Sites of reaction of pilocarpine

Analysis of the sites of reaction of a biologically important compound, pilocarpine, a molecule with imidazole and butyrolactone rings connected by a methylene bridge, has been accomplished in a quadrupole ion trap with the aim of characterizing its structure/reactivity relationships. Ion-molecule reactions of pilocarpine with chemical ionizing agents, dimethyl ether (DME), 2-methoxyethanol, and trimethyl borate (TMB), along with collision-activated dissociation elucidated the reaction sites of pilocarpine and made possible the comparison of structural features that affect sites of reaction. Based on MS/MS experiments, methylation occurs on the imidazole ring upon reactions with CH3OCH2+ or (CH3OCH2CH2OH)H+ ions but methylation occurs on the lactone ring for reactions with (CH3O)2B+ ions. Bracketing experiments with two model compounds, alpha-methyl-gamma-butyrolactone and N-methyl imidazole, show the imidazole ring to have a greater gas-phase basicity and methyl cation affinity than the lactone ring. The contrast of methylation by TMB ions on the lactone ring is explained by initial addition of the dimethoxyborinium ion, (CH3O)2B+, on the imidazole ring with subsequent collisional activation promoting an intramolecular transfer of a methyl group to the lactone ring with concurrent loss of CH3OBO. Semiempirical molecular orbital calculations are undertaken to further address the favored reaction sites.     

Photolysis of organopolysilanes. The reaction of photochemically generated methylphenylsilylene with conjugated dienes

Photolysis of 2-phenylheptamethyltrisilane (I) in the presence of acyclic and cyclic conjugated dienes has been investigated using both a high-pressure mercury lamp with a quartz filter and a low-pressure mercury lamp with a Vycor filter. Irradiation of I in the presence of 1,3-butadiene, isoprene or 2,3-dimethylbutadiene with a high-pressure mercury lamp gave a product arising from photochemical isomerization of a silacyclopropane derivative and a compound apparently formed by 1,4-silylene addition, along with a 1/1 “ene” adduct of the diene to a photo-rearranged intermediate containing the silicon—carbon double bond. Irradiation of I in the presence of the conjugated diene with a low-pressure mercury lamp, followed by treatment of the product with methanol, afforded a methoxysilane arising from methanolysis of the corresponding silacyclopropane, together with the isomerization product, silacyclopentene and rearranged addition product. Irradiation of I in the presence of cyclopentadiene with a high-pressure mercury lamp produced methylphenylsilylcyclopentadiene, while irradiation of a similar mixture with a low-pressure mercury lamp followed by treatment with methanol gave 4-(methoxymethylphenylsilyl)-1-cyclopentene. With 1,3-cyclooctadiene, the photochemically generated methylphenylsilylene afforded many types of addition product. Photolysis of I in the presence of 1,3-cyclohexadiene, however, afforded none of the silylene addition products.     

四氟乙烯齐聚体的反应 四氟乙烯四、五聚体和芳香胺的反应

前已报道四氟乙烯四聚体(全氟-3,4-甲基己烯-3)(1)、五聚体(全氟-3,4-二甲基-4-乙基己烯-2)(2)和脂肪烷氧以及脂肪胺的亲核反应.本文报道化合物1,2和芳香胺如苯胺、β-萘胺的反应.由于烯烃1、2双键处于分子中间,因而当亲核试剂进攻时,双键容易发生重排,生成的末端基烯烃更具反应性,故导致一取代、二取代、三取代以及环化降解等复杂产物.     

Unprecedented Halogenation of Biginelli Compounds of Salicyldehyde

Unprecedented results are obtained from bromination of the normal/non-bridged and abnormal/bridged Biginelli compound of salicyldehyde. Bromination with 2,4,4,6-tetrabromocyclohexa-2,5-dienone yields a bridged monobromo-derivative and a bridged dibromo-derivative, respectively, whereas bromination with bromine and acetic acid gives a mixture of those two products in each case. Bromination with N-bromosuccinimide gives a third product, namely a tribromo-derivative, in addition to those two products. Iodination of both the substrates using iodine monochloride (Wijs solution) furnishes a bridged monoiodo-derivative only.     

