Chain extension and biodegradation of poly(butylene succinate) with maleic acid units

Unsaturated groups were introduced into the main chains of poly(butylene succinate) (PBS) by the condensation polymerization of 1,4‐butanediol with succinic acid and maleic acid (MA). The resulting aliphatic polyesters were subjected to chain extension via the unsaturated groups with benzoyl peroxide (BPO), BPO/ethylene glycol dimethacrylate (DF), or BPO/triallyl cyanurate (TF). During the condensation polymerization, some of the cis‐structured maleate was isomerized into the trans‐structured fumarate. The trans‐structured fumarate participated in the chain‐extension reaction with BPO more than the cis‐structured maleate. However, the trans/cis ratio remained practically unchanged when bridging molecules such as DF and TF were used along with BPO. Chain extension of PBS containing 5.7 mol % MA units (PBSM57) resulted in gel formation. Chain extension with BPO/TF made more gels in PBSM57 than chain extension with the other crosslinking agents. Chain extension increased the glass‐transition temperature, decreased the melting temperature and crystallinity, and improved mechanical properties such as elongation and tensile strength. The results of the modified Sturm tests showed that the biodegradability of the unsaturated aliphatic polyesters decreased greatly because of the chain extension. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 2240–2246, 2000     

Enzymatic resolution, desymmetrization, and dynamic kinetic asymmetric transformation of 1,3-cycloalkanediols

An efficient desymmetrization of cis-1,3-cyclohexanediol to (1S,3R)-3-(acetoxy)-1-cyclohexanol ((R,S)-2a) was performed via Candida antarctica lipase B (CALB)-catalyzed transesterification, in high yield (up to 93%) and excellent enantioselectivity (ee's up to >99.5%). (R,R)-Diacetate ((R,R)-3a) was obtained in a DYKAT process at room temperature from (1S,3R)-3-acetoxy-1-cyclohexanol ((R,S)-2a), in a high trans/cis ratio (91:9) and in excellent enantioselectivity of >99%. Metal- and enzyme-catalyzed dynamic transformation of cis/trans-1,3-cyclohexanediol using PS-C gave a high diastereoselectivity for cis-diacetate (cis/trans = 97:3). The (1R,3S)-3-acetoxy-1-cyclohexanol (ent-(R,S)-2a) was obtained from cis-diacetate by CALB-catalyzed hydrolysis in an excellent yield (97%) and selectivity (>99% ee). By deuterium labeling it was shown that intramolecular acyl migration does not occur in the transformation of cis-monoacetate to the cis-diacetate.     

Molecular structure of the trans and cis isomers of metal-free phthalocyanine studied by gas-phase electron diffraction and high-level quantum chemical calculations: NH tautomerization and calculated vibrational frequencies

The molecular structure of the trans isomer of metal-free phthalocyanine (H2Pc) is determined using the gas electron diffraction (GED) method and high-level quantum chemical calculations. B3LYP calculations employing the basis sets 6-31G**, 6-311++G**, and cc-pVTZ give two tautomeric isomers for the inner H atoms, a trans isomer having D2h symmetry and a cis isomer having C2v symmetry. The trans isomer is calculated to be 41.6 (B3LYP/6-311++G**, zero-point corrected) and 37.3 kJ/mol (B3LYP/cc-pVTZ, not zero-point corrected) more stable than the cis isomer. However, Hartree-Fock (HF) calculations using different basis sets predict that cis is preferred and that trans does not exist as a stable form of the molecule. The equilibrium composition in the gas phase at 471 degrees C (the temperature of the GED experiment) calculated at the B3LYP/6-311++G** level is 99.8% trans and 0.2% cis. This is in very good agreement with the GED data, which indicate that the mole fraction of the cis isomer is close to zero. The transition states for two mechanisms of the NH tautomerization have been characterized. A concerted mechanism where the two H atoms move simultaneously yields a transition state of D2h symmetry and an energy barrier of 95.8 kJ/mol. A two-step mechanism where a trans isomer is converted to a cis isomer, which is converted into another trans isomer, proceeds via two transition states of C(s) symmetry and an energy barrier of 64.2 kJ/mol according to the B3LYP/6-311++G** calculation. The molecular geometry determined from GED is in very good agreement with the geometry obtained from the quantum chemical calculations. Vibrational frequencies, IR, and Raman intensities have been calculated using B3LYP/6-311++G**. These calculations indicate that the molecule is rather flexible with six vibrational frequencies in the range of 20-84 cm(-1) for the trans isomer. The cis isomer might be detected by infrared matrix spectroscopy since the N-H stretching frequencies are very different for the two isomers.     

