Kinetics and mechanism of the liquid-phase oxidation of tert-butyl phenylacetate

The oxidation of tert-butyl phenylacetate in ortho-dichlorobenzene at 140°C occurs with short chains. The primary nonperoxide reaction products (tert-butyl α-hydroxyphenylacetate, tert-butyl α-oxophenylacetate, and benzaldehyde) are formed by the decomposition of a hydroperoxide (tert-butyl α-hydroperoxyphenylacetate) and (or) by the recombination of peroxy radicals with and without chain termination. Benzaldehyde and tert-butyl α-hydroxyphenylacetate undergo radical chain oxidation in a reaction medium to result in benzoic acid and tert-butyl α-oxophenylacetate. Homolytic hydroperoxide decomposition is responsible for process autoacceleration and results in benzaldehyde, which is also formed from hydroperoxide by a nonradical mechanism, probably, via a dioxetane intermediate. Both of the reactions are catalyzed by benzoic acid. Benzoic acid has no effect on hydroperoxide conversion into tert-butyl α-oxophenylacetate, which most likely occurs as a result of hydroperoxide decomposition induced by peroxy radicals. The rate constants of the main steps of the process and kinetic parameters have been calculated by solving an inverse kinetic problem.     

Selected Species of the Cucurbitaceae Family Used in Mexico for the Treatment of Diabetes Mellitus

In Mexico, Diabetes mellitus (DM) is a serious health problem, and although the current pharmacological treatments for DM such as insulin and oral hypoglycemics are available, the Mexican population continues to use medicinal plants in the treatment of DM. The antidiabetic properties of the plant species that belong to the Cucurbitaceae family has already been recognized worldwide. Since Mexico is one of the most important centers of diversity of Cucurbitaceae, the present work contributes to the review of the most used species of Cucurbitaceae in the treatment of DM in Mexico. The reviewed species (Cucurbita ficifolia, C. maxima, C. moschata, C. pepo, Ibervillea sonorae, Sechium edule, Citrullus lanatus, Cucumis melo, and C. sativus) revealed that the antidiabetic effects exerted are effective in a number of mechanisms involved in the complex pathogenesis of DM: hypoglycemic, antioxidant, anti-inflammatory, anti-obesity, protective effects on diverse organs and cells, as well as in the control of dyslipidemias; furthermore, the select species of the Cucurbitaceae family could also be essential components of diets for the control of DM in patients with the disease. Thus, the Cucurbitaceae species selected in the present work represent a source of antidiabetic agents that perhaps establish the bases for novel clinical treatments.     

Synthesis and conformational analysis of glycomimetic analogs of thiochitobiose

The synthesis of six analogs of N,N′-diacetylchitobiose is reported, including a novel transglycosylation reaction for the preparation of S-aryl thioglycosides. The conformations of the compounds were studied by a combination of NMR spectroscopy and molecular modeling, using force field calculations. In the case of the S-aryl thioglycosides with exclusively S-glycosidic linkages, dihedral angles of the disaccharidic S-glycosidic bonds, Φ′ and Ψ′ and of the S-arylglycoside bonds, Φ and Ψ, were found to be similar, whereas they were different in mixed glycosides and in a thiazoline derivative. An adequate correlation between the calculated H,H-distances of the local minima and the measured NOE contacts was achieved by applying population-weighted averages over participating conformers based on weighted relative energies.     

RuO4-mediated oxidative polycyclization of linear polyenes. A new approach to the synthesis of the bis-THF diol core of antitumour cis-cis adjacent bis-THF annonaceous acetogenins

The RuO₄-catalyzed oxidative polycyclization of some selected linear polyenes, possessing a repetitive 1,5-diene structural motif, has been investigated. The all-trans triene (E,E,E)-acetic acid henicosa-2,6,10-trienyl ester gave the expected bis-tetrahydrofuranyl diol product possessing a threo-cis-threo-cis-threo relative configuration, along with a mixture of the corresponding bis-THF ketols. These compounds can be seen as useful intermediates in the synthesis of the bis-THF diol core of adjacent bis-THF antitumour acetogenins possessing a threo-cis-threo-cis-erythro relative configuration, such as rolliniastatiin-1, membranacin, rollimembrin and membrarollin. Oxidation of the related all-trans tetraene (E,E,E,E)-acetic acid pentacosa-2,6,10,14-tetraenyl ester stops at the second cyclization step giving a mixture of a threo-cis-threo-cis-threo bis-THF diol and the corresponding ketol products. Oxidation of the triene (E,Z,E)-acetic acid 12-acetoxy-dodeca-2,6,10-trienyl ester stops at the monocyclization level failing to give bis-cyclized products, as previously observed for the related isoprenoid triene (E,Z)-farnesyl acetate. This result confirms the difficulty of closing a second THF ring when the central double bond of the triene possesses a cis configuration. Based on the collected results, a plausible model is proposed that both explains the observed cis/trans stereoselectivity for each ring-closing step in these processes, and rationalize the stereochemical course of the previously studied polycyclization of the isoprenoid polyenes (E,E)-farnesyl acetate, geranylgeranyl acetate and squalene.     

