(5R,6S)-4-Isopropyl-5-methyl-6-phenyl-3-propanoyl-2H-1,3,4-oxadiazinan-2-one

The title compound, C16H22N2O3, was synthesized during the course of a study on (1R,2S)-norephedrine-derived 1,3,4-oxadiazinan-2-ones. The conformation adopted by the isopropyl group is pseudo-axial relative to the oxadiazinan core. The allylic strain contributes to this conformational arrangement.

The title compound, C 16 H 22 N 2 O 3 , was synthesized during the course of a study on (1R,2S)-norephedrine-derived 1,3,4oxadiazinan-2-ones. The conformation adopted by the isopropyl group is pseudo-axial relative to the oxadiazinan core. The allylic strain contributes to this conformational arrangement.

Comment
The production of enantiomerically pure compounds has become increasingly important in the pharmaceutical industry.
The high demand for a single enantiomer of a chiral intermediate has led to a wealth of methods for asymmetric synthesis (Hitchcock et al., 2004). While asymmetric catalysis and other methods have been functional in asymmetric synthesis, the important role of chiral auxiliaries in asymmetric synthesis is patent. Oxazolidin-2-ones, chiral auxiliaries first introduced by Evans et al. (Evans et al., 1981), have been of substance in areas of alkylation reactions, pericyclic reactions, and asymmetric aldol condensation reactions. Related, 1,3,4-oxadiazinan-2-one heterocycles have received little interest since their disclosure (Trepanier et al., 1968). It was not until recently that synthetic (Hitchcock et al., 2001) and conformational studies (Casper, Burgeson et al., 2002) of 1,3,4-oxadiazinan-2-one have been thoroughly performed.
Herein we report the X-ray structure of the N3-propanoyl acylated norephedrine-derived 1,3,4-oxadiazinan-2-one. The imide carbonyls adopt a syn-periplanar orientation, with an O21-C2-C17-O20 torsion angle of 23.67 (17)°. This result is consistent with those of previously reported N3 substituted oxadiazinan-2-ones (Casper, Blackburn et al., 2002;Casper, Burgeson et al., 2002;Ferrence et al., 2003). It is believed that in the oxadiazinaneone systems the syn-periplanar conformation arises from the lone pair repulsion interaction between the N4-nitrogen lone pair and the N3-carbonyl lone pair (Casper, Blackburn et al., 2002). However, in the case of the title compound, the repulsive interactions between the N3-substituent and the N4-isopropyl could also be held accountable for the syn-periplanar orientation. In fact, ring puckering analysis using PLATON (Spek, 2009;Cremer & Pople, 1975;Boeyens, 1978) indicates θ = 62.7 (2)° and Φ = 196.9 (2)° for the O1-C2-N3-N4-C5-C6 ring, which is consistent with a formal conformational assignment close to an idealized E 4 envelope with N4 being the flap apex. Such a conformation possesses a pseudo-axial C5-methyl group, a typical pseudoequatorial C6-phenyl ring, and a typical pseudo-axial N4-iso-propyl group. The imide carbonyls, although not syn-parallel, indicate the existence of resonance delocalization due to their approximately planar conformation [torsion angle 23.67 (17) Based on previous studies (Casper, Blackburn et al., 2002), the N3-substituent is held rigidly due to resonance interactions, while the N4-isopropyl group adopts a pseudoaxial orientation to relieve allylic strain on the system. Both intra-and intermolecular non-classical H-bonding interactions exist. Those interactions shorter than 2.7 Å with a >90° D-H···A angle are shown in Table 1 (Steiner, 1996). It appears that such non-classical H-bonding interactions may constitute the dominant packing forces in this structure; however, evaluation of additional related structures will be necessary before any particular rational for these interactions is defensible.

Experimental
The title compound was synthesized by acylation of norephedrine derived 1,3,4-oxadiazinan-2-one using propanoyl (Hitchcock et al., 2004). Single crystals were grown by vapor diffusion of hexane into a methylene chloride solution of the title compound.