Esters of 2,5-multisubstituted-1,3-dioxane-2-carboxylic acid: their conformational analysis and selective hydrolysis

The carbomethoxy group at the C2 position of the 2,5-multisubstituted 1,3-dioxanes prefers the axial conformation rather than the equatorial one due to an anomeric effect. The trans isomers of the 5-monosubstituted compounds are more selectively hydrolyzed than the cis isomers. Based on the calculated results, hydrolysis to the trans isomers is attributed to the larger carbonyl charges of the trans than those of the cis isomers. The anomeric and homoanomeric effects will explain the axial preference of the carbomethoxy group and selective hydrolysis to the trans isomers. Furthermore, the calculated stability between the cis and trans isomers is in good agreement with the experimental results in the equilibrium state.     

葡萄酒中白藜芦醇的固相萃取GC/MS法测定及其生理活性的初步研究

用反向C₁₈固相萃取小柱对葡萄酒进行了预处理,然后用BSTFA硅烷化试剂对萃取物进行了衍生化处理,用气相色谱/质谱对葡萄酒中顺式、反式白藜芦醇进行了同时测定,建立了定量分析方法.对方法的精密度和回收率进行了测定,方法的相对标准偏差分别为4.48%(顺式)和5.23%(反式).顺式、反式白藜芦醇的平均回收率分别为94.4%和97.6%.并对10种国产葡萄酒和两种进口葡萄酒样品进行了测定.对白藜芦醇的生理活性也进行了实验,初步探讨了白藜芦醇对大鼠血管平滑肌细胞(VSMC)、大肠癌细胞株Lovo细胞和小鼠成纤维株3T3细胞的生长生殖活性的影响,观察了其形态变化.     

1.1](3,3')-azobenzenophane: novel crystal structure and cis-trans isomerization of distorted azobenzene

[formula: see text] [1.1](3,3')-Azobenzenophane, in which two azobenzenes are cyclically connected by -CH2- chains at the meta positions, has been synthesized. The crystal structures of all isomers have been revealed. This is the first report on the crystal structure of the cis isomer of macrocyclic azobenzenes. The trans,trans isomer was slightly distorted, the trans,cis isomer highly deformed, and the cis,cis isomer unstrained. The thermal stability of cis isomers in solutions are deducible from the crystal structures.     

Rotation about the C?N bond in 2-aza-1,3-butadiene and the N?N bond in 2,3-diaza-1,3 butadiene: A molecular orbital study

The geometry and energy of 2-aza-1,3-butadiene and 2,3-diaza-1,3-butadiene have been calculated using the 6-31G* basis set as a function of the CNCC and CNNC dihedral angles, respectively. With the 2-aza derivative potential minima are located at 0° (trans) and at about 130° for a gauche structure approximately 9.5 kJ mol^?1 less stable than the trans. Potential maxima are at about 75° giving a gauche barrier height of approximately 19 kJ mol^?1 relative to the trans structure, and at 180° (cis) giving a barrier height of approximately 14.5 kJ mol^?1 relative to the 130° gauche structure. With the 2,3-diaza derivative the gauche barrier has disappeared and there are a series of gauche structures in the region 70°–100° of almost equal energy 12.5-15 kJ mol^?1 less stable than the trans. In addition the cis barrier is much greater, nearly 70 kJ mol^?1 relative to the trans structure. Inclusion of electron correlation, accounting for about 50% of the correlation energy, produces no significant changes in the shape of the potential energy curves. There are systematic and progressive changes in almost all the geometrical parameters as the ?CH? groups in butadiene are replaced by ?N? . The outward tilt and compression within the methylene groups show adverse steric interactions to be operative in the cis structures. The values of V_nn indicate that gauche structures of both the 2-aza and the 2,3-diaza derivatives near the cis structure are more compact (as with butadiene), and gauche structures of the 2-aza derivative near the trans structure are less compact (as with butadiene). Originating in the changes in bond lengths and bond angles, rotation-independent nuclear–nuclear interactions again play an important role.     

