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  • Title: Toward the development of a structurally novel class of chiral auxiliaries. Conformational properties of the aldol adducts of oxadiazinones: observation of unusual shielding effects.
    Author: Burgeson JR, Renner MK, Hardt I, Ferrence GM, Standard JM, Hitchcock SR.
    Journal: J Org Chem; 2004 Feb 06; 69(3):727-34. PubMed ID: 14750797.
    Abstract:
    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.
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