Enantioselective cyanosilylation of aldehydes catalysed by a diastereomeric mixture of atropisomeric thioureas

New bifunctional atropisomeric thioureas 1 were synthesised and tested as both a mixture of diastereomers (aR/aS)-(R,R)-1 and as single diastereomers (aR)-(R,R)-1 and (aR)-(S,S)-1, in the organocatalysed, enantioselective, cyanosilylation of a range of aldehydes (aromatic and aliphatic). Moderate enantiomeric excesses (up to 69% ee) and quantitative yields were obtained. The best results were achieved using a mixture of thiourea diastereomers (aR/aS)-(R,R)-1 instead of the single diastereomers alone.     

LCST behavior of copolymers of N-isopropylacrylamide and N-isopropylmethacrylamide in water

The lower critical solution temperature (LCST) behavior of copolymers of N-isopropylacrylamide (NiPA) and N-isopropylmethacrylamide (NiPMA) in water was studied as a function of the copolymer composition, using a combination of turbidity measurements and differential scanning calorimetry (DSC). The copolymers were prepared by free radical polymerization using N,N-dimethylformamide as a solvent and α,α′-azobis(isobutyronitrile) as an initiator. The copolymer composition was determined by elemental analysis. It was found that the temperature (T _c) at which the copolymer undergoes a phase transition, i.e., LCST, increases linearly with increasing the mole fraction (f _m) of NiPMA in the copolymer, within the T _c range from 32?°C (at f _m?=?0; NiPA homopolymer) to 42?°C (at f _m?=?1; NiPMA homopolymer). Also found from heating DSC thermograms were the linear dependencies of the enthalpy (ΔH) and entropy (ΔS) changes at T _c upon f _m. However, the ΔH (5.5?kJ/unit-mol) at f _m?=?1 was slightly smaller than that (5.7?kJ/unit-mol) of poly(N-n-propylacrylamide) but considerably smaller than that (7.8?kJ/unit-mol) of poly(N-n-propylmethacrylamide). The same trend was observed in the f _m dependence of ΔS. These results were discussed in terms of the structural effects of the NiPMA monomer unit on the heat-induced phase transition in water of poly(NiPA-co-NiPMA)s. It was suggested that a strong interaction of water with the amide group in the NiPMA would raise the transition temperature, but a local dehydration which occurs around the isopropyl side chain would not lead to large changes in the enthalpy and entropy at T _c.     

Synthesis of macrocyclic precursors of lankacidins using Stille reactions of 4-(2-iodo-alkenyl)azetidinones and related compounds for ring closure

In the context of a proposed total synthesis of lankacidins, the synthesis of 4-(2-iodo-alkenyl)azetidinones and their participation in Stille coupling reactions have been investigated. 1-tert-Butyldimethylsilyl-4-(2-iodoethenyl)azetidinone was found to undergo a Stille coupling reaction with a 3-hydroxy-1-tributylstannylhepta-1,5-diene to give an acceptable yield of the corresponding conjugated diene but the analogous reaction with a 3-tert-butyldimethylsilyloxy-1-tributylstannylhepta-1,5-diene was unsuccessful. A series of 4-[(E)-2-iodoprop-1-enyl]azetidinones, a ring-opened ester and a lactone were also found to undergo Stille reactions with 3-tributylstannylprop-2-enol albeit with variable yields. Asymmetric syntheses of methyl (2R,3R,5S)-3-tert-butyldimethylsilyloxy-2-methyl-5-(2-trimethylsilylethoxy)methoxy-6-oxohexanoate, (3R,4S)-1-tert-butyldimethylsilyl-4-[(E)-2-iodoprop-1-enyl]-3-methylazetidin-2-one, and (5S,2E,6E)-5-tert-butyldimethylsilyloxy-2-methyl-1-phenylsulfonyl-7-tributylstannylhepta-2,6-diene and their incorporation into macrocyclic precursors of the lankacidins were then investigated. Key reactions were a Julia reaction between the aldehyde and the sulfone to form the 12,13-double-bond, a stereoselective acylation of the azetidinone, and formation of macrocycles using intramolecular Stille reactions in the presence of a free hydroxyl group at C(8) (lankacidin numbering).     