PHOTOCHEMICAL AND FREE RADICAL INITIATED REACTIONS OF1,3-DIMETHYLTHYMINE WITH ISOPROPANOL

Abstract— A photochemically induced reaction of 1 ,3-dimethylthymine (DMT) with isopropanol leads to the formation of four alcohol adducts. The products have been identified as the cis and trans isomers of 5 ,6-dihydro-1,3-dimethyll-6-(2-hydroxy-2-propyl) thymine (I and II), 2.4-diaza-8-hydroxy-2.4,6.8-tetramethylbicyclo[4.2.0]octan-1,3-dione (III), and 5 ,6-dihydro-1,3-dimethyl-6-(2-oxo-l-propyl)-thymine (IV). An acetone photosensitized reaction of DMT with isopropanol gives the same products in a similar relative yield distribution. In both of these reactions, cyclobutane dimers of DMT are produced as well. Free radical reactions of 2-hydroxyisopropyl radicals with DMT, initiated by decomposition of di- t -butyl peroxide, leads to formation of only one of the cis and trans isomers described above. along with 1 ,3-dimethyl-5-(2-hydroxy-2-methyl-1-propyl)uracil (V).     

Synthesis of 3-Ethoxy-2-Oxo-3-Phenyl-1,2-Oxaphosphorinane-3,5-Diene

Abstract

In view of the limited number of examples of 2-oxo-l,2-oxaphosphorinane3,5diene derivatives^1,2, we investigated the preparation of the title compound and derivatives of the same. Of additional interest were the spectroscopic propertits of this class of compounds, their stability, and the outcome of their reaction with nucleophilts. Prior work in our laboratory on the synthesis of simple phostones and their derivatives³, led us to use these as starting mataials. The presence of the phenyl substituent in la, (cis and trans isomers) permitted facile introduction of a bromo group through free radical bromination with NBS/AIBN to give 2a,b in 79% overall yield. The individual isomers of 2 were separated; tentative stereochemical assignments were made using NMR spectroscopy. Treatmnt of 2 with LiCl/DMF gave 3; the trans isomer (phenyl and OEt trans) 2b reacted much faster than the cis isomer. Treatmnt of 3 with NBS/AIBN gave 4a,b and 5 which were separated by flash chromatography. Dehydrobromination of 4 (71% yield) was achieved by heating with an excess of Et₃N in toluenc at 95°C to produce 6; likewise, dehydrobromination of 5 at 70°C gave 7.     

Polymerization in lyotropic liquid crystals. I. Change of structure during polymerization

Polymerization was made at 60°C in a lyotropic liquid crystal of sodium undecenoate and water. The liquid crystalline structure prior to polymerization was identified by optical microscopy and low-angle x-ray diffraction as an array of hexagonal closely packed cylinders with the hydrophobic part of the soap in the center of the cylinders. During polymerization the structure became isotropic at 60°C. Cooling to 20°C transformed the structure to a lamellar liquid crystal–a reversible transition. The structure of the lamellar phase was interpreted as a polyethylene backbone from which deformed decanoate chains reached toward the aqueous layer. Molecular models showed the model to accept head-tail, head-head, and tail-tail configurations in cis and trans conformations with the exception of the cis tail-tail configuration.     

硝酰氯和顺/反亚硝酸氯及其互变异构反应的理论研究

用密度泛函理论（DFT）的B3LYP方法和6－311＋G（3df）基组，计算了气态下硝酰氯和顺／反应硝酸氯的几何构型、电子结构、红外光谱以及热力学性质，并讨论了它们的互变异构反应，分析了过渡态的结构。结果表明，B3LYP／6－311＋G（3df）计算得到的结果与实验值及CCSD（T）方法计算结果吻合，且更适应于研究反应机理，ClNO2转变为cis-ClONO的过渡态（TS1）偏离平面构型；cis-ClONO和trans-ClONO互变反应的过渡态（TS2）属于内旋转位垒；高水平计算表明不存在由ClNO2直接转变为trans-ClONO的过渡态，而是得到了一个十分接近异裂产物的二级马鞍点（2SP）。根据得到的热力学函数计算了气态时各温度下互变异构反应的平衡常数。     

Synthesis of 3,4-disubstituted piperidines by carbonyl ene and prins cyclizations: switching between kinetic and thermodynamic control with Brønsted and Lewis acid catalysts

