NOESY studies

The relative stereochemistry of hyperaspine 93 was determined by 2-D NMR spectroscopic and mass spectrometry (MS) methods. It has a m-fused bicyclic conformation 93a <2001TL4621>. The trans-fused one is disfavored by an axial pentyl group at C-8 and by a destabilizing dipole-dipole interaction between the N- and O-atoms, which does not exist in the alternative //.(-conformation. The geminal coupling constant of C( 1 )H2 in 93 (11.0 Hz), and that of its 6-hydroxy derivative (11.2 Hz), indicates that they exist preferentially in / //-conformations, whereas their 6-epimers adopt trans-conformations (9.3 and 8.4 Hz, respectively) <2005EJ01378>. Nuclear Overhauser enhancement spectroscopy (NOESY) studies also confirmed the stereochemistry of 93 by the marked nuclear Overhauser effect (NOE) correlation between H-3 and H-4a <20030L5063>. 

The bidentate oxazoline ligands 85 and 86 (and derivatives thereof) are excellent reporter ligands, and several studies have used NOEs to determine the nature of their chiral pockets [61, 113, 114, 126]. NOESY studies on the cations [Ir(l,5-COD)(86)]+ and several cationic tri-nudear Ir(iii)(hydrido) compounds [110], e. g. [Ir3(p3-H)(H)5(86)3] +, 87, in connection with their hydrogenation activity, allowed their 3-D solution structures to be determined. In addition to the ortho P-phenyl protons, the protons of the oxazoline alkyl group are helpful in assigning the 3-D structure of both the catalyst precursors and the inactive tri-nudear dusters. Specifically, for one of these tri-nudear Ir(iii) complexes, 87 [110], with terminal hydride ligands at d -17.84 and d -21.32 (and a triply bridging hydride at 5 -7.07), the P-phenyl and oxazoline reporters define their relative positions, as shown in Scheme 1.5. 

The NOESY spectrum of 3 6 showed that one end of this pair was locked by intermolecular H bonds, whereas the other end around the mismatched site consisting of the two amide carbonyl groups at that end was open. In contrast, the NOESY study indicated that both ends of 3 7 were locked, but no or very weak NOEs were detected for protons close to the internal mismatched site. These results indicated that, in spite of the presence of mismatched sites, these mutant duplexes still formed. More detailed analysis showed five interstrand H bonds in 3 6 or 3 7, suggesting that these strands were still fully registered. However, the oli-goamide backbones must be twisted near the mismatched sites to alleviate any unfavorable interactions. 

Heimer, N. E., Del Sesto, R. E., and Carper, W. R., Evidence for spin diffusion in a H,H-NOESY study of imidazolium tetrafluoroborate ionic liquids, Magn. Reson. Chem., 42, 71-75,2004. 

Similar NOESY studies in water (90% H2O, 10% D2O) on water-soluble tetramer 3 showed the same amide-side chain NOEs (Fig. 6), suggesting that these foldamers are stably folded in water. These oligoamides, with rigidified backbones and persistent shape, possess large, amide oxygendecorated cavities that should serve as hosts for large cations and polar organic molecules. 

Based on the backbone-rigidification strategy, oligomers with sufficiently long (> 7 subunits) backbones should adopt helical conformations in which one end of the molecule lies above the other. Extensive NOESY studies on the symmetrical nonamer 4 confirmed this expectation [50]. In addition to the presence of numerous well-resolved amide-side chain NOEs, the NOESY spectrum of 4 also revealed an end-to-end NOE cross peak between the end methyl (Me) protons and aromatic proton b 1 (Fig. 7). Due to the symmetrical... 

High-pressure two-dimenskmal NOESY study of complex liquid of 2-ethyIhexyl benzoate ... 

Scheme 50). ° A further three steps afforded ketone (+)-365, reductive amination of which produced a mixture of the two diastereomeric indolizidines (—)-366 and (—)-367 in ratios ofbetween 76 24 and 84 16. The orientation of the new stereocenter was confirmed by NOESY studies. An alternative cyclization by reducing the ketone of 365 to the alcohol, mesyl-ation and cyclization was less favorable, giving approximately equal amounts of the isomers. Hydrogenolysis of 366 and 367 completed the syntheses of (+)-steviamine ent-352) and the aforementioned diastereomer (—)-361, respectively. The weak inhibitory activity of natural (—)-steviamine toward various P-glucosidases and its stronger inhibition of p-galactosidase was estabhshed in this work, while weak inhibition of a-L-rhamnosidase by the (+)-enantiomer and by 361 was also demonstrated. 

Because base protons and sugar protons are separated by a minimum of four bonds, spin couplings are not usually observed between these units and recourse is made to the use of NOE measurements often as 2D NOESY studies. Thus, for example, NOEs observed between the sugar anomeric proton and FI-6 and H-8 of a base serve to identify the base and sugar units of a single residue. NOE measurement can also be used to gain information on the sequence of residues in a nucleic acid. 

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