Crystal structure of (1S,2S,5R)-5-acetylamino-4-oxo-2,3-diphenyl-1,3-thiazinan-1-ium-1-olate

The crystal structure of the enantiopure sulfoxide of a 2,3,5,6-tetrahydro-1,3-thiazin-4-one exhibits a twisted half-chair pucker for the thiazine ring. Intermolecular N—H ⋯O hydrogen-bonding interactions form a two-dimensional layered structure lying parallel to (001).


Structural commentary
The crystal structure of the title compound has two independent homochiral molecules (A and B) in the asymmetric unit ( Fig. 1), which have almost identical conformational features, having an alignment-r.m.s. deviation value of 0.3 Å . Both have the thiazine rings in a twisted half-chair configuration, with puckering amplitudes = 0.6753 (19)/0.653 (2) Å and = 131.05 (17)/135.66 (18) in molecules A/B, respectively (Cremer & Pople, 1975). The O atom on the S atom of the ring is pseudo-axial on the thiazine ring and trans to both the 2-phenyl group and the acetamide group in each case. The two phenyl rings in each molecule are almost orthogonal to one another, with dihedral angles of 83.79 (17) and 86.95 (16) in molecules A and B, respectively. The acetamide group is pseudo-equatorial and the 2-phenyl group is pseudo-axial on the thiazine ring. A weak intramolecular C-HÁ Á ÁO hydrogen bond between the 2-phenyl ring and the O atom of the acetamide group is seen in both molecules (C10A-HÁ Á ÁO3A and C10B-HÁ Á ÁO3B), as detailed in Table 1.

Figure 2
Crystal packing diagram with red dotted lines for intermolecular N-HÁ Á ÁO contacts between 2 1 -related molecules, forming helical chains along the b-axis direction, as well as the interaction with an independent molecule. Blue dotted lines represent the intramolecular C-HÁ Á ÁO contacts.

Figure 1
The molecular structures of the two independent molecules (A and B) in the asymmetric unit of the title compound, with displacement ellipsoids drawn at the 50% probability level. Dashed lines indicate intramolecular C-HÁ Á ÁO interactions.

Supramolecular features
In the crystal, the B molecule and its 2 1 -related symmetry neighbours form a continuous hydrogen-bonded chain along the b-cell direction through N-HÁ Á ÁO interactions involving the acetamide N atom and the thiazin-1-ium-1-olate O atoms [N2B-HÁ Á ÁO1B ii ; symmetry code: (ii) Àx, y + 1 2 , Àz; Table 1] (Fig. 2). Molecules A and B interact, wherein the O atom in the 4-position of molecule B accepts a proton from the acetamide N atom of molecule A [N2A-HÁ Á ÁO1B i ; symmetry code: (i) x + 1, y, z]. The sulfoxide O atom of molecule A does not participate in any hydrogen bonding. A two-dimensional sheet structure lying parallel to (001) is generated. No benzene ring in either of the molecules participates in face-to-facestacking interactions.

Synthesis and crystallization
A 5 ml round-bottomed flask was charged with 53.9 mg of N-[(2S,5R)-4-oxo-2,3-diphenyl-1,3-thiazinan-5-yl]acetamide 0.375-hydrate, whose configuration was established previously (Yennawar, Singh & Silverberg, 2015), and 1.4 ml of methanol and stirred. A solution of 79.5 mg of Oxone 1 and 1 ml of distilled water was added dropwise and the mixture was stirred until the reaction was complete, as determined by thin-layer chromatography (TLC). The solids were dissolved by the addition of 5 ml of distilled water. The solution was extracted with 10 ml of dichloromethane. The organic layer was washed with 5 ml of distilled water and then with 5 ml of saturated sodium chloride. The solution was dried over Na 2 SO 4 and concentrated under vacuum giving a crude solid. This was chromatographed on flash silica gel, eluting with a gradient of 0-60% acetone in ethyl acetate, giving 55.8 mg of product [98.6% yield; m.p. 449-452 K; R F = 0.20 (30% acetone/70% ethyl acetate)]. Crystals suitable for X-ray crystallography were grown by slow evaporation from propan-2-ol.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. The H atoms, excepting those on N atoms, were placed geometrically and allowed to ride on their parent C atoms during refinement, with C-H distances of 0.93 (aromatic), 0.96 (methyl), 0.97 or (methylene) and 0.98 Å (methyl), and with U iso (H) = 1.2U eq (aromatic or methylene C) or 1.5U eq (methyl C). H atoms on N atoms were located in a difference Fourier map and were refined isotropically. The absolute configuration for the chiral centres in the molecule was determined as (1S,2S,5R) (for the arbitrarily numbered atoms C1A/B,C3A/B), with a Flack absolute structure parameter (Flack, 1983) of 0.07 (6) for 4160 Friedel pairs.

Computing details
Data collection: SMART (Bruker, 2016); cell refinement: SAINT (Bruker, 2016); data reduction: SAINT (Bruker, 2016); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: OLEX2 (Dolomanov et al., 2009); software used to prepare material for publication: OLEX2 (Dolomanov et al., 2009). where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max = 0.001 Δρ max = 0.37 e Å −3 Δρ min = −0.27 e Å −3 Absolute structure: Flack (1983), 4160 Friedel pairs Absolute structure parameter: 0.07 (6) Special details Experimental. The data collection nominally covered a full sphere of reciprocal space by a combination of 4 sets of ω scans each set at different φ and/or 2θ angles and each scan (10 s exposure) covering -0.300° degrees in ω. The crystal to detector distance was 5.82 cm. Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds 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 > 2sigma(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.  (2) O1B-S1B 1.497 (2)