Roseophilin的合成研究进展

Roseophilin是一种从灰绿链霉菌中分离出来的、由一个十三元大环并一个杂环组成的具有抗肿瘤活性的新型抗生素的活性物质。Roseophilin独特的结构及活性引起了很多学者对其合成的兴趣。通过十三元大环和杂环合成Roseophilin,其中十三元大环的主要合成方法有末端双键的RCM关环、扩环、烷基化关环三种反应类型。     

Effect of the sequence of the introduction of foreign substances on the action of papain

Summary In general, aniline, aniline hydrochloride, aniline sulfate, and triethylamine act as inhibitors in the hydrolysis of casein or of a suspension of flour from grain of Cicer arietinum by means of papain. The order in which the enzyme and the substrate is added to the inhibitor often has a significant effect. As a rule, there is a greater retardation when the enzyme is added last to a complex of the substrate and the inhibitor.     

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.     

Study of the mechanism of formation of transparent polyacrylonitrile

The kinetics of bulk polymerization of acrylonitrile, leading to a transparent polymer, was studied. The reaction was followed by a dilatometer specially designed for high degrees of conversion and for continuous supply of monomer during the reaction. The last stages of polymerization were followed by density determinations. It was found that the kinetic scheme did not show any irregularities, and that the major condition for obtaining a transparent polymer is a continuous supply of monomer to fill up the gaps formed by the contraction during the polymerization process.     

Method of computer-aided determination of the type of normal vibrations of polyatomic molecules

Correct assignment of calculated and experimental frequencies, as well as corservation of the initial assignment in case of a random coincidence of two frequencies during the solution of an inverse spectral problem, are important problems in spectroscopy of polyatomic molecules. Vibrations are classified according to their form, which is thought to be a more reliable basis for assignment than frequency. We offer a PC program to determine the type of vibrations according to their form (obtained by solving a direct vibrational problem) in a given system of vibrational basis set functions. Optimization of systems of basis set functions for molecules with six-membered rings is discussed. L. Ya. Karpov Physicochemical Scientific Research Institute. Translated fromZhurnal Strukturnoi Khimii, Vol. 36, No. 2, pp. 298–303, March–April, 1995. Translated by L. Smolina     

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.     

Reprint of: Synthesis of a tricyclic core of rameswaralide

A tricyclic core containing a 5,7-fused bicyclic unit of rameswaralide was prepared starting from a 1,6-enyne. The synthetic sequence involved (i) ruthenium-catalyzed [5+2]-cycloaddition of 1,6-enyne, (ii) an acyl radical based approach to construct the lactone, and (iii) a regioselective installation of the conjugated double bond by a concomitant sulfenylation-dehydrosulfenylation sequence.     

Total synthesis of pinnatoxins A and G and revision of the mode of action of pinnatoxin A

Pinnatoxins belong to an emerging class of potent marine toxins of the cyclic imine group. Detailed studies of their biological effects have been impeded by unavailability of the complex natural product from natural sources. This work describes the development of a robust, scalable synthetic sequence relying on a convergent strategy that delivered a sufficient amount of the toxin for detailed biological studies and its commercialization for use by other research groups and regulatory agencies. A central transformation in the synthesis is the highly diastereoselective Ireland-Claisen rearrangement of a complex α,α-disubstituted allylic ester based on a unique mode for stereoselective enolization through a chirality match between the substrate and the lithium amide base. With synthetic pinnatoxin A, a detailed study has been performed that provides conclusive evidence for its mode of action as a potent inhibitor of nicotinic acetylcholine receptors selective for the human neuronal α7 subtype. The comprehensive electrophysiological, biochemical, and computational studies support the view that the spiroimine subunit of pinnatoxins is critical for blocking nicotinic acetylcholine receptor subtypes, as evidenced by analyzing the effect of a synthetic analogue of pinnatoxin A containing an open form of the imine ring. Our studies have paved the way for the production of certified standards to be used for mass-spectrometric determination of these toxins in marine matrices and for the development of tests to detect these toxins in contaminated shellfish.     

