Organocatalytic asymmetric allylic carbon-carbon bond formation

Organocatalytic allylic C-C bond-forming addition of activated alkylidenes to alkyl and aryl nitroalkenes has been achieved with high diastereo- and enantioselectivity. Chiral tertiary amine catalysts are used to give allyl intermediates which exhibit gamma-selectivity in the C-C bond forming step. The reactions proceed with up to >99:1 syn:anti ratio for both the alkyl- and aryl nitroalkenes with up 96% and 98% ee, respectively. The products of this conjugate addition are transformed into a range of intermediates, such as optically active conjugated dienes and 1-substituted tetralones, which are difficult to access via alternative methods.     

Estimation of the method evaluation function for the determination of hydride-generating arsenic compounds in urine by flow-injection atomic-absorption spectrometry

A direct flow-injection atomic-absorption spectrometric (FIA-AAS) method for the assessment of inorganic arsenic compounds and their metabolites was developed and statistically evaluated by the estimation of the method evaluation function (MEF), which provides detailed information on the analytical performance of the method, i.e., the average combined uncertainty and the magnitude of potential systematic errors. The method evaluation study demonstrated that the use of standard addition was a necessity to obtain an acceptable method performance at low concentrations typical for low dose exposure. In contrast the use of calibration curves resulted in a method with reduced sensitivity and high systematic error. The developed method, using standard addition, had a limit of detection (2.9 microg/l.) sufficiently low for the determination of hydride-generating arsenic species in urine from non-exposed and low exposed persons. Organoarsenicals such as arsenobetaine and arsenocholine are not detected by this method. Hence, the contribution of these compounds derived from a diet containing seafood does not affect the monitoring of inorganic arsenic compounds after occupational or environmental exposure. The high capacity of the FIA-AAS system (three minutes per sample measured by standard addition) together with the low limit of detection makes this method suitable for biological monitoring of inorganic arsenic exposure even though standard addition is required.     

A concise synthetic route to the conduritols from pentoses

A short synthetic strategy for preparation of the conduritols is described. The key step employs a zinc-mediated fragmentation of protected methyl 5-deoxy-5-iodo-d-pentofuranosides followed by an allylation of the intermediate aldehyde in the same pot. The allylation is performed with 3-bromopropenyl benzoate and occurs with good diastereoselectivity. An amino group can be introduced in the product by trapping the intermediate aldehyde as the imine prior to the allylation. The functionalised 1,7-octadienes, thus obtained, are converted into protected conduritols by ring-closing olefin metathesis.     

The crystal structure of racemic and meso-diastereoisomers of aqua[2,6-bis(3-carboxy-1,2-dimethyl-2-azapropyl)pyridine]-Co(III) · Hexafluorophosphate · Di- and monohydrate,respectively

Synthesis of the pentadentate ligand 2,6-bis(3-carboxy-1,2-dimethyl-2-azapropyl)pyridine yields a mixture of the racemic and meso-isomers which it was difficult to separate by column chromatography. When the cationic Co(III)-complex of this ligand was crystallized with hexafluorophosphate as anion, two distinct crystalline forms were produced. The complex of the racemic ligand, 1 , has C₂ symmetry and is a dihydrate; a = 8.999(8), b = 12.047(6), c = 20.65(1) Å, orthorhombic, space group Peen,Z = 4, R = 0.074 for 1439 observed reflections. The complex of the meso-ligand, 2 , shows two independent molecules ( 2A and 2B ) per asymmetric unit, both monohydrates with a resolved disordered H₂O molecule in 2A ; a = 10.109(4), b = 12.835(2), c = 16.651(3) Å, α = 89.5(1)°, β = 84.7(3)°, γ = 88.6(3)°, triclinic, space group P1 , Z = 4, Rs = 0.054 for 4198 observed reflections. The coordination around the Co-atom is distorted octahedral in both complexes, with the coordinated H₂O molecule trans to the pyridine N-atom. In the racemic form of the complex, 1 , the pyridine ring is twisted about the Co-N(1) bond with respect to the plane defined by atoms Co, N(1), O(W1), N(2) and N(2P) by 17.2(2)°. In the meso-form of the complex, 2 , the CH₃ substituent C(8P) on atom C(4P), is now axial with respect to the 5-membered chelate ring. As a result of steric hinderance between atom O(1) and CH₃(8P), the pyridine ring has been displaced from the best mean-plane formed by atoms Co, O(W1), N(2) and N(2P). The principal axis of the pyridine ring C(3)…N(1), makes an angle of 14.1(1)° (mean) with this plane. At the same time the pyridine ring is twisted about axis C(3)…N(1) with respect to this plane by 19.7(1)° (mean).     