Analysis of MS/MS fragmentation of taxoids

The fragmentation pathways of seven types of taxoids were investigated by using a LC-MS/MS method, namely: (1) neutral taxoids with a C-4(20) double bond; (2) taxoids with a C-4(20) double bond and oxygenation at C-14; (3) 5-cinnamoyl taxoids with a C-4(20) double bond; (4) a basic taxoid with a C-4(20) double bond; (5) a taxoid with a C-4(20) epoxide; (6) taxoids with an oxetane ring; and (7) taxoids with an oxetane ring and a phenylisoserine C-13 side chain. Depending on the class of core structure and the substitution pattern, each taxoid gave either the molecular adduct ion [M+NH4]+ or [M+H]+. In the MS/MS, the molecular adduct ion gave characteristic product ions corresponding to the loss of water, acetic acid, benzoic acid, and cinnamic acid or the phenylisoserine group. These could reflect the difference of the substitutions and structural modifications and should be utilized for the structure elucidation oftaxoids by LC-MS.     

Selection of anion exchangers for detoxification of dilute-acid hydrolysates from spruce

Six anion-exchange resins with different properties were compared with respect to detoxification of a dilute-acid hydrolysate of spruce prior to ethanolic fermentation with Saccharomyces cerevisiae. The six resins encompassed strong and weak functional groups as well as styrene-, phenol-, and acrylic-based matrices. In an analytical experimental series, fractions from columns packed with the different resins were analyzed regarding pH, glucose, furfural, hydroxymethylfurfural, phenolic compounds, levulinic acid, acetic acid, formic acid, and sulfate. An initial adsorption of glucose occurred in the strong alkaline environment and led to glucose accumulation at a later stage. Acetic and levulinic acid passed through the column before formic acid, whereas sulfate had the strongest affinity. In a preparative experimental series, one fraction from each of six columns packed with the different resins was collected for assay of the fermentability and analysis of glucose, mannose, and fermentation inhibitors. The fractions collected from strong anion-exchange resins with styrene-based matrices displayed the best fermentability: a sevenfold enhancement of ethanol productivity compared with untreated hydrolysate. Fractions from a strong anion exchanger with acrylic-based matrix and a weak exchanger with phenol-based resin displayed an intermediate improvement in fermentability, a four- to fivefold increase in ethanol productivity. The fractions from two weak exchangers with styrene- and acrylic-based matrices displayed a twofold increase in ethanol productivity. Phenolic compounds were more efficiently removed by resins with styrene-and phenol-based matrices than by resins with acrylic-based matrices.     

Effect of precursors of mesoporous silica on the adsorption properties of a ceramic material

Nitrogen low-temperature adsorption-desorption is employed to investigate the adsorption and textural properties of a ceramic material coated with silica, which is produced by the sol-gel method from oxide and alkoxide precursors. It is shown that a mesoporous silica coating with a thickness of ≈300–400 nm and a denser layer with a thickness of 13–34 nm are formed from deionized silica sol and hydrolyzed tetraethoxysilane, respectively.     

Modification of the pitch of chiral nematic liquid crystals by means of photoisomerization of chiral dopants

New photoisomerizable chiral dopants have been studied. The dopants used were menthone derivatives, a chiral stilbene derivative and a nematic copolymer of a menthone derivative and a benzoyloxybenzonitrile derivative. NMR, HPLC and UV results showed that the E-Z-isomerization of all the compounds indicated proceeded rapidly upon UV exposure, without the formation of undesired by-products. Isomerization of the menthone derivatives, including the copolymer, induced a substantial decrease in the helical twisting power. The changes in helical twisting power induced by the isomerization of the chiral stilbene derivative were limited to a factor of 2. In mixtures of the photoisomerizable dopants with commercial dopants of opposite twisting senses and a nematic host mixture, the sign of the twisting sense could be reversed by illuminating the mixture with UV light. The viewing angle dependence of irradiated regions of a 90 -twisted nematic cell was rotated 90 with respect to the viewing angle dependence of the non-irradiated regions. It is expected that this approach may be useful in the preparation of dual domain TN cells with a reduced viewing angle dependence.     

Role of the conformation in the reactivity of 1,3-diphosphapropenes

The symmetrical and unsymmetrical 1,3-diphospha-propenes 3 and 4 were obtained from the corresponding diposphiranes 1 and 2 . The chemical behavior of these compounds has been studied. Phosphonium-phosphaalkenes 7a and 10a have been obtained in the reactions with aluminium trichloride. Whereas the symmetrical diphosphaallene 13 can be obtained by reaction of 1 or 3 with lithio compounds, the unsymmetrical diphosphaalene 14 cannot be prepared by a similar route. Reduction of 3a and 4a (obtained with a different conformation) by lithium aluminum hydride afforded phosphino-phosphaalkenes 17a and 18a (with a similar conformation); further dehydrochlorination with amines led to the symmetrical and unsymmetrical diphosphaalenes 13 and 14 , respectively. The formation of allenes strongly depends on the conformation of the starting diphosphapropenes.     

