The 1,3-thia­zin-4-ones are a group of six-membered heterocycles with a wide range of biological activity (Ryabukhin et al., 1996 ▸). Surrey’s research (Surrey et al., 1958 ▸; Surrey, 1963a ▸,b ▸) resulted in the discovery of two drugs, the anti-anxiety and muscle relaxant chlormezanone [2-(4-chloro­phen­yl)-3-methyl-2,3,5,6-tetra­hydro-4H-1,3-thia­zin-4-one 1,1-dioxide] (O’Neil, 2006 ▸; Tanaka & Horayama, 2005 ▸) and muscle relaxant di­chloro­mezanone [2-(3,4-di­chloro­phen­yl)-3-methyl-2,3,5,6-tetra­hydro-4H-1,3-thia­zin-4-one 1,1-dioxide] (Elks & Ganellin, 1990 ▸). These sulfones showed greater activity than the sulfides from which they were synthesized (Surrey et al., 1958 ▸). Surrey also prepared a variety of other sulfoxides and sulfones of 3-alkyl-2-aryl-2,3,5,6-tetra­hydro-4H-1,3-thia­zin-4-ones (Surrey, 1963a ▸,b ▸). We have reported previously the crystal structure of the first N-aryl sulfoxide in this family, racemic 2,3-diphenyl-2,3,5,6-tetra­hydro-4H-1,3-thia­zin-4-one 1-oxide (Yennawar et al., 2016 ▸).

A sulfoxide typically has an S—O bond that is between a double bond and a single bond, with one of the lone pairs that was on the sulfide coordinating to the O atom, while O atom contributes electrons from a lone pair to a d orbital of the S atom. The geometry of a sulfoxide is pyramidal, with a high energy barrier for inversion, making it possible to isolate stable enanti­omers (Bentley, 2005 ▸). Herein, we report the crystal structure of the sulfoxide of N-[(2S,5R)-4-oxo-2,3-diphenyl-1,3-thia­zinan-5-yl]acetamide (Yennawar, Singh & Silverberg, 2015 ▸), C₁₈H₁₈N₂O₃S, prepared using the method we have reported previously for the oxidation of other 2,3-diphenyl-1,3-thia­zin-4-ones (Yennawar et al., 2016 ▸; Yennawar, Noble et al., 2017 ▸) and 1,3-thia­zolidinones (Yennawar, Hullihen et al., 2015 ▸; Cannon et al., 2015 ▸). The oxidation of the confirmed enanti­opure sulfide N-[(2S,5R)-4-oxo-2,3-diphenyl-1,3-thia­zinan-5-yl]acetamide 0.375-hydrate (Yennawar, Singh & Silverberg, 2015 ▸), derived from N-acetyl-l-cysteine, yielded a single stereoisomer as the only product.

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 θ = 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—H⋯O hydrogen bond between the 2-phenyl ring and the O atom of the acetamide group is seen in both mol­ecules (C10A—H⋯O3A and C10B—H⋯O3B), as detailed in Table 1 ▸.

We reported previously the crystal structure of the starting sulfide, N-[(2S,5R)-4-oxo-2,3-diphenyl-1,3-thia­zinan-5-yl]ace­t­amide 0.375-hydrate (Yennawar, Singh & Silverberg, 2015 ▸), which also had two independent homochiral mol­ecules in the asymmetric unit. However, they were not identical: in one mol­ecule, the thia­zine ring was in a half-chair conformation in which the 2-phenyl ring was nearly pseudo-axial and the acetamide group was nearly pseudo-equatorial. The other mol­ecule had the thia­zine ring in a boat conformation in which both substituents were pseudo-equatorial.

In the crystal, the B mol­ecule and its 2₁-related symmetry neighbours form a continuous hydrogen-bonded chain along the b-cell direction through N—H⋯O inter­actions involving the acetamide N atom and the thia­zin-1-ium-1-olate O atoms [N2B—H⋯O1B ⁱⁱ; symmetry code: (ii) −x, y + , −z; 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—H⋯O1B 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-face π–π stacking inter­actions.

Crystal structures of a number of 1,3-thia­zolidin-4-one 1-oxides have been reported (Wang et al., 2010 ▸; Johnson et al., 1983 ▸; Chen et al., 2011 ▸; Colombo et al., 2008 ▸; Yennawar, Hullihen et al., 2015 ▸) and the structure of chlormezanone [2-(4-chloro­phen­yl)-3-methyl-2,3,5,6-tetra­hydro-4H-1,3-thia­zin-4-one 1,1-dioxide] has also been reported (Tanaka & Horayama, 2005 ▸). We have reported previously the crystal structure of 2,3-diphenyl-2,3,5,6-tetra­hydro-4H-1,3-thia­zin-4-one 1-oxide (Yennawar et al., 2016 ▸). We have also reported recently the crystal structures of 2,3-diphenyl-2,3-di­hydro-4H-1,3-benzo­thia­zin-4-one 1-oxide (Yennawar, Fox et al., 2017 ▸) and 2,3-diphenyl-2,3-di­hydro-4H-pyrido[3,2-e][1,3]thia­zin-4-one 1-oxide (Yennawar, Noble et al., 2017 ▸).

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^® 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 Na₂SO₄ 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.

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.