On the Nature of the 7-Norbornadienyllithiums. Preliminary Communication

Lithium reductions of 7-chloronorbornadiene and of bis(7-norbornadienyl)mercury both provide (C₇H₇)₂Li₂ ( 5a ). This product is accompanied by C₇H₇Li₂Cl ( 5c ) in the first case, and by C₇H₇Li ( 5b ) in the second. The theoretically anticipated properties of all three organolithiums are apparent in the consistent C_s symmetry of their hydrocarbon ligands, their protolytic destruction by 12-crown-4, and their significant J(C(7), Li) ( 5a , 7.6; 5b , 16.0; 5c 8.9 Hz).     

One-step cleanup for PAH residue analysis in plant matrices using size-exclusion chromatography

A new one-step cleanup procedure, based on size-exclusion chromatography (SEC), usable for the extracts from accelerated solvent extraction (ASE), Soxhlet extraction, or ultrasonic extraction (USE), is described. The method is suitable for the determination of polycyclic aromatic hydrocarbons (PAHs), especially from very complicated plant matrices (e.g. pine needles, deciduous leaves, mosses). The main improvement compared with previous conventional procedures is that analyte peaks barely overlap with matrix peaks in the chromatograms and that it is a very rapid and simple one-step procedure with clearly improved analytical performance. Essential advantages of this SEC procedure are the sharper GC-MS chromatograms for the PAH fraction at retention times between 9.2 and 12.0 min, distinctly separated substance peaks resulting in better analysis, shorter running times, and lower solvent consumption.     

The electric field gradient at111Cd in vanadium oxides

The electric field gradient (efg) of¹¹¹Cd in polycrystalline V₂O₅ was studied using perturbed angular correlation (PAC) spectroscopy, with the¹¹¹In activity ion-implanted at 400 keV. Between the individual steps of an isochronal annealing program, a distinct efg (v _Q 1=88.1(3) MHz, ₁=0.62(2)) was recorded the contribution of which increased with annealing temperature up to 74% at 870 K. Corresponding X-ray analysis of inactive V₂O₅ samples, which underwent the same annealing treatment, proved that the sample always stayed as V₂O₅. Since V₂O₅ has only one equivalent cation site, it is concluded that this efg belongs to¹¹¹Cd at this site. Oxidation of a vanadium foil atT=675 and 800 K at =200 mbar also yielded this efg. From PAC measurements in VO₂, two well-defined efg's were found above and below the metal-semiconductor transition at 340 K, which are tentatively attributed to the monoclinic and the tetragonal phase.Supported by Deutsche Forschungsgemeinschaft and DAAD.On leave from the University of Durban-Westville, South Africa.     

Configuration interaction (CI): Approximate inclusion of fourfold and threefold excitations,an application of knowledge engineering

It is shown that even the CPMET approximation for the contributions of fourfold excitations can be replaced by a more sophisticated method. Furthermore, an approximation is presented for the contributions of threefold excitations which are neglected in CPMET . Finally, it is described how the advantages of different CEPA methods can be united in a single CEPA scheme (called CEPA -U). The various approximations are discussed in terms of Slater determinants, and it is shown how the relatively large amount of details can be handled with the help of computer programs, i.e., how knowledge engineering can be successfully applied here.     

Spectrophotometric determination of phosphate by extraction of reduced molybdoantimonylphosphoric acid with acetophenone-chloroform

A 90% ($\text{v}\text{v}$) mixture of acetophenone-chloroform is an effective extractant for reduced molybdoantimonylphosphoric acid. The extraction is quantitative over the acidity range of 0.04 to 4.8 N sulfuric acid. The mixed solvent extractant is virtually immiscible with water and can be used at aqueous/organic volume ratios from 1:1 to 40:1 with no loss of solvent or decrease in % extraction. Phosphate is easily determined from 5 to 1000 ppb. No interference from silicate was experienced.     

Structures and cytotoxic properties of sponge-derived bisannulated acridines

A reinvestigation of sponge natural products from additional Indo-Pacific collections of Xestospongiacf. carbonaria and X. cf. exigua has provided further insights on the structures, biological activities, and biosynthetic origin of bisannulated acridines. These alkaloids include one known pyridoacridine, neoamphimedine (2), and three new analogues, 5-methoxyneoamphimedine (4), neoamphimedine Y (5), and neoamphimedine Z (6). A completely new acridine, alpkinidine (7), was also isolated. A disk diffusion soft agar assay, using a panel of five cancer cell lines (solid tumors and leukemias) and two normal cells, was used to evaluate the differential cytotoxicity (solid tumor selectivity) of the sponge semipure extracts and selected compounds including amphimedine (1), 2, 4, and 7. While all four compounds were solid tumor selective, 1 and 2 were the most potent and 4 was the most selective. The rationale used to characterize the new structures is outlined along with the related biosynthetic pathways envisioned to generate 2 and 7.     

Chiral recognition in NMR spectroscopy using crown ethers and their ytterbium(III) complexes

Chiral crown ethers 1 and 5 are useful enantiomeric discriminating agents in ¹H NMR spectroscopy for neutral and protonated primary amines, amino acids, and amino alcohols. The presence of the carboxylic acid groups in 1 and 5 provide sites at which ytterbium(III) can bind. Adding ytterbium(III) nitrate to crown–substrate mixtures in methanol-d₄ causes shifts in the spectra of substrates and often enhances the chiral discrimination in the ¹H NMR spectrum. The enhancement in enantiomeric discrimination that occurs in the presence of ytterbium(III) allows lower concentrations of the crown ether to be used in chiral recognition studies. Several amide derivatives of 1 were prepared and evaluated as chiral NMR discriminating agents, although except for 1e, these were less effective than 1.     

Water-soluble calix[4]resorcarenes as enantioselective NMR shift reagents for aromatic compounds

A tetra L-prolinylmethyl derivative of a tetra-sulfonated calix[4]resorcarene (1) is an effective chiral NMR solvating agent for water-soluble compounds with phenyl, pyridyl, bicyclic aromatic, or indole rings. These aromatic compounds form host-guest complexes with the calix[4]resorcarene in water. Complexation of substrates with the calix[4]resorcarene is likely promoted by hydrophobic effects, and bicyclic substrates have association constants with the calix[4]resorcarene larger than those of similar phenyl-containing compounds. Aromatic resonances of the substrates show substantial upfield shifts because of shielding from the aromatic rings of the calix[4]resorcarene, and several resonances in the 1H NMR spectra typically exhibit enantiomeric discrimination. The extent of enantiomeric discrimination depends in part on interactions of the substituent groups of the substrates with the prolinylmethyl groups of the calix[4]resorcarene. The effectiveness of a calix[4]resorcarene prepared from N-methyl-L-alanine (2) as a chiral NMR discriminating agent is compared to the L-prolinylmethyl derivative.