Determination of Absolute Configurations of Light-Atom Molecules by Means of Direct Detection of Bijvoet Differences

The absolute configurations of two light-atom molecules were determined sufficiently well by direct detection of Bijvoet differences. The compounds examined were ( I ) -cytidine C 9 H 13 N 3 O 5 and ( II ) (S)-3-{(R)-4,4,4-Trifluoro-3-[4-methoxyphenyl]butanoyl}-4-(phenylmethyl)oxazolidin-2-one, C 21 H 20 NO 4 F 3 . Both compounds crystallize in orthorhombic system with the space group P 2 1 2 1 2 1 and Z = 4. The crystal structures were carefully refined by the technique of conventional structure analysis. All possible reflections were measured on a laboratory diffractometer with Cu K radiation. The multiple-diffraction effect was often observed especially as remarkable intensity enhancement in weak reflections. After such unreliable reflections were eliminated by comparisons among the equivalent mates, data were averaged to a set of Bijvoet pairs. Afterwards, additional measurements by the -scan technique were tested. Since -scan data showed a slight systematic error probably owing to some shape-effect, an artificial absorption correction DIFABS was adopted to remove the error. Small but significant intensity differences could be detected for many Bijvoet pairs, and the absolute configurations were correctly determined without ambiguity in all cases. The R and wR values for separate refinements of enantiomorphs supported the results with slight differences. Flack parameters indicated no contradictions as well.

Keywords:

• Absolute Configurations
• Bijvoet Differences
• Light-atom Molecules
• X-ray Structure Analyses