Search results for "stereochemistry"
showing 10 items of 4831 documents
Untersuchungen zu einem modell der herstellung von oligopeptiden mittels einer synthetischen matrizenreaktion
1970
Aktivierte Ester von Carbobenzoxyaminosauren mit drei phenolischen Zweikernverbindungen wurden dargestellt. Die anhydridmethode unter Verwendung von Chlorameisensaureathylester stellte sich dabei als gunstiger heraus als die Carbodiimidmethode unter Verwendung von Dicyclohexylcarbodiimid. Dies mag durch sterische Grunde bedingt sein. Uber gemischte Anhydride gelang auch die Verlangerung von Aminosaureestern zu Dipeptidestern. Die Ester von phenolischen Mehrkernverbindungen mit Aminosauren oder Peptiden konnen zur Darstellung von Oligopeptiden benutzt werden. Diese Reaktionsfolge, die als Modell einer Matrizenreaktion betrachtet werden kann, wurde an ihrem einfachsten Beispiel untersucht. Ac…
(2aRS,3RS,4aSR,6aRS,6bSR)-3-Hydroxy-2a,3,4a,6,6a,6b-hexahydro-1,4-dioxacyclopenta[cd]pentalen-2(5H)-one
2007
The molecular structure of the title compound [enantiomers (VIII) and (VIIIa)], C8H10O4, was determined in the course of our studies on the synthesis of cyclopenta[1,2-b]furan-4-one derivatives. Tricyclic (VIIIa) consists of a planar bridged lactone unit and the two other ring systems in the envelope conformation. It contains five chiral C atoms and was obtained as a racemic mixture. The X-ray analysis showed the compound to possess a half-acetal unit with an endo orientation of the half-acetal ether bridge with respect to the lactone unit.
β-Branched α-Halo Carboxylic Acid Derivatives via Stereoselective 1,4 Addition of Dialkylaluminum Chlorides to α,β-Unsaturated N-Acyloxazolidinones
1997
The stereoselective synthesis of β-branched α-halo carboxylic acids containing two newly formed chiral centers is accomplished by a reaction cascade consisting of the 1,4 addition of dialkylaluminum chlorides to α,β-unsaturated N-acyloxazolidinones and subsequent reaction of the intermediate aluminum enolates with N-halosuccinimide. The most efficient stereocontrol in these tandem processes has been achieved with oxazolidinones derived from glucosamine. Not only aryl substituted but also purely aliphatic β-branched α-halo carboxylic acids can be stereoselectively synthesized by this method. However, the reactions of β-aryl α,β-unsaturated N-acyloxazolidinones show the highest diastereoselec…
Neo-clerodane diterpenoids from Teucrium kotschyanum
1989
Abstract From the aerial parts of Teucrium kotschyanum three new neo-clerodane diterpenoids have been isolated. Their structures, (12R)-15,16-epoxy-19-nor-10α-neo-cleroda-4,13(16),14-triene-18,6β; 20,12-diolide (12-epiteucvidin), (12R)-15,16-epoxy-19-nor-neo-cleroda-4,13(16),14-triene-18,6β; 20,12-diolide (12-epiteufiin) and (12S,18R)-15,16-epoxy-6β-hydroxy-neo-cleroda-13(16),14-dien-20,12-olide-l8,19-hemiacetal (teukotschyn), have been established by chemical and spectroscopic means. In addition, ursolic acid, the flavones cirsimaritin and cirsiliol, and six previously known neo-clerodane diterpenoids (teucvidin, teuflin, teuscorodin, teucrin H2, teuscorodonin and montanin D) were also fou…
Conformational studies of hexapeptides containing two dehydroamino acid residues in positions 3 and 5 in peptide chain
2008
Abstract Synthesis and structural studies of hexapeptides containing two dehydroamino acid residues in positions 3 and 5 in a peptide chain were performed. All the investigated peptides adopted bent conformations, stabilized by intramolecular hydrogen bonding, and could exist as two different conformers in solution. Only in the case of the peptide containing ΔAla residues, expected 3 10 -helical conformation was found.
Conformation of dehydropentapeptides containing four achiral amino acid residues - controlling the role of L-valine.
2014
Structural studies of pentapeptides containing an achiral block, built from two dehydroamino acid residues (ΔZPhe and ΔAla) and two glycines, as well as one chiral L-Val residue were performed using NMR spectroscopy. The key role of the L-Val residue in the generation of the secondary structure of peptides is discussed. The obtained results suggest that the strongest influence on the conformation of peptides arises from a valine residue inserted at the C-terminal position. The most ordered conformation was found for peptide Boc-Gly-ΔAla-Gly-ΔZPhe-Val-OMe (3), which adopts a right-handed helical conformation.
Crystal structure of diethyl (E)-2-[(benzofuran-2-yl)methylidene]succinate
2015
The title compound, C17H18O5, was synthesized by a base-free catalytic Wittig reaction. The molecule consists of a diethyl itaconate unit, which is connectedviathe C=C double bond to a benzofuran moiety. The benzofuran ring system (r.m.s. deviation = 0.007 Å) forms dihedral angles of 79.58 (4) and 12.12 (10)° with the mean planes through thecisandtransethoxycarbonyl groups, respectively. An intramolecular C—H...O hydrogen bond involving the O atom of the benzofuran moiety is observed. In the crystal, molecules are linked into ribbons running parallel to thebaxis by C—H...O hydrogen bonds.
Crystal structure of (E)-4-methoxy-2-{[(5-methylpyridin-2-yl)imino]methyl}phenol
2015
The molecule of the title Schiff base compound, C14H14N2O2, displays anEconformation with respect the imine C=N double bond. The molecule is approximately planar, with the dihedral angle formed by the planes of the pyridine and benzene rings being 5.72 (6)°. There is an intramolecular hydrogen bond involving the phenolic H and imine N atoms.
Crystal structure of 1-hydroxy-2,2,6,6-tetramethylpiperidin-1-ium trifluoromethanesulfonate
2015
In the cation of the title salt, C9H20NO+·CF3O3S−, the six-membered heterocyclic ring displays a chair conformation. In the crystal, centrosymmetric pairs of cations and anions are linked by N—H...O and O—H...O hydrogen bonds to form rings with aR44(14) graph-set motif.
rac-1,1,1,6,6,6-Hexachlorohex-3-yne-2,5-diol hemihydrate
2017
The asymmetric unit of the title compound, C6H4Cl6O2·0.5H2O, contains one molecule of 1,1,1,6,6,6-hexachlorohex-3-yne-2,5-diol and half a water molecule located on a twofold rotation axis. In the crystal, pairs of hexachlorohexynediol molecules form centrosymmetric dimers connectedviapairwise O—H...O hydrogen bonds. These dimers are connected by water molecules, resulting in layers parallel to theabplane.