Disordered modifications of cobalt molybdate

The structures of recently discovered new high-temperature modifications of cobalt molybdate, a′-and a″-CoMoO₄, were determined. a′-and a″-CoMoO₄ appear after the phase a-CoMoO₄ is heated above the temperature range 700–1000°C. They seem to be the disordered modifications of a-CoMoO₄ with metal atoms distributed at random in an a-CoMoO₄ oxygen network.The F(hkl) values, calculated for variously disordered a-CoMoO₄ structure, were compared with the observed intensities of diffraction lines changing in the course of a → a′ and a → a″ transitions. It was concluded that a″-CoMoO₄₄ has a completely disordered structure with random distribution of both Co and Mo atoms in oxygen interatomic voids. The a′-CoMoO₄ is a partly disordered modification, with random distribution of some cations only.The temperature and the kind of order-disorder transition depend on the method of preparation of a-CoMoO₄ samples.The disordered modifications of cobalt molybdate may be supercooled—even to room temperature—before it transforms rapidly into low-temperature b-CoMoO₄ form.     

On maximal resonance of polyomino graphs

A polyomino graph is a finite plane 2-connected bipartite graph every interior face of which is bounded by a regular square of side length one. Let k be a positive integer, a polyomino graph G is k-resonant if the deletion of any i ≤ k vertex-disjoint squares from G results in a graph either having perfect matchings or being empty. If graph G is k-resonant for any integer k ≥ 1, then it is called maximally resonant. All maximally resonant polyomino graphs are characterized in this work. As a result, the least integer k such that a k-resonant polyomino graph is maximally resonant is determined.     

Fine structure and transition probabilities of the 2p 5 nl bound excited states of Ne atom

We present a semi-empirical calculational procedure for thep ⁵ nl bound excited states of rare gas atoms based on the use of an LS-dependent orbital for an excitednl electron outside a frozen Hartree-Fockp ⁵ core. The spin-orbit interaction is accounted for approximately using a localized Hartree potential. The contribution from the long-range core polarization is estimated by using a parametrized potential. A model potential is also introduced to represent partially the short-range multi-electron interactions for thep ⁵ np levels. The energy levels are calculated by diagonalizing the Hamiltonian matrix following anLS →jl transformation. The calculated fine structure of the Nep ⁵ ns,p ⁵ np,p ⁵ nd, andp ⁵ nf ¦ (jl)KJ〉 levels are in close agreement with the observed level splittings. The transition probabilities are also in agreement with earlier theoretical and experimental results.     

Amber suppressors of Erwinia chrysanthemi

Mutations trpl and thyA1, both of a polyauxotrophic derivative of the Erwinia chrysanthemi strain B374, were characterized as amber mutations with an Escherichia coli suppressor, supA1P2, which inserts a glutamine in response to UAG. Simultaneous reversion of both mutations allowed us to isolate amber suppressor mutants of E. chrysanthemi. These suppressors were tested with a set of amber mutants of bacteriophage Mu which had been previously characterized on E. coli. The two independently isolated suppressors behaved as supD and supE mutants, respectively, of E. coli.     

Two-photon ionization of an inner shell electron of the Cl atom

We have calculated, using second order perturbation theory, the two photon ionization cross section of a K-shell electron of chlorine forE=1.6 keV incident photons. Two classes of intermediate states must be considered, those in which a 1s electron moves to an emptyp-orbital, and those in which an electron from an occupiedp-orbital moves into the continuum. The first class of intermediate states is followed by the ejection of ap-electron into the continuum. The second class of intermediate states is followed by the transfer of a 1s electron into an emptyp-state. The largest contribution comes from 3p →d-continuum followed by 1s → 3p transition. Our result is σ⁽²⁾/I=2.06×10⁴¹ cm⁴/W where σ⁽²⁾ is the two photon ionization cross-section andI is the light intensity.     

Fractal nature and thermal analysis of powders

F(a) functions (wherea is the rate of conversion), frequently referred to when considering non-isothermal heterogeneous processes, are reconsidered from a fractal viewpoint. This is achieved on the basis of previous studies on the fundamental properties of powders, which show that any powder obtained by mechanical size reduction yields a fractal particle-size distributionP(X,t), whereX is a scaled particle size, with a material-dependent powern asP(X,t)∝X _n , and that the obtained powder has a specific surface area,S, expressed with the fractal particle sizex asS∝x ^D-3 with the fractal dimensionD. This can be interpreted to show that a powder obtained by mechanical grinding has a uniqueD for a specified particle-size range, and, in fact, TA results dependent on thisD were obtained. We also show that a mechanical size reduction process produces fractal surfaces. The phenomenologically known laws which relate input energy and the powder product are theoretically derived by assuming that the energy is consumed in producing fractal surfaces. The well-known reaction functions which relate the conversion rate with the physical and geometrical factors governing a reaction process are reconsidered from a fractal viewpoint. The validity of conventionalF(a) expressions based on integer dimensions are questioned.     

