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-3,4,5,6tetrahydro-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 N-1 NMR spectroscopy and X-ray crystallography. When evaluating the N-1 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 H-1 NMR spectroscopy, X-ray crystallography, and computational methods.