Conformational effects on circular dichroism in the photoelectron angular distribution.

The B-spline density-functional method has been applied to the conformers of the (1R, 2R)-1,2-dibromo-1,2-dichloro-1,2-difluoroethane molecule. The cross section, asymmetry, and dichroic parameters relative to core and valence orbitals, which do not change their nature along the conformational curve, have been systematically studied. While the cross section and the asymmetry parameter are weakly affected, the dichroic parameter appears to be rather sensitive to the particular conformer of the molecule, suggesting that this dynamical property could be a useful tool for conformational analysis. The computational method has also been applied to methyl rotation in methyloxirane. Unexpected and dramatic sensitivity of the dichroic-parameter profile to the methyl rotation, both in the core and valence states, has been found. Boltzmann averaging over the conformers reproduces quite closely the profiles previously obtained for the minimum-energy conformation, which is in good agreement with the experimental results.     

Enzymatic resolution of (±)-5-phenyl-4,5-dihydroisoxazole-3-carboxylic acid ethyl ester and its transformations into polyfunctionalised amino acids and dipeptides

Enantiomerically pure (5R)-(?)-5-phenyl-4,5-dihydroisoxazole-3-carboxylic acid ethyl ester was obtained via enzymatic resolution of the corresponding racemic mixture using a lipase from hog pancreas (PPL). The following reduction of the ester group to the corresponding alcohol and the oxidation of the latter led to (5R)-(?)-5-phenyl-4,5-dihydroisoxazole-3-carbaldehyde, and the reaction between this and Schöllkopf’s reagent, (2R)-(+)-2,5-dihydro-3,6-dimethoxy-2-isopropylpyrazine, gave mixtures of adducts with a good syn/anti ratio. The steric configurations of the major diastereoisomer were assigned on the basis of spectroscopic data and X-ray analysis. The subsequent controlled hydrolysis of the pyrazine ring led to β-(5-phenyl-4,5-dihydroisoxazol-3-yl)-serine methyl esters and the corresponding dipeptides with (R)-valine. Finally, reductive cleavage of the 4,5-dihydroisoxazole ring under hydrolytic conditions made it possible to obtain the corresponding polyfunctionalised dipeptides.     

Synthesis of phenyl-substituted 2H-3,4-dihydro-3-aminomethyl-1,4-benzoxazines. Intermediates for 1H-2,3,3a,4-tetrahydroimidazo[5,1-c][1,4]benzoxazin-1-one derivatives. Part II

The synthesis of 2H-3, 4-dihydro-3-aminomethyl-1, 4-benzoxazines substituted by one phenyl group in the 2, 3 or α-position is reported. The two diastereoisomers of the 2-phenyl-derivative were independently synthesized from ethyl trans-3-phenylglycidate by a stereospecific route. A previous attempt at the synthesis of these same compounds gave rise to an unexpected aliphatic-aromatic ketone rearrangement.     

Baker's yeast mediated preparation of carbohydrate-like chiral synthons

Baker's yeast mediated reduction of the α-acetoxy ketones (1)–(3) proceeds with high enantio- and stereoselectivity to give the $\text{anti}$ carbinols (4)–(6), easily converted into the masked chiral deoxy sugars (12)–(14), from which {=D}- and {=L}-deoxysugars have been obtained.     

Spectropolarimetric determination of copper(II), nickel(II) and iron(III) ions with n-carboxymethylpyrrolidine-2-carboxylic acid

N-Carboxymelhyl pyrrolidine-2-carboxylic acid (CMPCA) is suggested as an optically active titrant. The values of the acidity constants of the ligand were determined and the order of magnitude of the stability constants of the complexes formed by CMPCA with some metal ions was evaluated. In order to determine the best conditions for the spectropolarimetric titrations, the dependence of the molar rotation of CMPCA and its complexes on wavelength and pH was examined. The spectropolarimetric titrations ofcopper(II), nickel(II) and iron(III) ions were carried out successfully.     

Homogeneous catalytic hydrogenation of free carboxylic acids in the presence of cluster ruthenium carbonyl hydrides

Saturated monocarboxylic acids up to C₆, several bicarboxylic acids and some of the corresponding anhydrides are hydrogenated in the homogeneous phase with H₄Ru₄(CO)₈(PBu₃)₄ as catalyst to give the corresponding alcohols (present among the reaction products as esters) or lactones at 100–200°C under a pressure of 100–200 atm of hydrogen. Anhydrides react at temperatures lower than those needed for acids. Esters are not reduced. Only δ-valerolactone is hydrogenated to 1,5-pentanediol. Ruthenium carbonyl carboxylates have been recovered at the end of the reaction and appear to be catalytically active intermediates.     

Application of stochastic computer simulation to a solution of a nonlinear Fokker-Planck equation governing bacteria chemotaxis

A stochastic computer simulation technique has been used to solve a one-dimensional time-dependent Fokker-Planck (F-P) equation governing spacetime distribution of bacteria in a substrate (food) gradient. Because of consumption of the substrate by the bacteria the F-P equation becomes nonlinear due to coupling of the distribution with the self-generated substrate gradient. The simulation is efficient and numerically accurate for generating transient solutions. We are able not only to produce a transient solution displaying approach to the steady state solitary wave solution known from an analytical result of Keller and Segal but also to show an interesting dependence of the solitary wave propagation speed on the concentration dependence of the substrate consumption rate.