Search results for "nuclear magnetic resonance"
showing 10 items of 1743 documents
High-Yield Synthesis of 20-, 24-, and 28-Membered Macropentolide, -hexolide, and -heptolide, Respectively, from (R)- or (S)-3-hydroxybutanoic acid un…
1988
The macrocyclic pentolide 1, hexolide 2, and heptolide 3 constitute ca. 80% of the oligomers formed in ca. 50% yield from enantiomerically pure 3-hydroxybutanoic acid under Yamaguchi's macrolactonization conditions. The FAB mass spectra of the MH+, M Na+, and MCs+ are reported (Figs. 2, 3, 5, and 6). No cyclic tetramer is detected. The 1H-NMR spectra of the cyclic oligomers, of the monomer, and of the polymer (PHB) are very similar (Fig. 4). Directed synthesis of the open-chain dimer and tetramer of 3-hydroxybutanoic acid and attempted cyclization do not lead to the isolation of the cyclic tetramer.
Intramolecularly Coordinated Bis(crown ether)-Substituted Organotin Halides as Ditopic Salt Receptors
2013
The synthesis of the bis(crown ether)-substituted organostannanes X2Sn(CH2-[16]-crown-5)2 (3, X = I; 4, X = Br; 5, X = Cl; 6, X = F) and X2Sn(CH2-[13]-crown-4)2 (10, X = I; 11, X = Br; 12, X = F) is reported. The compounds have been characterized by 1H, 13C, 19F, and 119Sn NMR spectroscopy, elemental analyses, and electrospray ionization mass spectrometry (ESI-MS). Single-crystal X-ray diffraction analyses reveal a distorted-octahedral cis,cis,trans configuration of the tin atoms in compounds 4–6 and 10–12 as a result of intramolecular O→Sn coordination. The ability of the host molecules to form mono- and ditopic complexes with various halide salts in different solvents, including water, ha…
Mannich Reactions with Amino Alcohols
2000
The condensation of resorcarenes 1 with various amino alcohols and an excess of formaldehyde was studied. The tetrabenzoxazines 2a−e were found as the only products in the reaction with 6-aminohexan-1-ol, 4-aminobutan-1-ol, and 2-aminoethanol, while 3-aminopropan-1-ol forms the tetraoxazine 3 as the main product. In the case of aminoethanols substituted at the 2-position with alkyl groups, the tetraoxazolidines 4 are the preferred reaction products, while 1-methyl aminoethanol (1-amino-propan-2-ol) yields predominantly the tetrabenzoxazine 2f. The structures of all these compounds have been confirmed by NMR spectroscopy and additionally by single-crystal X-ray analysis in the case of 2a and…
Tri- and tetraurea piperazine cyclophanes: synthesis and complexation studies of preorganized and folded receptor molecules.
2010
A series of symmetrical tri- and tetrameric N-ethyl- and N-phenylurea-functionalized cyclophanes have been prepared in nearly quantitative yields (86-99 %) from the corresponding tri- and tetraamino-functionalized piperazine cyclophanes and ethyl or phenyl isocyanates. Their conformational and complexation properties have been studied by single-crystal X-ray diffraction, variable-temperature NMR spectroscopy, and ESI-MS analysis. The rigid 27-membered trimeric cyclophane skeleton assisted by a seam of intramolecular hydrogen bonds results in a preorganized ditopic recognition site with an all-syn conformation of the urea moieties that, complemented by a lipophilic cavity of the cyclophane, …
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…
Diastereoselective formation of highly functionalised α-substituted amino acid derivatives via aldol addition
2005
Abstract Highly diastereoselective aldol additions of (2 R ,4 S )-3- tert -butyl 4-methyl 2- tert -butyloxazolidine-3,4-dicarboxylate ( 1 ) are reported. The utility of the highly substituted oxazolidines of type 1 for diastereoselective α-addition of the fully protected amino acid l -serine with achiral and chiral carbonyl compounds is demonstrated and the relative and absolute configuration of the aldol products are discussed on the basis of NOESY data and solid state structures of selected examples. The aldol products represent highly useful intermediates in the syntheses of sphingosine-related metabolites.
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…
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…
Cyclometallation reactions in complexes of the type Rh(oq)(CO)[P(o-BrC6F4)Ph2]. The molecular structure of F4)Ph2] (oq = 8-hydroxyquinolinate)
1984
Cyclometallation occurs when a solution of the complex Rh(oq)(CO)(PCBr), (PCBr = 2-bromo-3,4,5,6-tetrafluorophenyldiphenylphosphine; oq = 8-hydroxyquinolinate) in toluene is refluxed, giving Rh(oq)2(PC) (PC = P(C6F4)(C6H5)2) and a dimeric compound, not yet completely characterized, formulated as Rh2Br(oq)(PCBr)2. Rh(oq)2(PC) was characterized by elemental analysis, by conductance measurements, and by 19F, 31P NMR and infrared spectroscopy. Its molecular structure was determined by single-crystal X-ray methods and refined by standard procedures to final agreement factors R and Rw of 0.067 and 0.060 for 5346 observed data. Lattice constants are 15.8494(6), 14.7188(5), 14.6675(5) A and β 96.93…