Long-range shielding effects in the (1)H NMR spectra of Mosher-like ester derivatives.

The relative magnitudes of the chemical shift differences (Deltadeltas) in the two diastereomers of menthyl esters of known chiral derivatizing agents (CDAs) were compared to those of the alpha-methoxy-alpha-trifluoromethyl-1-naphthylacetyl (MTN((1))A) analogues I. Discrimination of the terminal diastereotopic methyl resonances in esters of the homologous, symmetrical carbinols II was evaluated. Remarkably, the methyls differed in the MTN((1))A esters III even when n = 15; an unexpected crossover in the sign of the Deltadelta values was also observed.

Toward the development of a structurally novel class of chiral auxiliaries. Conformational properties of the aldol adducts of oxadiazinones: observation of unusual shielding effects.

Asymmetric aldol reactions were conducted with the titanium enolate of N(3)-hydrocinnamoyl-3,4,5,6-tetrahydro-2H-1,3,4-oxadiazin-2-one to afford aldol adducts 5a-j. The dominant product of the asymmetric aldol reaction was the non-Evans syn adduct as determined by (1)H NMR spectroscopy and X-ray crystallography. When evaluating the (1)H NMR spectra of adducts 5a-j, a highly shielded signal with an average chemical shift of 0.05 ppm was observed. This signal was readily determined to be the C(5)-methyl group of the oxadiazinone. It is presumed that the overall conformation adopted by the aldol adducts in solution places an aromatic ring of the N(3)-substituent in close proximity to the C(5)-methyl group. An investigation of this conformational preference is conducted employing (1)H NMR spectroscopy, X-ray crystallography, and computational methods.