Search results for "Catalysis"
showing 10 items of 5944 documents
Controlling Multivalent Interactions in Triply-Threaded Two-Component Superbundles
2003
We have investigated the (1)H NMR spectra, the absorption spectra, the fluorescence spectra and decays, and the electrochemical properties of i). a tritopic receptor in which three benzo[24]crown-8 macrorings are fused onto a triphenylene core, ii). a trifurcated trication wherein three dibenzylammonium ions are linked 1,3,5 to a central benzenoid core, and iii). their 1:1 adduct which constitutes a triply-threaded, two-component supramolecular bundle. X-Ray crystallography has established the precise geometry of this paucivalent recognition motif in the solid state. In addition to [N(+)-H...O] hydrogen bonding and [C-H...O] interactions between the NH(2) (+) centers on the three dibenzylam…
Recognition-Directed Supramolecular Assemblies of Metal Complexes of Terpyridine Derived Ligands with Self-Complementary Hydrogen Bonding Sites
2000
The synthesis and X-ray structures of three metal complexes with terpyridine-derived ligands that contain amino-pyrimidine and amino-pyrazine moieties are presented. They have been designed in view of directing their self-assembly into specific supramolecular arrays through molecular recognition interactions. The solid-state structures indeed reveal extensive hydrogen-bonded networks. The Co complex 4a with PF6- counterions builds a two-dimensional infinite interwoven grid through strong double hydrogen bonds (d(N-H-N) =2.918-3.018 A) between the amino groups and the N atoms of the rings, with all H-bonding sites saturated. Changing the anions to BF4- in 4b leads to a similar infinite but p…
The cyclic [N(PiPr2E)2]+ (E = Se, Te) cations: a new class of inorganic ring system.
2006
The two-electron oxidation of [(tmeda)NaN(PiPr2E)2] with iodine produces the cyclic [N(PiPr2E)2]+ (E = Se, Te) cations, which exhibit long E–E bonds in the iodide salts. peerReviewed
On the Importance of Carbohydrate-Aromatic Interactions for the Molecular Recognition of Oligosaccharides by Proteins: NMR Studies of the Structure a…
2005
The specific interaction of a variety of modified hevein domains to chitooligosaccharides has been studied by NMR spectroscopy in order to assess the importance of aromatic-carbohydrate interactions for the molecular recognition of neutral sugars. These mutant AcAMP2-like peptides, which have 4-fluoro-phenylalanine, tryptophan, or 2-naphthylalanine at the key interacting positions, have been prepared by solid-phase synthesis. Their three-dimensional structures, when bound to the chitin-derived trisaccharide, have been deduced by NMR spectroscopy. By using DYANA and restrained molecular dynamics simulations with the AMBER 5.0 force field, the three-dimensional structures of the protein-sugar…
New Triterpene Saponins fromAcanthophyllum pachystegium
2004
Four new triterpenoid saponins, pachystegiosides A (1), B (2), C (3), and D (4), were isolated from the roots of Acanthophyllum pachystegium K. H. Their structures were elucidated by means of a combination of homo- and heteronuclear 2D-NMR techniques (COSY, TOCSY, NOESY, HSQC, and HMBC) and by FAB-MS. The new compounds were characterized as 3-O-{O-β-D-galactopyranosyl-(12)-O-[β-D-xylopyranosyl-(13)]-β-D-glucuronopyranosyl}quillaic acid 28-{O-β-D-xylopyranosyl-(13)-O-β-D-xylopyranosyl-(14)-O-α-L-rhamnopyranosyl-(12)-O-[3,4-di-O-acetyl-β-D-quinovopyranosyl-(14)]-β-D-fucopyranosyl}ester (1), 3-O-{O-β-D-galactopyranosyl-(12)-O-[β-D-xylopyranosyl-(13)]-β-D-glucuronopyranosyl}quillaic acid 28-{O-…
