Crystal structure of the phenylacetylene-choleic acid

Deoxycholic acid (C₂₄H₄₀O₄, M _r =392.58) and phenylacetylene (C₈H₆, M _r =102.14) form a 2:1 inclusion compound which crystallizes in space groupP2₁2₁2₁ witha=25.542(6),b=13.662(5),c=7.227(2) Å,Z=4. The finalR is 0.07₁ for 2168 observed reflections withI>1.5(I). The crystal packing is characterized by an assembly of antiparallel pleated bilayers of deoxycholic acid molecules which leave empty channels running alongc. Phenylacetylene molecules are accommodated within the cavities in an ordered fashion and are very probably arranged in only one orientation, which was determined by means of Van der Waals energy calculations. Atoms of deoxycholic acid belonging to ring A and to the side-chain are engaged in binding interactions with the guest molecule. The arrangement of the guest molecules supports the experimental results of irradiation, which show a failure of the phenylacetylene molecules to undergo inclusion polymerization. Supplementary Data relevant to this paper have been deposited with the British Library Lending Division under SUP No. 82024 (13 pages).     

Strain-dependence of the electronic properties in periodic quadruple helical G4-wires

The electronic structure of periodic quadruple helix guanine wires, which mimic G4-DNA molecules, was studied as a function of the stacking distance between consecutive planes, by means of first principles density functional theory calculations. We show that, whereas for the native DNA interplane separation of 3.4 A the HOMO- and LUMO-derived bands are poorly dispersive, the bandwidths can be significantly increased when compressive strain is applied along the helical axis. Our findings indicate that efficient band conduction for both holes and electrons can be supported by such wires for stacking distances below 2.6 A, which imply a huge axial deformation with respect to double and quadruple helices in solutions and in crystals.     

Comparison between cocoa and chocolate: characterisation and fatty acid content

The connection between chocolate and cocoa is studied. As cocoa is produced in different countries, samples coming from the same geographical region are analysed. The analyses focus on the characterisation of the considered samples for different aspects of quality and to individuate the original locations of production, also connecting this with the nutritional aspects. For this purpose, qualitative and quantitative traditional analysis and determination of fatty acid content are performed. The obtained experimental data show interesting correlations. The percentages of fatty acids computed allow for distinctions to be made among cocoa and chocolate samples. Furthermore, a relation can be assumed among the data obtained for primary matters and final products corresponding to countries being positioned on the same latitude, even if they are situated in different continents.     

A Carbohydrate Approach to the Enantioselective Synthesis of 1,3-Polyols

Treatment of per-O-benzoyl-D-glycero-D-gulo-heptono-1,4-lactone (2) with tertiary amines afforded selectively and with good yields the (5H)-furan-2-one derivatives 3, 4, and 5, formed by controlled elimination of one, two, or three molecules of benzoic acid, respectively. The stereochemistry for the exocyclic double bonds of 4 and 5 was determined by means of NMR techniques. Particularly, the furanone 4 was obtained from 2 ( approximately 90% yield) as a mixture of the E and Z diastereoisomers, which were separated by column chromatography or, more efficiently, by HPLC. The catalytic hydrogenation of compounds 4-E and 4-Z took place diastereoselectively, due to the chiral induction of the stereocenter located in the lateral chain. Thus, hydrogenation of 4-E led to a mixture of the 4,5-dihydro-(3H)-furan-2-ones having 3R,5S,2'S (D-xylo, 6) and 3S,5R,2'S (D-arabino, 7) configurations, with 6 as the major product; whereas the 4-Z isomer gave the same mixture, but being 7 preponderant. On hydrogenation of the original 4-E/Z mixture, compound 6 was obtained pure after recrystallization. O-Debenzoylation of 6gave 9, which was reduced with NaBH(4) to the 3,5-dideoxy-meso-xylo-heptitol (11). The peracetate (12) and perbenzoate (13) of the latter were prepared, and the 1-(tert-butyldiphenylsilyl)oxy derivative (16) was also synthesized via the 3'-(silyloxy)-4,5-dihydro-(3H)-furan-2-one 14. Chemoselective reduction of the lactone function of 6 with diisoamylborane gave the 2,5,6-tri-O-benzoyl-3,6-dideoxy-D-xylo-heptofuranose (17). The 3,5-dideoxy-D-arabino-heptitol (18), a diastereoisomer of 11, was also isolated and characterized.     

Quantum mechanical study of stereoselectivity in the oxazaborolidine-catalyzed reduction of acetophenone

Chiral oxazaborolidines, known as CBS catalysts after the work of Corey, Bakshi and Shibata, are used for the stereoselective reduction of prochiral ketones to secondary chiral alcohols. Due to their relative low cost, ease of use, and high selectivity, their popularity has remarkably grown in the last 15 years. Oxazaborolidine-catalyzed reductions have been much studied, both experimentally and computationally, by means of semiempirical methods. Though, a more accurate high level quantum mechanical study on the complete system, capable of elucidating reliably the origins of stereoselectivity, is still lacking. Therefore, the acetophenone (PhMK) reduction with Corey's oxazaborolidine has been modeled for the first time with ab initio and DFT-B3LYP calculations on the complete system as well as with AM1. Calculations on the complexation of BH(3) to CBS, which can occur only in a cis fashion with respect to the hydrogen on the stereogenic C-4 carbon atom, have allowed us to confirm the great rigidity of Corey's catalyst, possibly determining its excellent enantioselectivity. Acetophenone-CBS-BH(3) complexes were characterized at various levels of theory, and it was found that the picture obtained depends heavily on the method adopted. A computational strategy for identifying the hydride transfer transition states of the competing pathways was developed and tested, using a model system for which the transition state geometry was already known. The application of the TS search method to the reduction of acetophenone allowed the characterization of the TS's for the competing pathways in this reaction, making it possible to predict with good quantitative accuracy the stereochemical outcome of the reaction at all the levels of theory adopted. The characterization of the intermediate oxazadiboretane products confirmed that the highly exothermic hydride transfer provides the thermodynamical drive for the reaction.