X-ray crystallographic and C-13 nuclear magnetic resonance studies of 3,4,5,6-tetrahydro-2H-1,3,4-oxadiazin-2-ones derived from ephedrine and pseudoephedrine

3,4,5,6-Tetrahydro-2H-1,3,4-oxadiazin-2-ones derived from (1R,2S)-ephedrine and (1S,2S)-pseudoephedrine have been synthesized and their conformational properties have been examined. The ephedrine heterocycles 5-7a appear to f avor one set of equilibrating conformers while the pseudoephedrine heterocy cles 5-7b exist as multiple conformers at room temperature. The observed co nformational behavior of these heterocycles is attributed to allylic strain and a gauche effect arising from the torsional energy barrier between the lone pair electrons of the N-3- and N-4-nitrogens. (C) 2001 Elsevier Scienc e Ltd. All rights reserved.