Acylated Triterpene Saponins from Atroxima libericaStapf
2011
The four new acylated triterpene saponins 1–4, isolated as two pairs of isomers and named libericosides A1/A2 and B1/B2, one pair of isomers 5/6, the (Z)-isomer libericoside C2 (5) being new, one new sucrose ester, atroximoside (7), and eight known compounds were isolated from the roots of Atroxima liberica by repeated MPLC and VLC on normal and reversed-phase silica gel. Their structures were elucidated on the basis of extensive 1D- and 2D-NMR studies (1H- and 13C-NMR, DEPT, COSY, TOCSY, NOESY, HSQC, and HMBC) and mass spectrometry as 3-O-β-D-glucopyranosylpresenegenin 28-{O-α-L-arabinopyranosyl-(13)-O-β-D-xylopyranosyl-(14)-O-α-L-rhamnopyranosyl-(12)-4-O-[(E)-3,4-dimethoxycinnamoyl]-β-D-f…
Vergleichende dimerisierung der enantiomere von Oct-1-in-3-ol mit chiralen rhodium-phosphan-komplexen
1996
Abstract Three optically active phosphanes ([(S)-2-methylbutyl]3−nP[phenyl]n(n = 0, 1, 2), were tested as ligands in rhodium(I) complexes to form catalysts for the enantioselective dimerisation of (R)- and (S)-oct-1-yn-3-ol. The catalytic reaction provides enantioselectivity for this type of CC bond formation. For reactions at 45–75°C and depending on the P-ligand used the rates vary for both enantiomers from 6:1 to 50:1 in favour of the (S)-octynol.
Asymmetric synthesis of fluorinated cyclic beta-amino acid derivatives through cross metathesis.
2006
The asymmetric synthesis of several fluorinated cis-2-aminocycloalkane carboxylic acids (cis-2-ACACs) with a cross metathesis (CM) reaction as the key step has been carried out, constituting the first time a metathesis protocol has been undertaken with fluorinated imidoyl chlorides. Subsequent chemoselective hydrogenation of the olefin moiety, Dieckmann condensation, and stereoselective reduction of the iminic double bond afforded the corresponding beta-amino esters with several ring sizes. The asymmetric version of the process was achieved by using (-)-8-phenylmenthol as a chiral auxiliary.
The First Examples of a Crown Ether Intramolecularly Encapsulating Mono- and Diorganotin Dications: Synthesis and Structures of [PhSnCH2([16]crown-5)…
2010
The reaction of silver perchlorate with [PhI 2 SnCH 2 ([16]crown-5)] (1) and [I 3 SnCH 2 ([16]crown-5)] (2) gave the organotin(IV)-substituted crown ether complexes [PhSnCH 2 ([16]crown-5)][ClO 4 ] 2 (3) and [HOSnCH 2 ([16]crown-5)][Y] 2 (4: Y= ClO 4 , 5: Y=CF3 S 03 ) , respectively. All compounds have been isolated as air-stable materials and characterised by 1 H, 13 C, 119 Sn and 119 Sn MAS (5) NMR spectroscopy, ESIMS spectrometry, elemental analysis and by single-crystal X-ray diffraction analysis. The molecular structures of 3-5 show that the tin(IV) cation fits perfectly into the crown ether cavity and is coordinated by the five oxygen atoms of the ring to give a pentagonal bipyramidal…
NonclassicalPschorr andSandmeyer Reactions in Pyrazole Series
2005
The diazonium salt derived from 4-amino-N,1,3-trimethyl-N-(3-methyl-1-phenyl-1H-pyrazol-5-yl)-1H-pyrazole-5-carboxamide (14) was reacted with a mixture of CuSO4 and NaCl, with ascorbic acid as an initiator to afford the planar derivative 4,6-dihydro-1,4,6,8-tetramethyl-3-phenyldipyrazolo[3,4-b:4′,3′-d]pyridin-5(3H)-one (16) and its unexpected isomer 4,6-dihydro-3,4,6,8-tetramethyl-1-phenyldipyrazolo[4,3-b:4′,3′-d]pyridin-5(1H)-one (17), as well as the epimers (3S,4S)- (or (3S,4R)-) and (3S,4R)- (or (3S,4S)-) 4-chloro-2,4-dihydro-1′,3′,5,5′-tetramethyl-2-phenylspiro[pyrazole-3,4′(1′H)-pyrrolo[3,4-c]pyrazol]-6′(5′H)-one (18a and b, respectively). Epimers 18a and b were converted under basic c…