Optical Purity Determination and 1H NMR Spectral Simplification with Lanthanide Shift Reagents. II. Thiamylal, 5-Allyl-5-(1-methylbutyl)-2-thiobarbituric Acid

Abstract

A racemic sample of thiamylal, 5-allyl-5-(1-methylbutyl)-2-thiobarbituric acid, 1, has been shown to exhibit significant enantiomeric shift differences, ΔΔδ, for the CH3- CH proton absorptions when treated with the chiral lanthanide shift reagent, tris[3-(trifluoromethylhydroxy-methylene)- d-camphorato| europium (III), 2, in CDC13 at 28°. For example, at the relatively low molar ratio of 2:1 of 0.181, a value of ΔΔδ of 22 Hz (0.36δ) was observed, which increased to 26 Hz (0.43δ) at a molar ratio of 0.226. These large values, with relatively little lanthanide-induced line-broadening, should make possible the facile determination of optical purity of 1 even with a 60 MHz nmr spectrometer. Parallel studies were run using the achiral shift reagent, tris-(6,6,7,7,8,8,8-heptafluoro-2,2-dimethyl-3,5-octanedionato) europium (III), 3, to confirm peak assignments. The considerable values of ΔΔδ may reflect the “soft base” character of the sulfur atom in 1. Less effective binding by sulfur to the lanthanide atom could lead to coordination by europium on the carbonyl oxygen, relatively close to the chiral center, providing enhanced AA5 values compared to some related compounds. While several other proton absorptions of 1 appear to show some observable ΔΔδ, they are less valuable for optical purity evaluation because of lower intensity, greater multiplicity, or small ΔΔδ value. The relative slopes of the plots of chemical shift, δ, for runs with 2 or 3, are consistent with the full assignment of the proton absorptions of 1 based on expected proximity of the protons to the complexed europium atom. The observed coupling constants of the vinyl hydrogens of 1 further support the assignments.