Unravelling stereochemistry via mass spectrometry

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 The new technique can determine the stereochemical configuration of a molecule by the way it breaks apart © Science/AAAS

Technique reveals a molecule’s chirality without separating the enantiomers or growing pure crystals READ ALL AT http://www.rsc.org/chemistryworld/2013/09/molecules-stereochemistry-chirality-mass-spectrometry

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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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Stereochemistry of cis/trans Decalins

A useful application of long range couplings for the assignment of ring-fusion stereochemistry in decalin ring system bearing an angular methyl group has been developed (Williamson, K. L.; Howell, T.; Spencer, T. A. J. Am. Chem. Soc. 1966, 88, 325). In the trans-decalins, there are usually several ideal W-pathways for long range coupling between the methyl group and axial protons. In cis-decalins, there are fewer or no such pathways. Thus in a pair of cis/trans isomers, the methyl group in the trans isomer will usually be broader (or will actually show splitting), whereas the cis isomer will have a sharper (unsplit) methyl group.

  

VINCRISTINE SPECTRAL DATA

VINCRISTINE

1H NMR spectrum

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