The first evidence for a transient stibaallene ArSbCCR2

Dechlorofluorination of ArSb(F)-C(Cl)CR₂ (CR₂ = fluorenylidene, Ar = 2,4,6-tri-tert-butylphenyl) by tert-butyllithium afforded a 3,4-bis(fluorenylidene)-1,2-distibacyclobutane. The formation of the latter probably involves the transient stibaallene ArSbCCR₂ followed by a head-to-head dimerization via two SbC double bonds. Molecular orbital calculations at the ab initio and DFT levels support the head-to-head dimerization of ArSbCCR₂ with the formation of a 1,2-distibacyclobutane.     

Singlet Sigma: The "Other" Singlet Oxygen in Solution

The lowest excited electronic state of molecular oxygen, O₂(a^1-DL_g), is often called simply singlet oxygen. This singlet delta state is an acknowledged and well-studied intermediate in many solution-phase photosystems. However, the second excited electronic state of oxygen, O₂(b¹δ_g+), is also a singlet. It has recently become possible to monitor this singlet sigma state in solution, which, in combination with studies of the singlet delta state, contributes to a better understanding of a variety of general problems in chemistry.     

Pulse sequences for measurement of one-bond (15)N-(1)H coupling constants in the protein backbone.

A set of three improved two-dimensional (2D) NMR methods for measuring one-bond (15)N-(1)H coupling constants in the protein backbone is presented. They are tailored to suit the size of the TROSY effect, i.e., the degree of interference between dipolar and chemical shift anisotropy relaxation mechanisms. The methods edit 2D spectra into two separate subspectra corresponding to the two possible spin states of the coupling partner. Cross talk between the two subspectra is a second order effect in the difference between the actual coupling constants and the one used in setting the pertinent delays of the pulse sequences. This relatively high degree of editing accuracy makes the methods useful for applications to molecules subjected to weak alignment where the one-bond coupling constants are linear combinations of a scalar J and a residual dipolar contribution containing important structural information. A demonstration of the new methods is shown for the (15)N-labeled protein chymotrypsin inhibitor 2 in a lipid bicelle mixture.     

Identification and Linkage of Tarballs from the Coasts of Vancouver Island and Northern California Using GC/MS and Isotopic Techniques

During January and February 1996, a significant number of tarball/patty incidents occurred along the coasts of Vancouver Island, Washington, Oregon, and California. Samples of the tarballs were collected from the affected beaches and analyzed by gas chromatography/mass spectrometry (GC/MS) and gas chromatography/flame ionization detector (GC/FID) using a tiered analytical approach developed for determining the origin of oils. Selected samples were further analyzed using a carbon isotopic technique. Also, the relative abundances of a large number of “source-specific marker” compounds, in particular alkylated series of polycyclic aromatic hydrocarbons within the same alkylation isomeric groups, were compared. Results of the analysis revealed that (1) California/Oregon samples were chemically similar and consistent with the same source. They were identified to be bunker type fuel; (2) The tarball samples collected from British Columbia and Ocean Shores, Washington were chemically similar and consistent with the same source (also bunker type fuel). They were found to be similar to but may have a source different than the California/Oregon samples; (3) The source of the tarball/patty samples was neither Alaska North Slope oil nor California Monterrey Miocene oil; (4) The spilled oil samples have been highly weathered since release, and the California samples were more heavily weathered than the British Columbia samples.