DETERMINATION OF THE ENANTIOMERIC COMPOSITION OF CHIRAL Δ-2-OXAZOLINES-1,3 BY 1 H AND 19 F NMR SPECTROSCOPY USING CHIRAL LANTHANIDE-SHIFT REAGENTS
Résumé
The 300 MHz H NMR spectra of 2-ethyl Δ-oxazoline 1m have been studied in CCl4, CD3CN and C6D6 solutions, in the presence of the achiral lanthanide shift reagent (LSR), tris (6,6,7,7,8,8,8-heptafluoro-2,2-dimethyl-3,5-octanedionato)-europium III, 4 known as Eu(fod)3, (see Sch. 1).10.1081/SL-120005675-SCH0001Scheme 1. Oxazolines and lanthanides reagents used in this paper.All the protons of 1m were deshielded at various extent, and the sequence observed for their Δδ suggested a major complexation at the basic N(3) center of the heterocycle. Then the chiral monosubstituted oxazoline 1e and the disubstituted oxazolines 1Aa–d and 1Ba, were studied in the presence of chiral LSR, tris-[D, D dicampholylmethanato] europium III Eu(dcm)35 and tris-[3-(heptafluoropropylhydroxy-methylene)-d-camphorato] praseodym III Pr(hfc)36 H NMR, and eventually F[H] NMR spectra were mostly recorded in C6D6 solution. Substantial to very important enantiomeric shift difference ΔΔδ values were observed i/ for proton signals concerning the diastereomeric methyl group with all the oxazolines, for the ortho aryl protons of 1Aa and 1Ac, and i/ for the F[H] signals of the fluorinated oxazolines 1Aa–b and 1Ba.NMR results are discussed from substrate and CLSR structure standpoints.For all tested chiral oxazolines there is at least one possibility to proceed to their enantiomeric discrimination either by H or F NMR using Eu(dcm)3 or Pr(hfc)3 as CLSR.