Novel mass spectral fragmentation of heptafluorobutyryl derivatives of acyl analogues of platelet-activating factor

Direct derivatization of the acyl analogue of platelet-activating factor (acyl-PAF) with heptafluorobutyric anhydride results in replacement of the phosphocholine moiety with a heptafluorobutyryl (HFB) group. Electron capture (EC) mass spectrometric analysis of this compound that makes use of negative ion detection along with subsequent accurate mass measurement and tandem mass spectrometry studies revealed that in addition to expected fragmentation due to losses of elements of HF, ketene, and/or acetic acid, there is a rearrangement reaction between the HFB group and the substituent on carbon-2 of the glycerol backbone. For 2-acetyl isomers, this fragmentation yields a characteristic ion at m/z 237; for 1-acetyl isomers, the analogous ion is observed at [M-135]⁻, along with a corresponding carboxylate anion. The use of the HFB derivative is invaluable for analysis of PAF homologues and analogues because it provides detailed structural information in combination with the high sensitivity of a gas chromatography combined with EC-mass spectrometry assay.     

Influence of the stereochemistry of sugars on the selectivity of formation of carbohydrate-derived cyclopentadienyl and indenyl ligands

Interaction of lithium cyclopentadienide with a suitable partially protected alpha-D-allofuranose triflate, 4, epimer at C3 of glucose, gives as a major product, besides the expected glucose-cyclopentadiene, 5, a glucose-disubstituted cyclopentadiene, 6. This unprecedented behaviour, which does not occur with alpha-D-glucofuranose and other sugars, is tentatively explained by a complexation of LiCp and the oxygen atoms of the isopropylidene function of one molecule of 4 and one of 5, giving a termolecular structure as the result of a template effect. The results of other experiments, such as the use of MgCp2 in place of LiCp or the complexation of oxygen atoms by lithium triflate, which changed the selectivity of the reaction largely in favour of the monosubstitution product 5, support this hypothesis. When lithium indenide is reacted with 4, glucose-monosubstituted and glucose-disubstituted indenes, 8 and 9, respectively, are formed, and 9 is obtained with almost total diastereoselectivity. This result can also be rationalised by a stereoselective complexation of lithium, as shown by separate experiments and by molecular mechanics calculations. Methyltricarbonyl molybdenum(II) complexes have been synthesised and characterised from glucose-monosubstituted 5 and glucose-disubstituted 6 cyclopentadienes.     

Design and fabrication of a superhydrophobic glass surface with micro-network of nanopillars

The wetting property of a superhydrophobic glass surface with a micro-network of nanopillars fabricated from colloidal lithography and plasma etching is investigated in this paper. The micro-network distribution of nanospheres can be modulated by diluting the nanosphere concentration and controlling the spin rate. The micro-network of nanospheres spun on the glass surface serves as a mask for nanopillars during the plasma etching process. After the fabrication, the nano-structured surface is treated with fluoroalkylsilane self-assembled monolayers to obtain superhydrophobicity. Among several spin rates, the minimum colloidal network area density from a 100 nm polystyrene nanosphere solution diluted to 0.026% was found at a spin rate of 4000 rpm. The sample with the lowest network area density shows a good quality of superhydrophobicity, having the highest water contact angle and the lowest sliding angle among samples with other network area densities. In particular, samples with a micro-network of pillars also showed mechanical robustness against finger rubbing. To assess the superhydrophobic behavior in-depth, a size-dependent contact angle equation is proposed for use with a high contact angle (>135°) and with a Bo (Bond number) ? 1. Furmidge's sliding angle equation is also modified; it is derived considering a static contact angle to simplify the prediction of the sliding angle. The contact and sliding angle measurements from samples with a micro-network of nanopillars show good agreement with the proposed equations.     

