A comparative analysis of the total syntheses of the amphidinolide T natural products

In this article we compare and contrast the strategies and tactics used in the syntheses of the amphidinolide T family of natural products that have been reported by Fürstner, Ghosh and ourselves. Similar approaches to the trisubstituted THF ring present in the targets are utilized in all of the syntheses, but each strategy showcases a different means of macrocyclization.     

Line sources in absorption spectroscopy

Hollow-cathode tubes and discharge lamps are studied as spectral sources for absorption spectrophotometry. Comparisons are made of monochromatic versus conventional continuous sources with respect to several experimental parameters, particularly the effect of slitwidth. Working curves are shown to indicate the possible advantages of using line sources for narrow band absorption systems.     

The effect of bubbles on the measured electrochemical parameters during hydrogen evolution on nickel

The hydrogen evoluton reaction has been investigated at nickel rotating and vertical stationary electrodes in 1 M HC1. Steady state current—potential, impedance—potential and potential step—current—time transients results are reported. The impedance is analysed by equivalent circuit to produce double layr capacity—potential, ohrnic resistance—potential (and charge transfer resistance—potential) curves. Some deductions are made about the bubble layer and its effect on the electron transfer mechanism for the reaction.     

Charged polypeptide vesicles with controllable diameter

We report the preparation and characterization of charged, amphiphilic block copolypeptides that form stable vesicles and micelles in aqueous solution. Specifically, we prepared and studied the aqueous self-assembly of a series of poly(L-lysine)-b-poly(L-leucine) block copolypeptides, KxLy, where x ranged from 20 to 80 and y ranged from 10 to 30 residues, as well as the poly(L-glutamatic acid)-b-poly(L-leucine) block copolypeptide, E60L20. Furthermore, the vesicular assemblies show dynamic properties, indicating a high degree of membrane fluidity. This characteristic provides stimuli-responsive properties to the vesicles and allows fine adjustment of vesicle size using liposome-based extrusion techniques. Vesicle extrusion also provides a straightforward means to trap solutes, making the vesicles promising biomimetic encapsulants.     

A sequential stereocontrolled cyclopropane ring formation and semi-pinacol rearrangement

Treatment of an unsaturated 2,3-epoxy alcohol with SnBr₄ leads to a stereoselective formation of a cyclopropane ring, and an α-ketol unit as part of a subsequent ring expansion.     

Investigation of novel mediators for a glucose biosensor based on metal picolinate complexes

The metal complexes [Os(byp)(2)(pic)](+) and [Ru(byp)(2)(pic)](+) where byp is 2,2'-bipyridine and HPic is o-picolinic acid were synthesised and characterised using spectroscopic and electrochemical techniques. These complexes were then evaluated as mediators for a glucose oxidase (GOx)-based biosensor. Results demonstrate the electrocatalytic behaviour of both metal couples towards regeneration of the flavoprotein GOx (FADH(2)) group, when co-immobilised with glucose oxidase. Surface immobilisation was achieved by potential cycling in aqueous solutions of the metal complexes at a glucose oxidase (GOx)/Nafion modified electrode. This proved successful in terms of catalytic efficiency and stability of redox sites. Kinetic parameters associated with both enzymatic and mediator reactions were estimated and the stability/performance properties of the sensor were tested.     

An efficient closed-shell singles and doubles coupled-cluster method

A reformulated set of equations for the closed-shell singles and doubles coupled-cluster (CCSD) method is presented. A computational cost of n_v⁴n₀²+7n_v³n₀³+1n_v²n₀⁴ for the n⁶ steps is obtained, where n_v is the number of virtual molecular orbitals included in the CCSD procedure, n₀ is the number of doubly occupied molecular orbitals and n=n₀+n_v. Test calculations for the cis and trans isomers of FNNF and planar and pyramidal CH₃⁻ are presented. Equilibrium structures determined with large Gaussian basis sets at the second-order Møller-Plesset (MP2) perturbation level of theory are reported and used for the other electron correlation methods. With the largest one-particle basis set (144 contracted Gaussian functions), the equilibrium geometries of cis- and trans-FNNF agree with experiment. Based on analyses of planar and pyramidal CH₃⁻ wavefunctions and the calculated inversion barrier, it is suggested that the molecular anion may not exist in a planar configuration but that autodetachment of an electron occurs before the transition state is reached. Comparisons of our new CCSD procedure demonstrate that coupled-cluster methods are not significantly more expensive than similar electron correlation techniques.     

Evaporatively cooled M+ (H2O)Ar cluster ions: infrared spectroscopy and internal energy simulations

Rotationally resolved IR spectra of M+ (H2O)Ar cluster ions for M=Na, K, and Cs in the O-H stretch region were measured in a triple-quadrupole mass spectrometer. Analysis of the spectra yields O-H stretch vibrational band origins and relative IR intensities of the symmetric and asymmetric modes. The effect of the alkali-metal ions on these modes results in frequency shifts and intensity changes from the gas phase values of water. The A-rotational constants are also obtained from the rotational structure and are discussed. Experimentally, the temperatures of these species were deduced from the relative populations of the K-rotational states. The internal energies and temperatures of the cluster ions for Na and K were simulated using RRKM calculations and the evaporative ensemble formalism. With binding energies and vibrational frequencies obtained from ab initio calculations, the average predicted temperatures are qualitatively consistent with the experimental values and demonstrate the additional cooling resulting from argon evaporation.     

Three 4‐amino­quinolines of anti­malarial interest

The structures of three compounds with potential anti­malarial activity are reported. In N,N‐diethyl‐N′‐(7‐iodo­quinolin‐4‐yl)ethane‐1,2‐diamine, C₁₅H₂₀IN₃, (I), the mol­ecules are linked into ribbons by N—H⋯N and C—H⋯N hydrogen bonds. In N‐(7‐bromo­quinolin‐4‐yl)‐N′,N′‐diethyl­ethane‐1,2‐diamine dihydrate, C₁₅H₂₀BrN₃·2H₂O, (II), two amino­quino­line mol­ecules and four water mol­ecules form an R₅⁴(13) hydrogen‐bonded ring which links to its neighbours to form a T5(2) one‐dimensional infinite tape with pendant hydrogen bonds to the amino­quinolines. The phosphate salt 7‐chloro‐4‐[2‐(diethyl­ammonio)ethyl­amino]quinolinium bis­(dihydrogen­phosphate) phospho­ric acid, C₁₅H₂₂ClN₃²⁺·2H₂PO₄⁻·H₃PO₄, (III), was prepared in order to establish the protonation sites of these compounds. The phosphate ions form a two‐dimensional hydrogen‐bonded sheet, while the amino­quino­line cations are linked to the phosphates by N—H⋯O hydrogen bonds from each of their three N atoms. While the conformation of the quinoline region hardly varies between (I), (II) and (III), the amino side chain is much more flexible and adopts a significantly different conformation in each case. Aromatic π–π stacking inter­actions are the only supramolecular inter­actions seen in all three structures.     

Qualitative identification of alloys by energy-dispersive X-ray spectroscopy

Energy dispersive X-ray spectroscopy can be used to identify alloy types quickly and, in many cases, to distinguish between alloy systems within a certain alloy type. Characteristic X-rays of major components in an alloy are produced through excitation by an isotopic source. Ratios of the X-ray intensities of a pair of elements are plotted vs. ratios of the theoretical weight fractions of these elements. This provides a unique data point which is a function of the composition of the particular alloy system. In some cases, characteristic X-rays of minor constituents are used for total identification.