Acid catalyzed monodehydro-2,5-diketopiperazine formation from N-α-ketoacyl amino acid amides

We have developed a new synthetic method for monodehydro-2,5-diketopiperazines (monodehydroDKPs), which is based on an acid catalyzed cyclization of N-α-ketoacyl amino acid amides. Using this cyclization reaction, monodehydroDKP was formed with no or slight racemization in case that N-α-ketoacyl amino acid amides with β-aliphatic-α-ketoacyl groups and sterically unhindered N-substituting groups at the C-terminal amide nitrogen were used in the presence of catalytic amount of p-TsOH (3-5 mol %) or 10% TFA. In the case of β-aryl-α-ketoacyl amino acid derivatives, in which an enol form predominantly exists by conjugation with the aromatic ring, racemization could be minimized by optimizing the reaction conditions (5 mol % p-TsOH, reflux for 6 h), although the chemical yield could not be dramatically improved. However, this reaction condition was successfully applied to the synthesis of a tubulin depolymerization agent, (−)-tert-butyl-oxa-phenylahistin, with no racemization.     

Isomery of [Re6S6Br8] and [Re6S5Br9] Units in a Rhenium Cluster Thiobromide: Experimental and Theoretical Approaches

The rhenium cluster thiobromide Cs_1.95(1)Re₆S_5.82(3)Br_8.19(3), belonging to the solid solution Cs₂Re₆S₆Br₈–CsRe₆S₅Br₉, crystallizes in the trigonal system (P31c, a = 10.001(5) Å and c = 14.676(5) Å). It is built up from [Re₆L ₈ ⁱ ]Br ₆ ^a cluster units in which sulphur and bromine are randomly distributed on inner position (Lⁱ). From the structural refinement performed using single-crystal X-ray diffraction data, the isomers of the [Re₆Sⁱ ₆Br ₂ ⁱ ] and [Re₆S ₅ ⁱ Br ₃ ⁱ ] cluster cores present in the structure have been unambiguously determined, due to the non-centro symmetry of the structure. Density functional theory calculations have been performed for all possible di- and tri-substituted isomers in order to confirm experimental analyses. Slight differences between the stability of di-substituted and tri-substituted cluster unit isomers built from Mo₆ cluster and Re₆ clusters are evidenced.     

Dimerization of ionized 4‐(methyl mercapto)‐phenol during ESI,APCI and APPI mass spectrometry

A novel ion/molecule reaction was observed to occur under electrospray ionization (ESI), atmospheric pressure chemical ionization (APCI), and atmospheric pressure photo ionization (APPI) conditions, leading to dimerization of ionized 4‐(methyl mercapto)‐phenol followed by fast H^· loss. The reaction is particularly favored during ESI, which suggests that this ion/molecule reaction can occur both in the solution inside the ESI‐charged droplets and in the gas‐phase environment of most other atmospheric pressure ionization techniques. The dimerization reaction is inherent to the electrolytic process during ESI, whereas it is more by ion/molecule chemistry in nature during APCI and APPI. From the tandem mass spectrometry (MS/MS) data, accurate mass measurements, hydrogen/deuterium (H/D) exchange experiments and density functional theory (DFT) calculations, two methyl sulfonium ions appear to be the most likely products of this electrophilic aromatic substitution reaction. The possible occurrence of this unexpected reaction complicates mass spectral data interpretation and can be misleading in terms of structural assignment as reported herein for 4‐(methyl mercapto)‐phenol. Copyright © 2009 John Wiley & Sons, Ltd.     

Surface glass transition of miscible amorphous polymers blends

The surface glass transition temperature (T _g ^surface) of the bulk samples of miscible blends formed of amorphous polystyrene (PS) and poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) has been characterised in terms of an adhesion approach we proposed recently. T _g ^surface has been measured as the temperature transition “occurrence of autoadhesion–nonoccurrence of autoadhesion” by employing a lap-shear joint mechanical testing method. The effect of the reduction in T _g ^surface with respect to the glass transition temperature of the bulk (T _g ^bulk), which had been observed earlier in pure homopolymers, has been found to exist in the blends of PS with PPO as well. The values of this effect for the blends have been compared with those for pure homopolymers, and the differences found have been discussed.     

