Additions of enantiopure alpha-sulfinyl carbanions to (S)-N-sulfinimines: asymmetric synthesis of beta-amino sulfoxides and beta-alphamino alcohols

The addition of the lithium anions derived from (R)- and (S)-methyl and -ethyl p-tolyl sulfoxides to (S)-N-benzylidene-p-toluenesulfinamide provides an easy access route to enantiomerically pure beta-(N-sulfinyl)amino sulfoxides. Stereoselectivity can be achieved when the configurations at the sulfur atoms of the two reagents are opposite (matched pair), thus resulting in only one diastereoisomer, even for the case in which two new chiral centers are created. The N-sulfinyl group primarily controls the configuration of the carbon bonded to the nitrogen, whereas the configuration of the alpha-sulfinyl carbanion seems to be responsible for the level of asymmetric induction, as well as for the configuration of the new stereogenic C-SO carbon in the reactions with ethyl p-tolyl sulfoxides. An efficient method for transforming the obtained beta-(N-sulfinyl)amino sulfoxides into optically pure beta-amino alcohols, based on the stereoselective non-oxidative Pummerer reaction, is also reported.     

Characterisation and determination of indole alkaloids in frog-skin secretions by electrospray ionisation ion trap mass spectrometry

The characterisation of selected indole alkaloids in a quadrupole ion trap mass spectrometer is presented. Fragmentation profiles for tryptamine, 5-hydroxytryptamine (5-HT), N'-methyl 5-hydroxytryptamine (N'-methyl 5-HT), N',N'-dimethyl 5-hydroxytryptamine (bufotenine), N',N',N'-trimethyl 5-hydroxytryptamine (5-HTQ), and N',N'-dimethyl 5-methoxytryptamine (5-MeODMT) are presented with proposed structures given for each product ion observed. Such MS(n) experiments can be used to differentiate the isobaric molecular ions of the compounds 5-HTQ (M(+)) and 5-MeODMT (MH(+)). The quantitative determination of certain indole alkaloids in the skin secretions of the Australian Golden Bell frog, Litoria aurea, by LC/ESI-ion trap MS is also presented. The concentrations of 5-HT, N'-methyl 5-HT and 5-HTQ were found to be 2.68, 0.26 and 0.54 microg per mg of skin secretion, respectively.     

Esters of 5-carboxyl-5-methyl-1-pyrroline N-oxide: a family of spin traps for superoxide

Apparent rate constants, at acidic pH and neutral pH for the reaction of a family of ester-containing 5-carboxyl-5-methyl-1-pyrroline N-oxides with superoxide (O2*-) were estimated, using ferricytochrome c as a competitive inhibitor. It was of interest to note that the rate constants were similar among the different nitrones and not that significantly different from that found for 5-(diethoxyphosphoryl)-5-dimethyl-1-pyrroline N-oxide. At acidic pH, the rate constant for spin trapping O2*- was 3-fold greater than that at physiological pH. Subsequent experiments determined the half-life of aminoxyls, derived from the reaction of these nitrones with O2*-. The EPR spectra were modeled by using a global analysis method. The results clearly demonstrated that EPR spectra of all the aminoxyls were inconsistent with a model that included a single gamma-hydrogen splitting. A better interpretation modeled them as two diastereomers with identical nitrogen splittings and slightly different beta-hydrogen splittings. Detailed line width analyses slightly favored an equal line width-unequal population ratio for the two diastereomers.     

Selective excitation effects in microwave optical double resonance and phosphorescence spectroscopy of molecular crystals

The photoexcitation routes used to produce molecular crystal, triplet states are shown to have important optical and microwave spectral consequences. 2-benzoylpyridine crystals at 4.2 K have T₁ → S₀ phosphorescence spectra showing line width dependence on whether initial production of the T₁ state is through direct T₁ → S₀ absorption, or through S₁ ← S₀ absorption followed by S₁ → T₁ intersystem crossing. Striking differences are seen in the optically detected zero-field resonance spectra.     

Effect of mixed organic solvents on atomic absorption spectrophotometry of refractory metals

The effect of various organic solvents on the absorption characteristics of vanadium was studied in fuel-rich oxy-acetylene and nitrous oxide-acetylene flames. The absorption of the 3183.9 Å line of vanadium was greatly enhanced by the use of various mixed organic solvents when fed to oxy-acetylene flames. In the case of the nitrous oxide-acetylene flame, the addition of diethylene glycol (about 8% in the final solution) and similar compounds to the aqueous solution of vanadium increased the absorption by about 50%. The observations and the possible role of the mixed organic solvents are discussed.     

