methoxyphenyl substituent, stereospecific ring opening occurred. In contrast, switching the para-methoxy group to a para-N,N-dimethylamino group gave a racemic diamine product. Second, starting from N-Boc pyrrolidine, asymmetric lithiation-trapping-ring expansion (via an aziridinium ion) was used to synthesise a piperidine alcohol. In this way, a formal synthesis of (?)-swainsonine was completed.     

Noisy random graphs and their Laplacians

Spectra and representations of some special weighted graphs are investigated with weight matrices consisting of homogeneous blocks. It is proved that a random perturbation of the weight matrix or that of the weighted Laplacian with a “Wigner-noise” will not have an effect on the order of the protruding eigenvalues and the representatives of the vertices will unveil the underlying block-structure.Such random graphs adequately describe some biological and social networks, the vertices of which belong either to loosely connected strata or to clusters with homogeneous edge-densities between any two of them, like the structure guaranteed by the Regularity Lemma of Szemerédi.     

Analysis of Detached-Eddy Simulation for the Flow Around a Circular Cylinder with Reference to PIV Data

A detailed validation study is presented for the detached-eddy simulation (DES) of the flow around a circular cylinder at a high sub-critical Reynolds number. Good comparability with unsteady experimental field data is facilitated by the confined and clearly-defined geometry, although some uncertainty remains regarding the free stream turbulence intensity. The combination of DES with an appropriate low-dissipative hybrid numerical convection scheme and high temporal resolution delivers excellent agreement with the experiment for the time and phase-averaged fields as well as the spectral content. A strong sensitivity of the solution to the numerical time step size has been identified, which is attributed to time-filtering effects damping the development of resolved turbulence in the early shear layer. Recommendations are made concerning a CFL-type criterion for the temporal resolution of DES and future studies involving a still finer time step are outlined.     

Structural elucidation and NMR assignments of a new pyrrolizidine alkaloid from Crotalaria vitellina Ker Gawl

A new pyrrolizidine alkaloid, named crotavitelin, was isolated from fruits of Crotalaria vitellina, Fabaceae (Papilionoideae). The structure was established by spectroscopic techniques such as one‐dimensional and two‐dimensional NMR, IR, and MS. Copyright © 2013 John Wiley & Sons, Ltd.