Kinetics of a second order reaction involving two intermediate second order reactions

We have studied the kinetics of a reaction in which starting materials A and B react to form the products C and D directly, as well as by way of two additional intermediate reactions. In the first intermediate reaction, A and C react to form D and E, and in the second intermediate reaction, B and E react to form 2C. There are three conservation laws, which effectively reduce the number of variables to two, resulting in nonlinear coupled first order differential equations for [A](t) and [B](t). These equations are readily solved by standard numerical procedures, for various values of the relative reaction rates and starting concentrations. Our results are used to fit experimental data for the case in which A and B are 1-acetyl-4-(1-ethoxycarbonyl-1-cyano)methylene-1,4-dihydroquinoline and water, for which the first and third reverse reactions can be neglected.     

Leucascandrolide A: a second generation formal synthesis

A convergent, second generation formal synthesis of (+)-Leucascandrolide A (1) has been efficiently achieved by providing a flexible, enantiocontrolled strategy toward the bioactive macrolactone component. Advancements for stereocontrol in asymmetric allylation methodology are discussed. Efforts feature novel results for reductions using the Terashima hydride reagent. [structure: see text]     

Microwave spectrum of a second conformer of cyclobutylsilane

Rotational spectra of the ground and some excited vibrational states of the equatorial and the axial conformer of cyclobutylsilane have been measured and evaluated. The assignment of the spectra to a particular conformation is discussed. For the axial conformer the internal rotation barrier potential of the silyl group was determined as 2150 ± 30 cal/mole.     

Dendri-RAFTs: a second generation of cyclopeptide-based glycoclusters

Synthetic glycoclusters and their related biological applications have stimulated increasing interest over the last decade. As a prerequisite to discovering active and selective therapeuticals, the development of multivalent glycoconjugates with diverse topologies is faced with inherent synthetic and structural characterisation difficulties. Here we describe a new series of molecularly-defined glycoclusters that were synthesized in a controlled manner using a robust and versatile divergent protocol. Starting from a Regioselectively Addressable Functionalized Template (RAFT) carrier, either a polylysine dendritic framework or a second RAFT, then 16 copies of βGal, αMan, βLac or cancer-related Thomsen-Freidenreich (αTF) antigen were successively conjugated within the same molecule using oxime chemistry. We thus obtained a new generation of dendri-RAFTs glycoclusters with high glycosidic density and variable spatial organizations. These compounds displaying 16 endgroups were unambiguously characterized by NMR spectroscopy and mass spectrometry. Further biological assays between a model lectin from Canavalia ensiformis (ConA) and mannosylated glycoclusters revealed a higher inhibition potency than the tetravalent counterpart, in particular for the hexadecavalent polylysine skeleton. Together with the efficiency of the synthetic and characterisation processes, this preliminary biological study provided clear evidence of promising properties that make the second generation of cyclopeptide-based glycoclusters attractive for biomedical applications.     

Measurement of the second moment of a Raman band

A method for determining the second moment of a Raman band has been proposed. The band shape is assumed to be expressed in terms of a mathematical function, which is derived from the Fourier transform of the Kubo correlation function. Parameters in the expression are adjusted with the least squares method to fit the experimental band shape to the theoretical expression. The second moment can be obtained from those parameters determined. The advantage of the proposed method lies in that the second moment can be determined even for overlapped bands.     

(+)-saxitoxin: a first and second generation stereoselective synthesis

A stereoselective synthesis of the bis-guanidinium toxin (+)-saxitoxin (STX), the agent infamously associated with red tides and paralytic shellfish poisoning, is described. Our approach to this unique natural product advances through an unusual nine-membered ring guanidine intermediate 39 en route to the tricyclic skeleton that defines STX. The effectiveness of this strategy is notable, as only four steps are needed to transform 39 into the target molecule, including a four-electron alkene oxidation catalyzed by OsCl3. Construction of the critical monocyclic guanidine has been achieved through two channels, the first of which makes use of Rh-catalyzed C-H amination and highlights a novel class of heterocyclic N,O-acetals as iminium ion equivalents for crafting functionalized amines. A second route to 39 relies on a stereoselective acetylide dianion addition to a serine-based nitrone, thereby facilitating the preparation of STX in just 14 linear steps from commercial material.     