Fluorescence correlation spectroscopy of water-in-oil microemulsions: an application in specific characterisation of droplets containing biomolecules

Fluorescence correlation spectroscopy (FCS) has been successfully used to characterise water-in-oil (w/o) microemulsions. The investigated systems were stabilised by sodium bis-2-ethylhexyl sulphosuccinate (AOT) and the measured diffusion times have been related to the radii of the aggregated species, which for some systems, were separately determined by small-angle neutron scattering (SANS). We demonstrate that FCS is capable of measuring hydrodynamic radii of microemulsions rapidly and at surfactant concentrations lower than previously reported for other techniques. FCS was also used to specifically interrogate microemulsion droplets containing a fluorescently-labelled biomolecule, specifically phalloidin, a peptide fungal toxin from Amanita phalloides, and the enzyme -chymotrypsin (-CT). The microemulsion droplets are only marginally increased in size if a small peptide (phalloidin) is included in the water phase, whereas the droplet size is significantly increased when a larger protein (-CT) is included.     

Characterisation and proposed origin of mass spectrometric ions observed 30 Th above the ionised molecules of per-O-methylated carbohydrates

Derivatisation of carbohydrates by permethylation significantly improves the mass spectrometric intensity of carbohydrate-derived ions and allows more readily interpretable fragmentation; in addition, samples are conveniently separated from salts, and larger oligosaccharides are more readily ionised. It has previously been recognised that, in the mass spectra of permethylated carbohydrates, a series of ions indicating species 30 Da larger than the fully methylated carbohydrate molecules are also observed. These species have not been characterised in the literature despite their apparently ubiquitous occurrence in the mass spectra of permethylated carbohydrates. Tandem mass spectrometry (MS/MS) experiments were performed on permethylated carbohydrates and reduced permethylated carbohydrates that exhibit the artefact, demonstrating that the artefact is not reducing terminal specific, and that the artefact can be introduced at any hydroxyl residue. It was further demonstrated through the use of different alkylation reagents that the origin of this artefact group is the alkylating reagent itself. It is proposed that side reactions that occur between the permethylation reagents allow the production of small amounts of iodomethyl methyl ether. This reagent can then compete with methyl iodide for reaction with the carbohydrate -OH groups. The result is partial incorporation of a methoxymethyl moiety instead of a methyl group, detected as '+30' artefact ions.     

Ligand close packing and the geometry of the fluorides of the nonmetals of periods 3, 4, and 5

This paper discusses the geometry of the fluorides of the nonmetals of periods 3, 4, and 5 in terms of the ligand close packing (LCP) model according to which molecular geometry is determined primarily by ligand-ligand repulsions (Pauli closed shell repulsions) rather than by the bonding and lone pair Pauli repulsions of the VSEPR model. The LCP model becomes the dominant factor in determing geometry when the ligands are sufficiently crowded that they may be regarded as essentially incompressible. Ligand close packing is a modification of the VSEPR model in which ligand-ligand repulsion (Pauli closed shell repulsion) is given more emphasis than bonding and nonbonding electron pair Pauli repulsion. The nonmetals of period 3 are large enough to form octahedral six coordinated molecules in which the ligands are close packed. The larger nonmetals of period 4 also have a maximum coordination number of six and an octahedral geometry although the ligands are not close packed. Ligand radii derived from the interligand distances in the molecules of period 3 depend only on the charge of the fluorine ligands and are consistent with the previously derived radii obtained from the fluorides of the close packed tetrahedral molecules of the period 2 elements. Although the ligands in the molecules of the period 4 nonmetals are not close packed, these elements are not large enough to form molecules with a higher coordination number. However, the larger period 5 nonmetals may have coordination numbers of seven and eight. The seven coordinated molecules have a pentagonal bipyramidal geometry in which the equatorial ligands are close packed. The eight coordinated molecules have a square antiprism geometry, which is not a close packed geometry although the fluorine interligand distances are only a little larger than expected for close packing. The difference between the axial and equatorial bond lengths in the trigonal bipyramidal pentafluorides and the pentagonal bipyramidal pentafluorides can be understood on the basis of ligand close packing. Ligand packing prevents the lone pair in AF(6)E molecules from fully entering the valence shell and thereby exerting its full stereochemical effect so that these molecules have a C(3)(v)() distorted octahedral geometry rather than a geometry based on pentagonal bipyramidal seven coordination.