Home About us Contact
|
Home About us Contact | ||
|
|
||
Shift Reagent (shift + reagent)
Selected AbstractsPreparation, structure, and optical properties of chiral sulfoxides and disulfoxide with a trithiole ringHETEROATOM CHEMISTRY, Issue 1 2003Takeshi Kimura Optically active 4,9-diethyl[1,4]-dithiino[5,6- f]benzo[1,2,3]trithiole 5-oxide (3) and 4,9-diethyl[1,4]dithiino[5,6- f]benzo[1,2,3]trithiole 5,8-dioxide (4) were obtained by the asymmetric oxidation of 6,11-diethyl[1,4]dithiino[5,6- h]benzo[1,2,3,4,5]pentathiepin (1). The reaction was accompanied by desulfurization and ring-contraction reactions of the pentathiepin ring. Similarly, optically active 4,8-diethyl[1,3]dithiolo[4,5- f]benzo[1,2,3]trithiole 5-oxide (7) was produced by the analogous asymmetric oxidation of 6,10-diethyl[1,3]dithiolo[4,5- h]benzo[1,2,3,4,5]pentathiepin (2). The specific rotations of 3, 4, and 7 were measured in chloroform, and their optical purity was verified by 1H NMR with a shift reagent [Eu(hfc)3]. The structures of 4 and 7 were determined by X-ray crystallography using Cu K, radiation, and the absolute configuration of the sulfinyl group was examined based on the Flack parameter, which revealed that 4 has an RR configuration, while 7 has an S configuration. The circular dichroism spectra of 3, 4, and 7 were measured in chloroform. © 2003 Wiley Periodicals, Inc. Heteroatom Chem 14:88,94, 2003; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/hc.10104 [source] Analysis of pD-Dependent complexation of N -benzyloxycarbonylaminophosphonic acids by ,-cyclodextrin.CHIRALITY, Issue 10 2007Enantiodifferentiation of phosphonic acid pKa values Abstract The influence of aqueous solution pD on stereoselective complexation of N -benzyloxycarbonylaminophosphonic acids with ,-cyclodextrin was investigated by means of nuclear magnetic resonance spectroscopy. The highest enantiodiscrimination was achieved at pD close to the pKa of less acidic hydroxyl group of the phosphonic moiety of analytes (6.5,7.5). This effect results from the stereoselective differentiation of pKa (up to 0.28 pD unit) upon complexation with applied chemical shift reagent. Moreover, analysis of 2D-ROESY spectra proved that the host,guest inclusion mode is strongly influenced by pD. Chirality, 2007. © 2007 Wiley-Liss, Inc. [source] Helical chirality in hexamethylene triperoxide diamineMAGNETIC RESONANCE IN CHEMISTRY, Issue 9 2006Chunlei Guo Abstract The primary explosive hexamethylenetriperoxide diamine has previously been found to exist in the solid state as a racemic mixture of helically chiral, threefold symmetric enantiomers; another enantiomeric pair of low-energy conformers has been predicted, but has never been observed. We show by solution 2D NMR at 14 T, in achiral solution and by addition of chiral shift reagents, that all four optically isomeric conformers coexist at slow equilibrium on the NMR timescale at room temperature, and can be observed. Calculations of the 1H and 13C NMR chemical shifts using gauge-including atomic orbital methods are in excellent agreement with experiment; thermochemical calculation of the free energies in solution are in somewhat worse agreement, but correctly predict the relative stability of the conformers. Analysis of the effects of chiral shift reagents on the NMR spectra suggests that discrimination between chiral isomers is primarily around the molecular equator, around which the enantiomeric gauche OO linkages are arrayed. Copyright © 2006 John Wiley & Sons, Ltd. [source] Paramagnetic Liposomes as Innovative Contrast Agents for Magnetic Resonance (MR) Molecular Imaging ApplicationsCHEMISTRY & BIODIVERSITY, Issue 10 2008Enzo Terreno Abstract This article illustrates some innovative applications of liposomes loaded with paramagnetic lanthanide-based complexes in MR molecular imaging field. When a relatively high amount of a GdIII chelate is encapsulated in the vesicle, the nanosystem can simultaneously affect both the longitudinal (R1) and the transverse (R2) relaxation rate of the bulk H2O H-atoms, and this finding can be exploited to design improved thermosensitive liposomes whose MRI response is not longer dependent on the concentration of the probe. The observation that the liposome compartmentalization of a paramagnetic LnIII complex induce a significant R2 enhancement, primarily caused by magnetic susceptibility effects, prompted us to test the potential of such agents in cell-targeting MR experiments. The results obtained indicated that these nanoprobes may have a great potential for the MR visualization of cellular targets (like the glutamine membrane transporters) overexpressing in tumor cells. Liposomes loaded with paramagnetic complexes acting as NMR shift reagents have been recently proposed as highly sensitive CEST MRI agents. The main peculiarity of CEST probes is to allow the MR visualization of different agents present in the same region of interest, and this article provides an illustrative example of the in vivo potential of liposome-based CEST agents. [source] | ||||||||||