Stereochemistry Information from NOESY/ROESY data

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Several NMR experiments offer tools to help determine the stereochemistry of a structure. Some typical experiments are 1D NOE (Nuclear Overhauser Effect), 2D NOESY (NOE Spectroscopy) and ROESY (Rotating-frame Overhauser Effect Spectroscopy). These experiments will produce signals for nuclei that are close to each other through space independent of the number of bonds separating the nuclei.
A simplified 1H-1H NOESY spectrum is shown below. The spectrum shows 2 correlations at (4.29,1.28) and (4.29,3.13) ppm. There is no correlation to the proton signal at 2.68 ppm.

 

NOESY, ROESY, COSY and TOCSY are all 2D NMR experiments that sound so similar but offer different pieces of information about the puzzle. When interpreting the NMR data, it is important to understand how the nuclei interact with each other. For example, the presence of a cross peak (a correlation off the diagonal) on a COSY dataset is a result of nuclei coupling through a bond(s) whereas a NOESY dataset measures NOE’s (Nuclear Overhauser Effect) through space regardless of the number of bonds separating the nuclei. An NOE is typically observed for nuclei that are separated no farther than 5 Å apart.
For the enantiomers example shown below, the NOESY and COSY experiments differ in the presence or absence of the cross peaks. A clear difference between the two experiments is the information provided on the diastereotopic protons of the CH2 group.

The NOESY spectrum, also outlined , shows 2 correlations at (4.29,1.28) and (4.29,3.13) ppm. There are no NOE's to the proton signal at 2.68 ppm. The DQF-COSY below shows two-bond and three-bond correlations at (4.29,3.13), (4.29,1.28) and (3.13,2.68) ppm. There are no four-bond correlations present as the 4J coupling constants are close to zero.

  

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 ALASKA,  USA

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The City and Borough of Juneau (/ˈdʒuːnoʊ/; Tlingit: Dzánti K'ihéeni [ˈtsántʰì kʼìˈhíːnì]) is the capital city of Alaska. It is a unified municipality located on ...

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Symmetry 2014, 6(3), 622-654; doi:10.3390/sym6030622

Symmetry Breaking in NMR Spectroscopy: The Elucidation of Hidden Molecular Rearrangement Processes

 http://www.mdpi.com/2073-8994/6/3/622
 

Michael J. McGlinchey

School of Chemistry and Chemical Biology, University College Dublin, Belfield, Dublin 4, Ireland; E-Mail: michael.mcglinchey@ucd.ie; Tel.: +353-1-716-2165; Fax: +353-1-716-1178

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 Variable-temperature NMR spectroscopy is probably the most convenient and sensitive technique to monitor changes in molecular structure in solution. Rearrangements that are rapid on the NMR time-scale exhibit simplified spectra, whereby non-equivalent nuclear environments yield time-averaged resonances. At lower temperatures, when the rate of exchange is sufficiently reduced, these degeneracies are split and the underlying “static” molecular symmetry, as seen by X-ray crystallography, becomes apparent. Frequently, however, such rearrangement processes are hidden, even when they become slow on the NMR time-scale, because the molecular point group remains unchanged. Judicious symmetry breaking, such as by substitution of a molecular fragment by a similar, but not identical moiety, or by the incorporation of potentially diastereotopic (chemically non-equivalent) nuclei, allows the elucidation of the kinetics and energetics of such processes. Examples are chosen that include a wide range of rotations, migrations and other rearrangements in organic, inorganic and organometallic chemistry.

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 Multiple Cope rearrangements equilibrate all ten CH positions in bullvalene.


 
Interconversion of distal and proximal ethyls, combined with rapid tripodal rotation, generates effective C_6v symmetry.

The role of chemical synthesis in structure elucidation of oxasqualenoids The role of chemical synthesis in structure elucidation of oxasqualenoids

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Recently, highly oxidized and structurally diverse triterpene polyethers, which are thought to be biogenetically squalene-derived natural products (oxasqualenoids), have been isolated from both marine and terrestrial organisms. However, it is often difficult to determine their stereostructures even by the current, highly advanced spectroscopic methods, especially in acyclic systems including stereogenic quaternary carbon centers. In such cases, it is effective to predict and synthesize the possible stereostructures. Herein, we report total assignments of the previously incomplete stereostructures of an epoxy tri-THF diol, intricatetraol and enshuol, members of the oxasqualenoids, through the first asymmetric total syntheses of the natural products, the configurations of which are difficult to determine by other means. Since this article is basically written as a communication without detailed experimental procedures and spectroscopic data, original papers with full data should follow.

Emerging Area

The role of chemical synthesis in structure elucidation of oxasqualenoids

Yoshiki Morimoto*^a  

*Corresponding authors

^aDepartment of Chemistry, Graduate School of Science, Osaka City University, Sumiyoshi-ku, Japan
E-mail: morimoto@sci.osaka-cu.ac.jp
Fax: +81 6 6605 2522
Tel: +81 6 6605 3141

Org. Biomol. Chem., 2008,6, 1709-1719

DOI: 10.1039/B801126E


 The Great Victoria Desert, Australia

 The Great Victoria Desert is next on the list, ranking eighth. The Great Victoria Desert spans about 647,000 square kilometers of Australia.

 

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1. The Great Victoria Desert, an interim Australian bioregion, is a sparsely populated desert area in Western Australia and South Australia.
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