XAFS investigation of [Pd(NH<Subscript>3</Subscript>)<Subscript>4</Subscript>][AuCl<Subscript>4</Subscript>]<Subscript>2</Subscript> and its thermolysis products

Thermolysis of double complex salt [Pd(NH₃)₄][AuCl₄]₂ has been studied in helium atmosphere from ambient to 350 °C. The XAFS of Pd K and Au L₃ edges and thermogravimetry measurements have been carried out to characterize the intermediates and the final product. In the temperature range 115–160 °C the complex is decomposed to form Pd(NH₃)₂Cl₂ and AuCl_4−x N _x species with x ranging from 2 to 3. Subsequent heating of the intermediate up to 300 °C leads to the total loss of NH₃. The Au–Cl and Au–Au bonds form the local environment of Au at the stage of decomposition while only four chlorine atoms are around Pd. At the temperature of 330 °C the Au and Pd nanoparticles as well as residues of palladium chloride are detected. The final product consists of separated Au and Pd nanoparticles.     

Geodesic flows and Neumann systems on Stiefel varieties: geometry and integrability

We study integrable geodesic flows on Stiefel varieties V _n,r ?=?SO(n)/SO(n?r) given by the Euclidean, normal (standard), Manakov-type, and Einstein metrics. We also consider natural generalizations of the Neumann systems on V _n,r with the above metrics and proves their integrability in the non-commutative sense by presenting compatible Poisson brackets on (T ^* V _n,r )/SO(r). Various reductions of the latter systems are described, in particular, the generalized Neumann system on an oriented Grassmannian G _n,r and on a sphere S ^n?1 in presence of Yang–Mills fields or a magnetic monopole field. Apart from the known Lax pair for generalized Neumann systems, an alternative (dual) Lax pair is presented, which enables one to formulate a generalization of the Chasles theorem relating the trajectories of the systems and common linear spaces tangent to confocal quadrics. Additionally, several extensions are considered: the generalized Neumann system on the complex Stiefel variety W _n,r ?=?U(n)/U(n?r), the matrix analogs of the double and coupled Neumann systems.     

Examples of frames on the Cantor dyadic group

We present two ways to construct frames on the locally compact Cantor dyadic group. The first approach gives a Parseval frame related to the generalized Walsh?CDirichlet kernel, while the second approach includes the Daubechies-type ??admissible condition?? and leads to dyadic compactly supported wavelet frames. The corresponding wavelet constructions on the Cantor and Vilenkin groups (as well as on the half-line $ {\mathbb{R}_{+} } $ ) requires an additional constraint related to the requirement that the masks have no blocking sets.     

Lie Superautomorphisms on Associative Algebras,II

We consider Lie superautomorphisms of prime associative superalgebras. A definitive result is obtained for central simple superalgebras: their Lie superautomorphisms are of standard forms, except when the dimension of the superalgebra in question is 2 or 4.     

Controlled synthesis of gold nanostars by using a zwitterionic surfactant

By replacing cetyltrimethylammonium bromide (CTAB) with the zwitterionic lauryl sulfobetaine (LSB) surfactant in the classical seed-growth synthesis, monocrystalline gold nanostars (m-NS) and pentatwinned gold asymmetric nanostars (a-NS) were obtained instead of nanorods. The main product under all synthetic conditions was a-NS, which have branches with high aspect ratios (AR), thus leading to LSPR absorptions in the 750-1150?nm range. The percentage of m-NS versus a-NS, the aspect ratio of the a-NS branches, and consequently the position of their LSPR absorption can be finely tuned simply by regulating the concentration of reductant, the concentration of surfactant, or the concentration of the "catalytic" Ag(+) cation. The m-NS have instead shorter and larger branches, the AR of which is poorly influenced by synthetic conditions and displays an LSPR positioned around 700?nm. A growth mechanism that involves the direct contact of the sulfate moiety of LSB on the surface of the nano-object is proposed, thereby implying preferential coating of the {111} Au faces with weak interactions. Consistent with this, we also observed the straightforward complete displacement of the LSB surfactant from the surface of the nanostars. This was obtained by the simple addition of thiols in aqueous solution to yield extremely stable coated a-NS and m-NS that are resistant to highly acidic, basic, and in similar to in vivo conditions.