Synthesis of optically active 4-hydroxymandelic acid and derivatives via Regio- and Stereoselective Friedel-Crafts alkylation.

Optically active 4-hydroxymandelic esters were prepared in good yields and very high diastereoselectivity (up to 96% d.e.) via para-hydroxyalkylation of phenolic ethers. Enantiomeric pure 4-hydroxymandelic acid is available by hydrolysis.     

Conformational aspects of physical phenomena in polymeric materials

Calculation of all possible conformations of a reasonably representative model of a real polymeric chain and consequent knowledge of the Boltzmann partition function provide information that emphasizes the importance of conformational aspects in explaining physical phenomena in polymeric materials. The study of changes in entropy and Helmholtz free energy leads to definitions of conformational transitions which are useful in problems concerned with structure-property relationships, e.g., optimization of a material property through evaluation of substituents, because these changes are related to structural and phase transitions in polymeric materials.     

Ab initio study of β-lactam antibiotics. II. Potential energy surface for the amidic CN bond breaking in the 3-cephem + OH− reaction and comparison with the β-lactam + OH− reaction

The potential energy surface for the β-lactam amidic CN bond breaking in the 3-cephem + OH^? reaction was investigated by using the ab initio Hartree—Fock method with a 9s6p/7s3p/3s basis set. The investigated reaction is a model of the reaction between an antibiotic cephalosporin and an enzymatic nucleophilic group, this last reaction being related to the mode of action of β-lactam antibiotics. The minimum-energy reaction path is characterized by a tetrahedral intermediate ≈ 116 kcal mol^?1 more stable than the reagents, by a barrier which corresponds to the partial breaking of the amidic bond and is ≈ 7 kcal mol^?1 above the intermediate and by a product ≈ 31 kcal mol^?1 more stable than the intermediate. The analysis of the wavefunction along the reaction path and the comparison with the β-lactam + OH^? reaction pointed out the role of electron-withdrawing groups on the height of the barrier and the role of intramolecular hydrogen bonds on the structure and energy of the product. The calculations suggest a model of the antibiotic activity of cephalosporins which is compared with previous qualitative pictures.     

Superconductivity and magnetic properties of YBa2Cu3O7-δ annealing and quenching effects

Summary Crystal structure, superconductivity and magnetic properties as functions of annealing and quenching temperatures for high-T _c YBa₂Cu₃O_7-δ superconductor were investigated. It was found that the ?orthorhombic character? of the superconducting phase is affected by the quenching process through a mechanism in which both density and order of oxygen vacancies in the Cu (X, Y)-plane of the unit cell may be involved. The quenching-induced effects manifest themselves onT _c and on the behaviour of normal-state resistivity and magnetic susceptibility, whose values are analysed and discussed.

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Riassunto Sono state studiate le proprietà strutturali, superconduttive e magnetiche del composto YBa₂Cu₃O_7-δ in funzione delle condizioni di annealing e tempra. Si è trovato che il ?carattere ortorombico? della fase superconduttrice è modificato dai processi di tempra attraverso un meccanismo nel quale possono essere coinvolti sia la concentrazione delle vacanze di ossigeno che la loro distribuzione nel pianoX-Y della cella unitaria. Gli effetti indotti dalla tempra si riflettono sul valore diT _c e sull'andamento della resistività e della suscettività magnetica nello stato normale, i cui valori sono analizzati e discussi.

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Резюме Исследуются кристаллическая структура, сверхпроводящие и магнитные свойства, как функция температур отжига и закалки для высокостемпературногоT _c сверхпроводника YBa₂Cu₃O_7-δ. Получено, что ?орторомбический характер? сверхповодящей фазы изменяется в процессе закалки через механизм, в котором учитываются и плотность и порядок кислородных вакансиь в CuX-Y плоскости элементарной ячейки. Эффекты закалки в явном виде влияют наT _c и на поведение сопротивления нормальной фазы и магнитной восприимчивости, значеня которых анализируются и обсуждаются.

     

Statistical mechanics of holonomic systems as a Brownian motion on smooth manifolds

The statistical mechanics of arbitrary holonomic scleronomous systems subjected to arbitrary external forces is described by specializing the Lagrange and Hamilton equations of motion to those of the Brownian motion on a manifold. In this context, the Klein‐Kramers and Smoluchowski equations are derived in covariant form, and it is demonstrated that these equations have equilibrium solutions corresponding to the Gibbs distribution, in agreement with standard thermodynamics. At last, the Langevin dynamics corresponding to the Smoluchowski limit is found to exactly correspond to the Brownian motion on a smooth manifold. These results find significant applications in the study of several statistical properties of constrained molecular assemblies (e.g. polymers) of interest in chemistry, physics and biology.     

Downward migration credit risk problem: a non-homogeneous backward semi-Markov reliability approach

International organizations evaluate credit risk and rank firms according to risk by assigning them a ‘rating’. The time evolution of a rating can be studied by means of Markov models. Some papers have outlined the problem pertaining to the unsuitable fitting of Markov processes in a credit risk environment. This paper presents a model that overcomes the problems given by the Markov rating models. It includes non-homogeneity, the downward problem and the randomness of time in the transitions of states, thus making it possible to consider the duration inside a state in a complete way. In this paper, both, the transient and asymptotic analyses are presented. The asymptotic analysis is performed by using a mono-unireducible topological structure. Moreover, a real data application is conducted using the historical database of Standard & Poor’s as the source.     

Basic alumina catalysed synthesis of substituted 2-amino-2-chromenes via three-component reaction

Substituted 2-amino-2-chromenes were obtained in excellent yield and selectivity simply by mixing malononitrile, α-naphthol and aromatic aldehydes in water in the presence of basic alumina as heterogeneous and reusable catalyst.     

Micellar Effects on S(N)2 Reactions of Alkyl Naphthalene-2-sulfonates:The Role of Hydrophobic Substituents

Reactivities of methyl naphthalene-2-sulfonate, MeONs, and its 6-alkyl derivatives, alkyl=Me, n-C(6)H(13) and n-C(12)H(25), 6-Me-MeONs, 6-Hex-MeONs, and 6-Do-MeONs, respectively, are compared for reactions in cetyl trialkylammonium bromide micelles, n-C(16)H(33)NR(3)Br, R=Me, Et, n-Pr, n-Bu, CTABr, CTEABr, CTPABr, CTBABr, respectively. Similar experiments were made on reactions of Br(-) with n-butyl and n-decyl naphthalene-2-sulfonates, BuONs, and DeONs, respectively. Reactions with OH(-) were followed in cetyl trialkylammonium hydroxide, alkyl=Me, Et, n-Pr, CTAOH, CTEAOH, and CTPAOH, solubility permitting. Some reactions with OH(-) or Br(-) were also followed in mixed-ion systems of CTAOMs or CTPAOMs (OMs=MeSO(3)). Micellar rate effects were analyzed by using pseudophase treatments including interionic competition in mixed-ion systems. Second-order rate constants in the micellar pseudophase increase systematically with increases in substrate hydrophobicity and for reactions with Br(-), but not OH(-), they also increase with bulk of the cationic head group. Copyright 2001 Academic Press.