Structure of water in microscopic fractures of a silver iodide crystal

The results of a computer simulation of flat fractures with widths of 0.63, 1.25, and 2.5 nm filled with vapor molecules in a silver iodide crystals at 260 K were presented. The two-dimensional gas of molecules adsorbed on the walls was found to be strongly clustered. Before the pore was filled, its walls had been covered with a monomolecular water film with a characteristic hexagonal structure. The perpendicular growth of the film was hindered by the hydrophobicity of its surface; the adsorbed molecules were bonded with the walls by interactions with the ions of the second crystal layer to form a specific orientational molecular order in the region of contact with the wall. On the wall with silver cations, the molecular energy was lower and the entropy higher than on the wall with iodide anions; on the wall with a lower energy, the adsorption started earlier and was more active. In an extremely narrow pore having room for only one molecular layer, the monomolecular film consists of spots held on opposite walls; in each spot, the orientational molecular order is the one characteristic of the wall with which the spot is in contact.     

复杂沉淀体系中沉淀剂合理用量的计算

在沉淀体系中,当有掩蔽剂存在时沉淀剂的合理用量及被沉淀离子在溶液中最小残留量的计算是个复杂问题。本文从理论上建立了解决这一问题的通用函数方程,并藉助于电子计算机进行实例处理,结果令人满意。     

Formation of gutingimycin: analytical investigation of trioxacarcin A-mediated alkylation of dsDNA

Formation and fragmentation of recognition complexes between trioxacarcin A and various DNA sequences were examined by temperature-dependent UV and CD spectroscopy, HPLC analysis, and ESI mass spectrometry with regard to reaction conditions, intermediates, products, mechanism, and sequence specificity. Cleavage of the trioxacarcin–DNA complexes provided the natural product gutingimycin by guanine abstraction. The resulting DNA with an abasic site was further cleaved into a DNA fragment with a furanyl unit at the 3′-end and an oligonucleotide with a phosphorylated 5′-end.     

Heats of Mixing of Mixed Solvent Solutions of Alkali Metal Salts of Substituted Benzenesulfonic Acids

The enthalpy change on mixing solutions of methyl-substituted benzenesulfonic acids and their salts, with salts having a common cation or anion, at constant ionic strength in mixtures of 1,4-dioxane with water, were measured at 25°C. The heat effects of mixing solutions having a common anion increase almost linearly with the reciprocal value of the dielectric constant of the solvent. The heat effects of mixing solutions having a common cation, which are all negative in water-rich solutions, become endothermic after a certain value of the dielectric constant is passed. The results are discussed in terms of the solute-solvent structural properties. This revised version was published online in July 2006 with corrections to the Cover Date.     

Molecular modeling simulations of the morphology of polyphenylene dendrimers

Three polyphenylene dendrimers were studied by molecular modeling techniques with the goal of better defining the notion of shape persistence often associated with these molecules. We show that despite the rigidity of the monomers, a large variety of dendrimer morphologies is accessible, in large part due to the asymmetrical branching pattern of the monomers (they have nodes in meta and para positions with respect to their anchor point). The folding ability of a branch depends on the number and sequence of the meta and para nodes it contains: while some of the branches are always straight, others can fold back, and the amplitude of that folding increases with generation. As a result, the range of accessible morphologies increases with the generation, from a dense-shell model in low generation to a model intermediate between a dense shell and a dense core in high generation. When the typical A2B monomer is replaced by an A4B monomer, the dense packing limit is reached as early as the second generation because of a higher density and the presence of nodes in the ortho position, which are oriented backward.     

分子动力学模拟研究纳米碳管中甲醇-水混合溶液的结构与输运性质

利用分子动力学模拟方法,对比考察了平衡条件、外压作用、梯度电场作用下,摩尔比为1：1 的甲醇-水混合溶液在纳米碳管（CNT）中的静态结构以及输运行为. 研究发现：在平衡体系与外压作用下,纳米碳管内甲醇与水呈现出明显的不混溶现象,甲醇主要分布于管壁附近,水分子主要分布于纳米碳管轴心附近;而在梯度电场作用下,纳米碳管由疏水性向亲水性转变,更多的水分子分布于管壁,导致纳米碳管内甲醇-水的不混溶现象消失. 另一方面,在外压作用下,纳米碳管内甲醇与水呈现单向移动;而在梯度电场下,甲醇与水呈现快速的双向移动,其流通量较相应外压作用体系高出近一个数量级,但由于双向的流通量大小相近,导致净流通量与外压作用下的净流通量差异不大.     

炔烃的钛氢化反应及产物的立体结构

通过色谱跟踪对炔烃的钛氢化反应进行了研究，反应前期可得到高选择性高产率的顺式烯烃，反应后期可得到高选择性高产率的反式烯烃，控制反应烯烃，控制反应时间，用酮类淬息方法分别得到了一系列新的专一立体构型的烯丙醇衍生物。