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.     

Regularities of binary condensation of vapors when one of them is undersaturated

Simple analytical expressions are derived for the stationary concentration of a binary solution in a markedly supercritical droplet growing exothermically in diffusion or free-molecular regimes in mixed vapors when one of condensing vapors is supersaturated and present in a small amount and another vapor is slightly undersaturated and present in a large amount. The condensation of sulfuric acid and water vapors on a droplet under the conditions of Earth atmosphere is considered as an example of practical importance. Under isothermic conditions, analytical expressions are obtained for the time of establishing a power law for variations in a droplet radius with time under the diffusion and free-molecular regimes of the droplet growth. The power laws are derived in an explicit form under these regimes, which describe the rapid establishment of a stationary concentration of a solution in a growing droplet.     

Total synthesis of a stereoisomer of bistramide C and assignment of configuration of the natural product

After the isolation of the bioactive polyether bistramide C from the marine ascidian Lissoclinum bistratum in 1988, NMR spectroscopic investigations over the next 12 years reduced the total number of possible stereoisomers of this compound from 1024 to 32. Based on the preparation of segments of the natural product as well as the total synthesis of a randomly selected stereoisomer of bistramide C, the stereochemical puzzle could be further simplified to eight possible stereoisomers. A convergent three-segment coupling strategy, the use of a common, D-glucose-derived intermediate for the preparation of pyran rings in two segments, a stereoselective photo-spiroketalization, and the use of azides to minimize protective group manipulations before segment couplings are highlights of the synthetic approach. The total synthesis also provided the key segments for a chiroptical analysis according to van't Hoff's principle of optical superposition, which was crucial for the assignment of a sole relative and absolute configuration of the natural product. Bistramide C represents therefore the first member of this class of structurally unusual marine polyethers whose configuration is known as a result of the combined use of synthetic and chiroptical tools.     

Stereoselectivity of electroreduction of a menthone-isomenthone mixture

Electroreduction of an equilibrium menthone-isomenthone mixture at a controlled potential in a diaphragm cell on various cathodes was studied. The influence exerted by the nature of a solvent and of an electrode, electrode material, and various additives on the reduction stereoselectivity of ketones was examined. Conditions for a predominant formation of the most stable of the four isomeric alcohols being formed, menthol, were found.     

Fast evaporation of spreading droplets of colloidal suspensions

When a coffee droplet dries on a countertop, a dark ring of coffee solute is left behind, a phenomenon often referred to as the coffee-ring effect. A closely related yet less-well-explored phenomenon is the formation of a layer of particles, or skin, at the surface of the droplet during drying. In this work, we explore the behavior of a mathematical model that can qualitatively describe both phenomena. We consider a thin axisymmetric droplet of a colloidal suspension on a horizontal substrate undergoing spreading and evaporation. In contrast to prior work, precursor films (rather than pinned contact lines) are present at the droplet edge, and evaporation is assumed to be limited by how quickly molecules can transfer out of the liquid phase (rather than by how quickly they can diffuse through the gas phase). The lubrication approximation is applied to simplify the mass and momentum conservation equations, and the colloidal particles are allowed to influence the droplet rheology through their effect on the viscosity. By describing the transport of the colloidal particles with the full convection-diffusion equation, we are able to capture depthwise gradients in particle concentration and thus describe skin formation, a feature neglected in prior models of droplet evaporation. The highly coupled model equations are solved for a range of problem parameters using a finite-difference scheme based on a moving overset grid. The presence of evaporation and a large particle Peclet number leads to the accumulation of particles at the liquid-air interface. Whereas capillarity creates a flow that drives particles to the droplet edge to produce a coffee ring, Marangoni flows can compete with this and promote skin formation. Increases in viscosity due to particle concentration slow down droplet dynamics and can lead to a reduction in the spreading rate.     

