Absolute stereochemistry : the merits of VCD and XRD

Abstract

The aims of this research are to make the use of VCD for absolute stereochemistry determination desirable within the pharmaceutical industry and to investigate the limits of single crystal XRD.

A reliable VCD sampling methodology was developed using the nujol mull technique and provided good quality VCD spectra.  Development of a straightforward method for prediction of accurate VCD spectra, proved to be a much harder target to achieve.  Using published prior information was shown to provide a quicker route to an accurate predicted VCD spectrum.  However, rotation of the non-chiral groups of the molecule was shown to invert bands in the VCD spectra, which had generally only been thought to occur when the absolute stereochemistry inverted.  This degree of added complexity may mean that using VCD for absolute stereochemistry determination within the pharmaceutical industry would only prove useful for a specific subset of chiral compounds.

Increasing the number of heavy atoms in the chiral compound improved the reliability of the absolute stereochemistry determination.  Increasing the redundancy of the data collection improved the Flack parameter and lowered its associated esd, thus making the absolute stereochemistry determination more reliable.  The data collection itself took several days, so may not prove practical for routine use.  However, it does provide an alternative route for accurate absolute stereochemistry determination when no other techniques are available.

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