Pseudohelical and helical primary structures of 1,2-spiroannelated four- and five-membered rings: syntheses and chiroptical properties

The pseudohelical hydrocarbons (R)-6, (S)-7, and (R)-8 and the helical hydrocarbon (P)-9, formally derived from the helical hydrocarbon (P)-4 by stepwise replacement of each of the four-membered rings by a five-membered ring, have been prepared. Their optical rotations vary systematically, both in magnitude and sign. Of the extremes, (P)-4 represents the usual case of a right-handed dextrorotatory helix, while (P)-9 represents the unusual case of a right-handed levorotatory helix. To rationalize these facts, DFT calculations of the rotatory power of (P)-helices of three-, four-, and five-membered rings have been performed. The results show a very good agreement with the experimental data for the rigid helices of three-membered rings and always show the correct sign and order of magnitude for the flexible helices of four- and five-membered rings for which Boltzmann-averaged optical rotations of up to six conformers had to be used. Within the conformers of the latter, a set of large dihedral angles for the bonds of the inner sphere correspond to a high specific rotation, and a set of small dihedral angles correspond to a low specific rotation. As a consequence, the Boltzmann-averaged values markedly depend on the geometry and weight of the conformers involved.     

Stereoselective Epoxidation of Cyclohexa-Anellated Triquinacenes with Iodine/Silver(I) Oxide As Compared to m-Chloroperbenzoic Acid(1)

Iodine/silver(I) oxide (I(2)/Ag(2)O) reacts highly stereoselectively in the single, double, and triple anti epoxidation of the spherical 1,4,7-triene, 10-methyl-2,3:5,6:8,9-tris(cyclohexano)triquinacene 1. All of the three epoxides 3, 4, and 5 obtained with this reagent contain the epoxy groups at the convex side of the triquinacene framework. The stereochemical course of the epoxidation with I(2)/Ag(2)O is clearly distinct from that observed with m-chloroperbenzoic acid (MCPBA), which gives the same monoepoxide (3) but exclusively anti,syn di- and triepoxides (6-8) bearing at least one epoxy group at the concave side of the triquinacene framework. Epoxidation of the related three-fold 1,4-cyclohexadiene, tris(cyclohexeno)triquinacene 2, with MCPBA occurs similarly to the conversion of 1, whereas I(2)/Ag(2)O reacts with high regioselectivity at the less electron-rich peripheral double bonds of 2 giving triepoxides 12 and 13. The molecular structure of triepoxide 8 has been elucidated in detail by X-ray crystal structure analysis.     

Changes of Working Styles in a Computer Algebra Environment – The Case of Functions

This study is an empirical investigation of 11th graders at a German high school (Gymnasium). Working over a 24-hour period in a computer lab, we investigated students' use of quadratic functions with `Derive', and trigonometric functions with `Mathplus' (or `Theorist' for Macintosh). We were particularly interested in the working styles of students while they solved problems and looked for changes in these styles, as compared to traditional paper and pencil activities. While students worked on the computer, their activities (such as inputs from the keyboard, menu choices or mouse movements) were saved by a special program, which ran in the `background'. We are interested in the possibilities of developing a research method based on these `computer protocols'. The study should be seen as an exploratory study for developing hypotheses for further empirical investigations.This revised version was published online in September 2005 with corrections to the Cover Date.     

Bildung siliciumorganischer Verbindungen. 95. Die Kristallstruktur eines Hexadecamethyl-octasila-dispiro-[5.1.5.1]-tetradecans,Si8C22H65

Formation of Organosilicon Compounds. 95. Crystal Structure of a Hexadecamethyloctasila-dispiro [5.1.5.1]tetradecane, Si₈C₂₂H₅₆ 1,1,3,3,5,5,7,7,9,9,11,11,13,13,14,14-Hexadecamethyl-1,3,5,7,9,11,13,14-octasila-dispiro[5.1.5.1]tetradecane crystallizes monoclinically in the space group P2₁/n (No. 14) with a = 1352.4 pm, b = 1215.5 pm, c = 1001.2 pm, β = 92.11° and Z = 2 molecules per unit cell. The dispiro system is formed by a central disilacyclobutane and two C-spiro connected trisilacyclohexane rings. The symmetry of the molecule is 2/m, with flattened six membered rings in chair conformation. The Si? C bonds are enlarged (192 pm) at the strained spiro region whereas the Si? C bonds are distinctly shortened (186 pm) at the opposite Si atoms in the six membered rings.     

Ionic liquid applications in peptide chemistry: synthesis, purification and analytical characterization processes

This review aims to provide a comprehensive overview of the recent advances made in the field of ionic liquids in peptide chemistry and peptide analytics.     

Formation of Graphene Oxide Nanocomposites from Carbon Dioxide Using Ammonia Borane

To efficiently recycle CO(2) to economically viable products such as liquid fuels and carbon nanomaterials, the reactivity of CO(2) is required to be fully understood. We have investigated the reaction of CO(2) with ammonia borane (AB), both molecules being able to function as either an acid or a base, to obtain more insights into the amphoteric activity of CO(2). In the present work, we demonstrate that CO(2) can be converted to graphene oxide (GO) using AB at moderate conditions. The conversion consists of two consecutive steps: CO(2) fixation (CO(2) pressure < 3 MPa and temperature < 100 °C) and graphenization (600-750 °C under 0.1 MPa of N(2)). The first step generates a solid compound that contains methoxy (OCH(3)), formate (HCOO) and aliphatic groups while the second graphenization is the pyrolysis of the solid compound to produce graphene oxide-boron oxide nanocomposites, which have been confirmed by micro-Raman spectroscopy, solid state (13)C and (11)B magic angle spinning-nuclear magnetic resonance (MAS-NMR), transmission electron microscopy (TEM), and atomic force microscopy (AFM). Our observations also show that the mass of solid product in CO(2) fixation process and raw graphene oxide nanocomposites is twice and 1.2 times that of AB initially charged, respectively. The formation of aliphatic groups without using metal-containing compounds at mild conditions is of great interest to the synthesis of various organic products starting from CO(2.).