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Ring Atom (ring + atom)

Distribution by Scientific Domains

Chemistry	80%

Selected Abstracts

Competition between Non-Classical Single and Double Epimerizations in Cyclitol Chemistry

EUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 9 2004
Ralf Miethchen
Abstract Two competitive regio- and stereoselective epimerization reactions were investigated in four cyclitols characterized by four contiguous OH groups with a cis - trans - trans sequence and by varied substituents (OMe, OBz, F, H) adjacent to this tetrol unit. The starting materials were synthesized from L -quebrachitol (compounds 5,7) and myo -inositol (compound 8). Their acetalization with the chloral/DCC reagent system gave cyclic acetals with one epimerized chiral ring atom and also with two epimerized chiral centres. The single epimerization takes place exclusively at the middle C-atom of the cis - trans triol unit in the tetrol sequence (products 15, 17, 19/20 and 24,27), whereas the double epimerization occurs at both of the "centrally located" C-atoms in the cis - trans - trans tetrol unit (products 16, 18, 21 and 28). The product ratios of singly to doubly inverted compounds change as follows: the lower the electron-withdrawing effect of the substituents adjacent to the tetrol unit, the higher the percentage of the corresponding doubly inverted product. However, the singly inverted products remain the major products in all cases. X-ray analyses are given for the starting material 1-fluoro-2- O -(methyl)cyclohexane-2,3,4,5,6-pentol (5) and for the products 1- O -cyclohexylcarbamoyl-2,3- O -(2,2,2-trichloroethylidene)5- O -(methyl)cyclohexane-1,2,3,4,5-pentol (17), 3- O -acetyl-1- O -benzoyl-6- O -cyclohexylcarbamoyl-2- O -methyl-4,5- O -(2,2,2-trichloroethylidene)- muco -inositol (22) and 2,3-di- O -ethylidene)-(+/-)- chiro -inositol (24). (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004) [source]

Synthesis of Chiral Imidazole Derivatives as Purine Precursors

EUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 6 2003
Jerzy Suwi
Abstract From commercially available chiral building blocks, we have developed methods for the syntheses of imidazole derivatives that contain a chiral alkyl substituent at ring atom. These compounds are suitable for further transformation into Nalkyl purine derivatives. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003) [source]

Methyl ,- d -galactopyranosyl-(1,4)-,- d -allopyranoside tetrahydrate

ACTA CRYSTALLOGRAPHICA SECTION C, Issue 9 2010
Wenhui Zhang
The title compound, C13H24O11·4H2O, (I), crystallized from water, has an internal glycosidic linkage conformation having ,, (O5Gal,C1Gal,O1Gal,C4All) = ,96.40,(12)° and ,, (C1Gal,O1Gal,C4All,C5All) = ,160.93,(10)°, where ring-atom numbering conforms to the convention in which C1 denotes the anomeric C atom, C5 the ring atom bearing the exocyclic hydroxymethyl group, and C6 the exocyclic hydroxymethyl (CH2OH) C atom in the ,Galp and ,Allp residues. Internal linkage conformations in the crystal structures of the structurally related disaccharides methyl ,-lactoside [methyl ,- d -galactopyranosyl-(1,4)-,- d -glucopyranoside] methanol solvate [Stenutz, Shang & Serianni (1999). Acta Cryst. C55, 1719,1721], (II), and methyl ,-cellobioside [methyl ,- d -glucopyranosyl-(1,4)-,- d -glucopyranoside] methanol solvate [Ham & Williams (1970). Acta Cryst. B26, 1373,1383], (III), are characterized by ,, = ,88.4,(2)° and ,, = ,161.3,(2)°, and ,, = ,91.1° and ,, = ,160.7°, respectively. Inter-residue hydrogen bonding is observed between O3Glc and O5Gal/Glc in the crystal structures of (II) and (III), suggesting a role in determining their preferred linkage conformations. An analogous inter-residue hydrogen bond does not exist in (I) due to the axial orientation of O3All, yet its internal linkage conformation is very similar to those of (II) and (III). [source]

Recent work on entropically-driven ring-opening polymerizations: some potential applications,

POLYMERS FOR ADVANCED TECHNOLOGIES, Issue 2-3 2005
Philip Hodge
Abstract The entropically-driven ring-opening polymerization of macrocyclic monomers (>ca. 14 ring atoms per repeat unit) and/or macrocyclic oligomers is a relatively new method of polymer synthesis that exploits the well-known phenomenon of ring-chain equilibria. It attracts interest because of its novel features. For example, these ring-opening polymerizations emit no volatiles and little or no heat. This review considers the principles of entropically-driven ring-opening polymerizations, gives selected examples and discusses potential applications. The latter include micromolding, high throughput syntheses and the synthesis of supramolecular polymers. Copyright © 2005 John Wiley & Sons, Ltd. [source]

Metal Ion Coordination to Azole Nucleosides

CHEMISTRY - A EUROPEAN JOURNAL, Issue 21 2005
Jens Müller Dr.
Abstract To evaluate the possibility of introducing azole nucleosides as building blocks for metal-mediated base pairs in artificial oligonucleotides, imidazole nucleoside, 1,2,4-triazole nucleoside and tetrazole nucleoside have been synthesized and characterized. The X-ray crystal structures of p -toluoyl-protected 1,2,4-triazole and tetrazole nucleosides are reported. Contrary to the situation primarily found for deoxyribonucleosides, the sugar moieties adopt C3,- endo conformations. The acidity of the , nucleosides increases with increasing number of nitrogen ring atoms, giving pKa values of 6.01±0.05, 1.32±0.05 and <,3, respectively. This decrease in basicity results in a decreasing ability to form 2:1 complexes with linearly coordinating metal ions such as Ag+ and Hg2+. In all cases, the Ag+ complexes are of higher stability than the corresponding Hg2+ complexes. Whereas imidazole nucleoside forms highly stable 2:1 complexes with both metal ions (estimated log ,2 values of >10), only Ag+ is able to reach this coordination pattern in the case of triazole nucleoside (log ,2 = 4.3±0.1). Tetrazole nucleoside does not form 2:1 complexes at all under the experimental conditions used. These data suggest that imidazole nucleoside, and to a lesser extent 1,2,4-triazole nucleoside, are likely candidates for successful incorporation as ligands in oligonucleotides based on metal-mediated base pairs. DFT calculations further corroborate this idea, providing model complexes for such base pairs with glycosidic bond distances (10.8,11.0 Å) resembling those in idealized B-DNA (10.85 Å). [source]