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Absolute structure of the chiral pyrrolidine derivative (2S)‐methyl (Z)‐5‐(2‐tert‐butoxy‐1‐cyano‐2‐oxoethylidene)pyrrolidine‐2‐carboxylate,a compound with low resonant scattering
Authors:Ai Wang  Ulli Englert
Abstract:The enantiopure monopyrrolidine derivative (2S)‐methyl (Z)‐5‐(2‐tert‐butoxy‐1‐cyano‐2‐oxoethylidene)pyrrolidine‐2‐carboxylate, C13H18N2O4, ( 1 ), represents a potential ligand and an attractive intermediate for the synthesis of chiral metal complexes. At the molecular level, the compound features an intramolecular N—H…O hydrogen bond; neighbouring molecules interact via N—H…N contacts to form chains along 100]. Due to its elemental composition, resonant scattering of the target compound is entirely insignificant for diffraction experiments with Mo Kα and small even for Cu Kα radiation. A preliminary study with the harder radiation type confirmed the chiral space group and the suitability of the single crystal chosen; as expected, the results concerning the absolute structure remained completely inconclusive. A second data collection with the longer wavelength gave satisfactory quality indicators for the correct handedness of the molecule, albeit with high standard uncertainties. The absolute configuration has been assessed independently: CD spectra for both enantiomers of the target molecule were calculated and the spectrum for the S‐configured stereoisomer was in agreement with the experiment. The Cotton effect of ( 1 ) may be ascribed to π–π* transitions from HOMO to LUMO and from HOMO to LUMO+1. As both independent techniques agree with respect to the handedness of the target molecule, the absolute structure may be assigned with a high degree of confidence.
Keywords:pyrrolidine derivative  absolute structure  resonant scattering  circular dichroism  crystal structure
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