Crystal structure of (4R,5S)-4-methyl-3-methylsulfinyl-5-phenyl-1,3-oxazolidin-2-one

The absolute structure of the chiral asymmetric indole precursor title compound, C11H13NO3S, was confirmed by refinement of the Flack and Hooft parameters and is that expected based on the starting materials for the synthesis. The phenyl group subtends a dihedral angle of 56.40 (5)° with the mean plane of the oxazolidinone ring, which adopts an envelope conformation, with the C atom bearing the methyl group as the flap. In the crystal, no significant directional interactions beyond van der Waals contacts are observed.

For general background to the preparation of naturally occurring alkaloids, see: Marino et al. (1992). For further synthetic details, see: Silveira & Marino, 2013. For related structures, see: Evans et al. (1992); ; Silveira et al. (2012); Clara-Sosa et al. (2004); Romanenko et al. (2003). A statistical analysis (Hooft et al., 2008) was used to corroborate that the correct enantiomorph of the space group and hence handedness of the molecule had been determined.

S1. Introduction
Our research group has interest in the development of new methodologies to the synthesis of chiral sulfur compounds (Silveira & Marino, 2013). Thus, we have been preparing chiral oxazolidinones (Silveira, Oliver & Noll, 2013) to the synthesis of asymmetric indole derivatives (Pozza Silveira et al., 2012) as precursors to the preparation of naturally occurring alkaloids (Marino et al., 1992).

S2.1. Synthesis and crystallization
411 mg of (4R,5S)-4-methyl-5-phenyloxazolidin-2-one (2.32 mmol) and 25 mL of dry THF were added to a 50 mL flame-dried round bottom flask charged with argon gas at 0 °C. To this 1.21 mL of n-buthyl lithium (1.83 M, 2.21 mmol) was added dropwise into the solution during five minutes and the mixture obtained cooled to -78 °C. Subsequently, 320 mg of sulfinyl chloride was added. After 10 min. the reaction was quenched with 6.5 mL of saturated NH 4 Cl solution.
The aqueous layer was extracted with 25 mL of ethyl acetate and the organic phase washed with 8 mL of saturated NaHCO 3 solution and 10 mL of saturated NaCl solution, respectively. The organic phase was dried over Na 2 SO 4 and the salt removed by filtration. The solvent was removed under reduced pressure to give a white solid. The crude solid was dissolved with ethyl acetate and hexanes were added dropwise to the solution until a cloudy suspension was observed.
The ethyl acetate / hexanes solution was left overnight to evaporate yielding 238 mg of clear colorless rods (45%).

S2.2. Refinement
Crystal data, data collection and structure refinement details are summarized in Table 1. All non-hydrogen atoms were refined with anisotropic thermal displacement parameters. Hydrogen atoms were included in geometrically calculated positions riding on the carbon to which they are bonded. C-H bond distances were restrained to 0.95 Å (aromatic), 0.98 Å (methyl) and 1.00 Å (methyne). Hydrogen thermal parameters were set as U iso (H) = 1.2 × U eq (C)aromatic/methyne and 1.5 × U eq (C)methyl.
The absolute stereochemistry was determined both by the known chiralty that was retained during synthesis and by comparison of intensities of Friedel pairs of reflections. Both a direct measurement in the differences in intensities (Flack x paramter = -0.012 (3), (Parsons et al., 2013)) and a statistical analysis (Hooft y parameter = -0.015 (17), Hooft et al., 2008) corroborate that the correct enantiomorph of the space group and hence handedness of the molecule were determined. All three techniques agree and the correct chirality is shown.

S3. Results and discussion
The structure of the oxazolidinone is as expected. The steroechemistry from the parent reactants was retained through the synthesis. Surprisingly, no significant intermolecular interactions are observed in the crystal. The phenyl group which could exhibit either π···π interactions or C-H···π interactions shows no sign or indication of such arrangements.