(2S,4S)-3-Benzoyl-4-benzyl-2-tert-butyl-1,3-oxazolidin-5-one

In the title compound, C21H23NO3, the central oxazolidinone ring is approximately planar, the maximum deviation from the plane through the central ring being 0.043 (1) Å. The tert-butyl and benzyl substituents are cis to each other and trans to the N-benzoyl group. The interplanar angle between the aromatic rings of the C-benzyl and N-benzoyl groups is 81.10 (4)°.

In the title compound, C 21 H 23 NO 3 , the central oxazolidinone ring is approximately planar, the maximum deviation from the plane through the central ring being 0.043 (1) Å . The tert-butyl and benzyl substituents are cis to each other and trans to the N-benzoyl group. The interplanar angle between the aromatic rings of the C-benzyl and N-benzoyl groups is 81.10 (4) .

Experimental
This structure reveals that the oxazolidinone ring is approximately planar, with the tert-butyl and benzyl groups occupying the same face of the ring plane. Observation of the cis isomer is in accord with previously reported NMR experiments (Seebach & Fadel, 1985) and the crystal structures of related compounds (Dungan et al. 2010;Barry et al. 2012). In the crystal, the angle between the oxazolidinone ring and the phenyl ring of the N-benzoyl group (C1 to C6) is 65.74 (4)°, while the angle between the oxazolidinone and the phenyl ring of the C-benzyl group (C16 to C21) is 25.66 (7)°.
Oxazolidinones of this type are of interest due to their capacity to undergo stereoselective α-alkylation via the cyclic enolate, a reaction that exploits the principle of "self-reproduction of chirality centres" introduced by Seebach and coworkers (Seebach & Naef, 1981, Seebach et al., 1984, Seebach & Fadel, 1985. Thus the tert-butyl group directs an incoming electrophile to the opposite face of the planar enolate, giving an enantiopure product with retention of stereochemistry from the original oxazolidinone. We have recently applied this strategy in the synthesis of ligand architectures designed to mimic the structure and function of non-heme iron enzymes (Krall et al. 2005, Barry & Rutledge, 2008, Dungan et al. 2010, Dungan et al. 2012)

Experimental
The title compound was prepared following the procedure reported by Seebach (Seebach & Fadel, 1985). Thus the sodium salt of L-phenylalanine was condensed with pivalaldehyde, by heating at reflux in pentane overnight under Dean-Stark conditions to affect azeotropic removal of water. The intermediate Schiff base thus formed was treated with benzoyl chloride in dichloromethane at 233 K, prompting cyclization to give the crude product. Recrystallization from methanol gave white needles in a low overall yield (31%).

Refinement
A floating origin restraint was automatically generated by SHELXL. Hydrogen atoms were added at calculated positions and refined using a riding model. Their isotropic displacement parameters were fixed to 1.2 (1.5 for methyl groups) times the equivalent one of the parent atom. C-H bond lengths range from 0.95 Å to 1.00 Å.

Special details
Experimental. R(int) for selected reflections was 0.037 before and 0.019 after correction for absorption. The Ratio of minimum to maximum transmission is 0.893567. The λ/2 correction factor is 0.0015. Friedel pairs were merged. Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes. Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > σ(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.