Fluxional additives: a second generation control in enantioselective catalysis

The concept of "fluxional additives", additives that can adopt enantiomeric conformations depending on the chiral information in the ligand, is demonstrated in enantioselective Diels-Alder and nitrone cycloaddition reactions. The additive design is modular, and diverse structures are accessible in three steps. Chiral Lewis acids from main group and transition metals show enhancements in enantioselectivity in the presence of these additives.     

Detection of a second triplet state in thiophosgene by electron-impact spectroscopy

The electron-impact energy-loss spectrum of thiophosgene was investigated at incident energies of 25 eV and 40 eV and scattering angles from 0° to 80°. In these spectra we observe a previously unreported triplet state at 3.1 ± 0.1 eV which is tentatively assigned as the 1³A₁ (π, gp^*) state. This state may play a role in intramolecular radiationless transitions in this molecule.     

Ultrafast dynamics of epicocconone, a second generation fluorescent protein stain

Femtosecond upconversion experiment has been carried out for epicocconone and its butylamine adduct in acetonitrile and tert-butanol. An ultrafast component is found to dominate the decay of fluorescence of epicocconone in acetonitrile solution. Upon reacting with butylamine, a model for the epicocconone-protein adduct, this ultrafast component remains almost unaffected but an additional rise time occurs, indicating the formation of a highly emissive species from the locally excited state. This phenomenon is central to the extraordinary applications of epicocconone in biotechnology. The magnitude of the rise time of the butylamine adduct is similar to that of the longer component of the decay of epicocconone in acetonitrile, suggesting that the dynamics of epicocconone and its butylamine adduct are similar. The ultrafast component is slowed upon increasing the viscosity of the solvent. This results in a marked increase in quantum yield and suggests that it corresponds to rapid bond isomerization, leading to a nonradiative decay. Surprisingly, in water/sucrose mixtures, the ultrafast component remains unaffected but there is still an increase in quantum yield, suggesting that there are at least two nonradiative pathways, one involving bond isomerization and another involving proton transfer. The correct interpretation of these data will allow the design of second generation protein stains based on the epicocconone scaffold with increased quantum yields and photostability.     

Oxysulfides formed by a rare earth and a second metal: II. Shear structures

Many of the oxysulfides containing two metals have layered structures of (RO) sheets and (M_xS_y) sheets (R = rare earth, M = Cu, Ag, Ga, In, Ge, Sn, As, Sb, Bi). In the new compounds (La₄O₃)(AsS₃)₂ and (La₅O₃)In₆S₁₇, parallel ribbons of stoichiometry (La₄O₃) and (La₅O₃) substitute for the (LaO) sheets. The formation of ribbons is explained by shear mechanisms involving two different orientations of the periodic shear planes in the tetragonal (LaO) sheet. In a third compound La₃GaS₅O or (La₂O)LaGaS₅, the formation of (La₂O) ribbons involves similar shear mechanism. All these shear structures preserve the main characters of the layered oxysulfides: oxygen is only bound to lanthanum, and the second metal is only bound to sulfur.     

Simulation of a movement of dispersed particles in view of a second gas circulation in a cyclone

The gas flow in the cyclone was simulated in view of a constant component of a gas velocity directed from periphery to center of a cyclone. An influence of this component of the gas velocity makes it possible a judgment that with an increase in a value of the radial flowing off the particle velocity can shift and achieve outside wall of a vortex body. Its occurrence was not observed earlier and its forecast was difficult since the radial flowing off was directed to other side. As shown from computations at certain intensity of the second circulation such a critical size of the particles occurs that the particles transfer from suspension to rapid movement toward the outside wall of the vortex chamber.