Fine tuning of the structure of phosphine–phosphoramidites: application for rhodium-catalyzed asymmetric hydrogenations

Diastereomers of (4-(diphenylphosphino)pentan-2-yl)-N-isopropyl-{dinaphtho[2,1-d:1′,2′-f][1,3,2]dioxa-phosphepin-2-yl}-4-amine, (S)-(2S,4S)-1, and (S)-(2R,4R)-3; the octahydro derivative 4 of (S)-(2S,4S)-1, and derivative 2 of (S)-(2S,4S)-1 containing a 1,3-propanediyl backbone, have been synthesized and used for rhodium-catalyzed asymmetric hydrogenations of prochiral olefins in order to study the role of the stereogenic elements in the backbone and in the terminal moiety. The central chirality in the bridge has been found to determine the configuration of the product with a cooperative effect from the terminal groups. Excellent ee’s (up to 99.9%) were obtained in the hydrogenation of methyl (Z)-α-acetamidocinnamate using a rhodium complex with the matched arrangement (S)-(2S,4S)-1. The hydrogenation is accomplished in a highly enantioselective manner by using green solvents such as propylene carbonate.     

Isothermal study of crystallization of Pd-Ni-Si amorphous metallic materials

The isothermal crystallization of amorphous alloys of the type (Pd_100?x Ni _x )₈₃Si₁₇ has been investigated by DSC. The character of the transformation depends upon the content of Ni. Ifx>30, material crystallizes probably without nucleation in a very narrow temperature range. Ifx?30, the crystallization process is controlled by a nucleationgrowth mechanism. The initial stages of transformation are characterized by a relatively short incubation period with an Arrhenius temperature-dependence. Afterwards, the time-dependence of the degree of conversion is sigmoidal. Ifα<0.1, thenp～1 characterizes a homogeneous nucleation. If 0.1<α<0.9 than the Avrami exponent isp(α)～3–4. Atα>0.9 so-called tails are observed. Two kinds of unseparable non-stationarities of the nucleation-growth process at the beginning of crystallization of the amorphous material are discussed.     

Study of the oxidation of 3-hydroxypyrroloindoles to pyrrolobenzoxazine alkaloids

This report describes a detailed study of the oxidation-Meisenheimer rearrangement of N-methyl-3-hydroxy-7-chloropyrroloindoline ethyl ester and the corresponding O-Boc and N-Boc derivatives. Experimental conditions were found, which allowed the selective Boc protection of either the tertiary alcohol substituent or the NH group of the aminal function. It was shown that both the parent compound and its O-Boc derivative yielded a mixture of oxazines and, in some cases, N-oxides upon treatment with m-CPBA. MS fragmentation (APCI) clearly differentiates formation of N-oxides and oxazines. The N-Boc derivatives exclusively yielded the N-oxides showing that the Meisenheimer rearrangement requires the presence of a high energy lone pair on the neighbouring nitrogen atom. Both the parent compound and the O-Boc derivative gave a mixture of rearranged products and N-oxide depending on the reaction conditions.     

Partition of symmetric powers of G-sets into orbits: application to the enumeration of force constants

Using the theory of actions of groups on sets this paper describes an efficient method to obtain the partition of the symmetric powers of a G-set into orbits, where G is a finite group. In this method, a generating function is obtained for each representative of the conjugacy classes of subgroups of G. The coefficients of the generating function corresponding to a representative subgroup H?≤ G give the number of orbits isomorphic to the coset G/H that are contained in the successive symmetric powers of the G-set. A direct application of this approach is the attainment of the number and isotropy group of the vibrational force constants associated with a set of internal coordinates for a given molecule. As illustration, the method has been applied to XYZ ₃?(C _3v ) molecules.     

Influence of diluent on radiolysis of amides in organic solution

The radiolytic behavior of N,N,N′,N′-tetraoctyldiglycolamide, N,N′-dimethyl-N,N′-dioctyl-2-(3′-oxapentadecyl)malonamide, and N,N-dioctylhexanamide in n-dodecane was investigated by irradiation with γ-rays and electron pulses. The results showed n-dodecane has a sensitization effect on the radiolysis of these amides, owing mainly to a charge transfer from radical cations of n-dodecane to the amide molecules in the primary process observed in pulse radiolysis. This result was supported by the difference in the ionization potentials between n-dodecane and these amides.     

Highly enantioselective bioreduction of 4-bromoacetophenone

Microorganisms were used to reduce 4-bromoacetophenone to (S)-4-bromophenylethanol and (R)-4-bromophenylethanol. After a fractional factorial design to identify the important variables for this reaction, Geotrichum candidum provided a 98.9% conversion with >99% ee of the (R)-isomer, while Rhodotorula rubra led to a 97.6% conversion with a 98.8% ee of the S-isomer.