Lipase-catalyzed Irreversible Transesterification of Secondary Alcohols Using Isopropenyl Acetate

Summary. Asymmetric acetylation of a set of secondary alcohols with the innocuous acyl donor isopropenyl acetate catalyzed by a lipase from Pseudomonas cepacia immobilized on ceramic particles (PSL-C) in toluene as organic medium afforded the chiral alcohols and the corresponding acetates in high enantiomeric excess (up to 99%). An effective baseline separation of the enantiomers of both substrate and product was performed in one analysis without derivatization using gas chromatography on a new chiral stationary phase (CSP) Chirasil--Dex containing an undecamethylene spacer (C11-Chirasil-Dex).Received November 27, 2002; accepted (revised) January 9, 2003 Published online June 2, 2003     

Crystal and molecular structure of DL-serine hydrochloride studied by X-ray diffraction, low-temperature Fourier transform infrared spectroscopy and DFT(B3LYP) calculations

The structure of dl-serine.HCl was studied by three complementary techniques. Experimental Fourier transform infrared (FT-IR) spectra of pure NH/OH polycrystalline dl-serine.HCl [HO-CH2-CH(NH3+)-COOH.Cl(-)] and the respective deuterated derivatives [ND/ODAlcohol/Acid (<10% and ca. 60% D)] were recorded in the region 4000-400 cm(-1) in the temperature range 300-10 K and interpreted. The assignments were confirmed by comparison with the vibrational spectra of crystalline dl- and l-serine zwitterions [HO-CH 2-CH(NH3+)-COO(-)]. Further insight into the structure of the title compound was provided by theoretical DFT(B3LYP)/6-311++G(d,p) calculations of the infrared spectra and energies of 13 different conformers. Potential energy distributions resulting from normal co-ordinate analysis were calculated for the most stable conformer ( I) in its hydrogenated and deuterated modification. Frequencies of several vibrational modes were used in the estimation of enthalpies of individual H-bonds present in the crystal, using empirical correlations between enthalpy and the frequency shift that occurs as a result of the establishment of the H-bonds. X-ray crystallography data for dl-serine.HCl were recorded for the first time and, together with the experimental vibrational spectra and the theoretical calculations, allowed a detailed characterization of its molecular structure.     

Photochemistry of 5-allyloxy-tetrazoles: steady-state and laser flash photolysis study

The photochemistry of three 5-allyloxy-tetrazoles, in methanol, acetonitrile and cyclohexane was studied by product analysis and laser flash photolysis. The exclusive primary photochemical process identified was molecular nitrogen elimination, with formation of 1,3-oxazines. These compounds were isolated in reasonable yields by column chromatography on silica gel and were fully characterized. DFT(B3LYP)/6-31G(d,p) calculations predict that these 1,3-oxazines can adopt two tautomeric forms (i) with the NH group acting as a bridge connecting the oxazine and phenyl rings and (ii) with the -N=bridge and the proton shifted to the oxazine ring. Both tautomeric forms are relevant in the photolysis of oxazines in solution. Secondary reactions were observed, leading to the production of phenyl vinyl-hydrazines, enamines, aniline and phenyl-isocyanate. Transient absorption, detected by laser flash photolysis, is attributed to the formation of triplet 1,3-biradicals generated from the excited 5-allyloxy-tetrazoles. The 1,3-biradicals are converted to 1,6-biradicals by proton transfer, which, after intersystem crossing, decay to generate the products. Solvent effects on the photoproduct distribution and rate of decomposition are negligible.     

Wick Rotation for D-modules

We extend the classical Wick rotation to D-modules and higher codimensional submanifolds.     

Design of the Central Solenoid of the HV Electron Cooling System for the NICA Collider

Physics of Particles and Nuclei Letters - The HV electron cooling system designed at the Budker Institute of Nuclear Physics for the NICA collider is the world’s only setup developed for a...