Non-Gaussian statistics of electrostatic fluctuations of hydration shells

This paper aims to understand the statistics of the electric field produced by water interfacing a non-polar solute of nanometer dimension. We study, by numerical simulations, the interface between SPC/E water and a Kihara solute, which is a hard-sphere core with a Lennard-Jones layer at its surface. The distribution of the interfacial electric field is monitored as a function of the magnitude of a point dipole placed close to the solute-water interface. The free energy surface as a function of the electric field projected on the dipole direction shows a cross-over with increasing dipole magnitude. While it is a single-well harmonic function at low dipole values, it becomes a double-well surface at intermediate dipole moment magnitudes, transforming into a single-well surface again, with a non-zero minimum position, at still higher dipoles. This transformation, reminiscent of a discontinuous phase transition in bulk materials, has a broad intermediate region where the interfacial waters fluctuate between the two minima. This region is characterized by intense field fluctuations, with non-Gaussian statistics and variance far exceeding expectations from the linear-response approximation. The excited state of the surface water is found to be lifted above the ground state by the energy required to break approximately two hydrogen bonds. This state is pulled down in energy by the external electric field of the solute dipole, making it readily accessible to thermal excitations. The excited state is a surface defect in the hydrogen-bond network, creating a stress in the nearby network, but otherwise relatively localized in the region closest to the solute dipole.     

Chemical investigation of biomass of a culture of ginseng cells III. Polysaccharides of a callus culture of ginseng

A water-soluble polysaccharide fraction has been isolated from a callus culture of ginseng with a yield of 16–20%. It has been shown that it included starch a and acidic polysaccharides — arabinogalactans and a xyloglucan.Pacific Ocean Institute of Bioorganic Chemistry, Far Eastern Branch, USSR Academy of Sciences, Vladivostok, All-Union Scientific Research Institute of Biochemical Machinery Design, USSR Ministry of the Medical Industry, Moscow. Translated from Khimiya Prirodnykh Soedinenii, No. 6, pp. 771–772, November–December, 1989.     

Electric birefringence of dispersions of platelets

This paper describes the electro-optic response of a suspension of disk-like colloids. We have considered aqueous suspensions of Gibbsite platelets and measured the electrically induced birefringence in the broad frequency range 10(2)-10(8) Hz. When simply dispersed in an electrolyte solution, these particles orient with their major axis parallel to the electric field at all frequencies. The spectral dependence of their Kerr coefficient features three regimes: an electrokinetic α-relaxation within the kHz range, a conductive Maxwell-Wagner-O'Konski (MWO) relaxation having characteristic frequency in the 1-10 MHz range, and a nonzero high frequency asymptote. We quantitatively analyze the MWO spectral component and the high-frequency asymptote on the basis of a model developed for oblate particles. This analysis enables us to obtain the relevant particle properties: surface conductivity, zeta potential, and intrinsic Gibbsite birefringence. When the particles are dispersed in a mixture that also contains smaller spherical particles that have a charge of the same sign, their electric birefringence becomes negative at low frequency. This anomalous orientation of the platelets is analogous to that observed in mixtures of prolate and spherical particles, and demonstrates the anomalous birefringence as a universal property of suspensions of nonspherical particles when surrounded by smaller charged particles.     

Mathematical Description of the Kinetics of the Heterophase Macromolecular Reaction of Hydrazidation of Grafted Polyglycidyl Methacrylate

Hydrazidation of a grafted copolymer of polycaproamide and polyglycidyl methacrylate was mathematically modeled by solving the diffusion equation with a boundary condition in the form of a flow.__________Translated from Zhurnal Prikladnoi Khimii, Vol. 78, No. 6, 2005, pp. 1010–1015.Original Russian Text Copyright © 2005 by Druzhinina, Efremov, Struganova.     

Synthesis of thermochromic formazans of a number of benzazoles

In a search for compounds that have thermochromic properties, a group of formazans that contain benzimidazolyl (benzothiazolyl), methyl (phenyl), and hydroxydinitrophenyl (anthraquinoyl) groups in the 1-, 3-, and 5-positions, respectively, were synthesized. Benzimidazolylformazans with a picramic acid residue in the 5-position have thermochromic properties.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 7, pp. 970–973, July, 1971.