A novel approach to cis and trans 3,4-disubstituted piperidines is described. Carbonyl ene cyclization of aldehydes 4a-e catalyzed by MeAlCl(2) in refluxing chloroform afforded the trans piperidines 7a-e with diastereomeric ratios of up to 93:7, while aldehyde 4f afforded solely the cis product 6f, which was resistant to isomerization to the trans isomer. It was demonstrated for 4a that the cyclization catalyzed by a variety of Lewis acids at low temperature proceeded under kinetic control to afford predominantly the cis piperidine 6a, and this isomerized to the thermodynamically more stable trans piperidine 7a on warming. In contrast, Prins cyclization of 4a-e catalyzed by concentrated hydrochloric acid in CH2Cl2 at low temperature afforded cis piperidines 6a-e with diastereomeric ratios of up to >98:2. The yield and diastereoselectivity of these cyclizations could be improved by using HCl-saturated CH2Cl2 to form the corresponding chloride, followed by elimination of HCl effected by ammonia. Aldehydes 4f and 4galso cyclized in good yield under the latter conditions. Mechanistic studies supported by DFT calculations (B3LYP/6-31G(d)) suggest that the cyclizations proceed via a mechanism with significant carbocationic character, with the cis carbocation being more stable than the trans carbocation. DFT calculations (B3LYP/6-31G(d)) of the transition state energies for concerted cyclization show that the cis piperidine is also the favored product from cyclization through a more concerted mechanism.     

Theoretical Studies of Distyrylbenzene and its Optical Properties

The electronic and geometrical properties of distyrylbenzene (DSB) are investigated by using chemistry theoretical calculation methods. Specifically, the excited state properties are studied by performing ab initio correlation interaction singlet (CIS) and time‐dependent density functional theory; the ground state and Raman activities are computed by density functional theory with the B3LYP method. Eight conformers of distyrylbenzene are found and they are derived from three isomers which are cis, cis‐, cis, trans‐, and trans, trans‐, respectively. The relative energy shows that each isomer of three types is separated with a large energy barrier, but a small energy difference of each conformer is found if they are in the same type. The transition state also shows the barrier between conformers is lower than isomers. The computed excited transition energies using ZINDO/S based on the optimized geometries at a DFT/B3LYP level with 6–31+G show an excellent agreement with experimental absorption spectra.     

Relationship of deformation behavior to thermal transitions of ethylene/styrene and ethylene/octene copolymers

The stress‐strain response of low‐crystallinity ethylene‐octene (EO) and ethylene‐styrene (ES) copolymers with 7–20 mol % comonomer was compared over a temperature range that spanned the glass‐transition and crystal melting regions. Above the onset temperature of the glass transition, the copolymers exhibited elastomeric behavior with low initial modulus, uniform deformation to high strains, and high recovery after the stress was released. In the glass‐transition range, an initial low‐stress elastomeric response was followed by a distinct “bump” in the stress‐strain curve. On the basis of the temperature and rate dependence of the stress‐strain curve, local strain‐rate measurements, local temperature changes, and recovery characteristics, the “bump” was identified as high strain yielding. Hence, the stress‐strain curve sequentially exhibited the features of elastomeric and plastic deformation. Following high strain yielding, strain hardening dramatically increased the fracture strength. This behavior was defined as elastomeric‐plastic. Elastomeric‐plastic behavior in the broad glass‐transition range constituted a gradual transition from elastomeric behavior at higher temperatures to low‐temperature plastic behavior with high modulus and macroscopic necking. Because of the lower glass‐transition temperature of EO, ?40 °C as compared with ?10 °C for ES, the onset of elastomeric‐plastic behavior occurred at a significantly lower temperature. The concept of a network of flexible chains with fringed micellar crystals serving as the multifunctional junctions that provides the structural basis for elastomeric behavior of low‐crystallinity ethylene copolymers was extended to elastomeric‐plastic behavior by considering a network with a fraction of rigid, glassy chains. © 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 40: 142–152, 2002     

Stereochemistry of heterocycles

Methods were developed for the synthesis of 2-ethoxy-1,4-butanediol and 2-ethoxy-1,4-pentanediol. The latter was a mixture of diastereomers. These diols were used to synthesize 5-ethoxy-1,3-dioxepan and 2,2,4-trimethyl-6-ethoxy-1,3-dioxepan. Gas-liquid chromatography was used to demonstrate that 2,2,4-trimethyl-6-ethoxy-1,3-dioxepan is a mixture of cis and trans isomers; the ratio of these isomers is similar to the ratio of diastereomers in the starting 2-ethoxy-1,4-pentanediol. The configurations of the indicated isomers were evaluated by conformational analysis.See [1] for communication XIII.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 1, pp. 3–6, January, 1972.     

