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Optical purity determination and 1H NMR spectral simplification with lanthanide shift reagents—VI. Methohexital, α-dl-5-allyl-1-methyl-5-(1-methyl-2-pentynyl) barbituric acid

1984; Pergamon Press; Volume: 40; Issue: 9 Linguagem: Inglês

10.1016/0584-8539(84)80178-6

1873-3824

Robert Rothchild, Paul Simons,

Analytical Chemistry and Chromatography

The 60 MHz 1H NMR spectra of racemic methohexital, 1, have been studied with the achiral shift reagent, tris(6,6,7,7,8,8,8-heptafluoro-2,2,-dimethyl-3,5-octanedionato) europium(III), 2, and the chiral tris[3-(trifluoromethylhydroxymethylene)-d-camphorato] europium(III), 3. Appreciable values of the enantiomeric shift differences, ΔΔδ, were observed at low molar ratios of 3:1 for the NHC3 and CH3CH in CDCl3 at 28°C for samples about 0.3 molal in 1. Optical purity determinations should easily be carried out using the CH3CH resonance at 3:1 ratios as low as 0.067 (ΔΔδ = 0.103 ppm) or within a range of 3:1 ratios from about 0.3 to 0.55. Enantiomeric shift differences of 0.43 and 0.25 ppm were found for the NCH3 and CH3CH, respectively, with a 3:1 ratio of 0.575. The latter absorption is preferable for quantitative assays because of freedom from overlap with other protons. Parallel studies with 2 as well as coupling constants support proton assignments.