(R)-4-Isopropyl-3-isopropylsulfanyl-5,5-diphenyl-1,3-oxazolidin-2-one

The title compound, C21H25NO2S, consists of a five-membered heterocyclic ring, with pendant phenyl groups, an isopropyl group and a thioether residue. The thioether bonds to the heterocycle via the N atom. The absolute configuration results from an inversion of the configuration of substrate during the synthesis.

The title compound, C 21 H 25 NO 2 S, consists of a five-membered heterocyclic ring, with pendant phenyl groups, an isopropyl group and a thioether residue. The thioether bonds to the heterocycle via the N atom. The absolute configuration results from an inversion of the configuration of substrate during the synthesis.
Eventually, these auxiliaries are applied to the synthesis of new sulfimines in a high enantiomeric ratio (Celentano et al., 1998). To the best of our knowledge, the only other N-thioether-containing oxazolindinone is a dione (Valle et al., 1992).
All other oxazoldininones that exhibit an N-S bond are sulfinyl-or sulfonyl-containing compounds (Allen, 2002).
An interesting feature of this compound is the conversion of S-isopropyl isopropanesulfonothioate to an R-isomer during the synthesis. Confirmation of the correct absolute stereochemistry of (I) was determined as described below.

Experimental
To a solution of the oxazolidinone (Hintermann & Seebach, 1998) (2.50 g, 8.80 mmol) in dry THF (40 ml) at 273 K was slowly added 1 equiv of n-BuLi (Celentano et al., 1998). The solution turned from colorless to dark-red. After the mixture was left to react for 30 min at 273 K, a solution of S-isopropyl isopropanesulfonothioate (Derbesy & Harpp, 1995) (1.58 g, 9.10 mmol) in dry THF (40 ml) was added by cannula, at once, and the reaction was left stirring overnight at room temperature. The white mixture was quenched with saturated NH 4 Cl (50 ml) and extracted with ethyl acetate (50 ml). The organic layer was washed with H 2 O (50 ml) and brine, dried with MgSO 4 and then filtered. The solvent was removed at reduced pressure on a rotovap and the colorless oil was purified through flash chromatography with elution by (1:9 ethyl acetate/hexanes) to provide 2.28 g of the oxazolidine sulfide (73% yield) as colorless prisms after slow evaporation.

Refinement
All hydrogen atoms were included in geometrically calculated positions with C-H distances constrained to 0.95 Å for aromatic C-H and 0.98-1.00 Å for aliphatic C-H bonds. Hydrogen thermal parameters were tied to the occupancy of the atom to which they are bonded. The U iso was set to 1.5 × U eq for methyl H atoms and 1.2 × U eq for all others.
The absolute configuration was determined by the known handedness of the molecule from synthesis, comparison of intensities of Friedel pairs of reflections (Flack, 1983) and by Bayesian analysis of Bijvoet pairs (Hooft et al., 2008). All three techniques agree and the correct configuration is depicted in Fig. 1. The Flack x parameter refined to 0.039 (15) based on 1165 Friedel pairs. The Hooft y parameter was 0.056 (6) based on 1170 Bijvoet pairs. P2(true) and P3(true) values were calculated at 1.000 and 1.000 indicative an an enantiopure crystal.

Figure 1
The molecular structure of (I) showing the atom-labelling scheme and displacement ellipsoids at the 50% probability level. H atoms are shown as idealized spheres of an arbitrary radius.

Special details
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.