Counter-current chromatographic estimation of hydrophobicity of Z-(cis) and E-(trans) enalapril and kinetics of cis/trans isomerization

The kinetics of Z-(cis)/E-(trans) isomerization of enalapril was investigated by reversed phase high-performance liquid chromatography (RP-HPLC) using a monolith ODS column under a series of different temperature and pH conditions. At a neutral pH 7, the rate (k(obs)) of Z-(cis)/E-(trans) isomerization of enalapril at 4 degrees C (9.4 x 10(-3)min(-1)) is much lower than at 23 degrees C (1.8 x 10(-1)min(-1)), while the fractional concentration of Z-(cis) isomer is always higher than that of E-(trans) isomer in the pH range 2-7. The fractional concentration of the E-(trans) isomer becomes a maximum (about 40%) in the pH range 3-6, where enalapril exists as a zwitterion. The hydrophobicity (logP(O/W)) of both isomers was estimated by high-speed counter-current chromatography (HSCCC). Normal phase HSCCC separation using a tert-butyl methyl ether-acetonitrile-20mM potassium phosphate buffer (pH 5) two-phase solvent system (2:2:3, v/v/v) at 4 degrees C was effective in partially separating the isomers, and the partition coefficient (K) of each isomer was directly calculated from the retention volume (V(R)). The logP(O/W) values of Z-(cis) and E-(trans) isomers were -0.46 and -0.65, respectively.     

Effect of proline and glycine residues on dynamics and barriers of loop formation in polypeptide chains

Glycine and proline residues are frequently found in turn and loop structures of proteins and are believed to play an important role during chain compaction early in folding. We investigated their effect on the dynamics of intrachain loop formation in various unstructured polypeptide chains. Loop formation is significantly slower around trans prolyl peptide bonds and faster around glycine residues compared to any other amino acid. However, short loops are formed fastest around cis prolyl bonds with a time constant of 6 ns for end-to-end contact formation in a four-residue loop. Formation of short loops encounters activation energies in the range of 15 to 30 kJ/mol. The altered dynamics around glycine and trans prolyl bonds can be mainly ascribed to their effects on the activation energy. The fast dynamics around cis prolyl bonds, in contrast, originate in a higher Arrhenius pre-exponential factor, which compensates for an increased activation energy for loop formation compared to trans isomers. All-atom simulations of proline-containing peptides indicate that the conformational space for cis prolyl isomers is largely restricted compared to trans isomers. This leads to decreased average end-to-end distances and to a smaller loss in conformational entropy upon loop formation in cis isomers. The results further show that glycine and proline residues only influence formation of short loops containing between 2 and 10 residues, which is the typical loop size in native proteins. Formation of larger loops is not affected by the presence of a single glycine or proline residue.     

Palladium-catalyzed alkylation of vinyl oxiranes with substituted allenes. Direct access to bifunctionalized allylic alcohols

The first palladium-catalyzed alkylation of vinyl oxiranes with substituted allenes to form functionalized allylic alcohols is described. The reaction of activated allenes 5 with vinyl oxiranes 1 in the presence of catalytic amounts of Pd(PPh(3))(4) (10 mol %) and 1,3-bis(diphenylphosphino)propane (dppp) (20 mol %) in THF at 60 degrees C gave the corresponding allylic alcohols 6 in good to excellent yields. The allylic alcohols were obtained in different ratios of trans/ cis isomers.     

Stereochemie des Übergangszustandes aliphatischer CLAISEN-Umlagerungen. Vorläufige Mitteilung

The stereochemistry of the transition state in the CLAISEN rearrangement of crotyl propenyl ethers ( 2 ) has been established. By heating trans, cis-, cis, cis- and trans, trans-crotyl propenyl ether at 142,5°, erythro and threo 2, 3-dimethylpent-4-en-1-al ( 3 and 4 ) were obtained. From the ratio 3/4 it was shown that the rearrangement of the three ethers largely involves (97–98%) a chair-like transition state.