Optical Purity Determination and 1H NMR Spectral Simplification with Lanthanide Shift Reagents: Glutethimide, 3-Ethyl-3-phenyl-2, 6-piperidinedione, 1
The 60 MHz 1H NMR spectra of racemic glutethimide, 1, have been studied with the chiral shift reagent, tris[3-(trifluoro-methylhydroxymethylene)-d-camphorato] europium (III), 2, and the achiral tris(6,6,7,7,8,8,8-heptafluoro-2,2-dimethyl-3,5-octanedionato) europium
(III), 3. Appreciable values of the enantiomeric shift difference, ΔΔδ, were observed at relatively low molar ratios of 2:1 for the CH3 group in CDCl3 solution at 28°C. Optical purity determinations should easily be carried out by using this absorption
with a 2:1 ratio of approximately 0.25; 3 to 5% of one enantiomer should be detectable in a sample. Lanthanide-induced line broadening was relatively low, thereby facilitating these measurements. A substantial ΔΔδ was also observed for the ortho hydrogens of the aromatic
ring and the NH. For example, at a 2:1 ratio of 0.254, the CH3 resonance displayed a value of ΔΔδ of 7.3 Hz (0.12 δ) and was free of overlap with the absorptions of the shift reagent, 2. The relative slopes of the plots of induced shift, Δδ, vs
molar ratios of 2:1 or 3:1, as well as values of coupling constants, support assignments for the proton absorptions of the aromatic ring.
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