Crystal structure of methyl 1-allyl-4-methyl-1H-benzo[c][1,2]thiazine-3-carboxylate 2,2-dioxide

In the title benzothiazine compound, the dihydrothiazine ring adopts a sofa-like conformation with the S atom displaced from the mean plane through the N and C ring atoms by 0.767 (1) Å. In the crystal, molecules are linked by C—H⋯π interactions, forming chains propagating along the a-axis direction.


Structural commentary
The molecular structure of the title compound, 3, is illustrated in Fig. 2. The dihydrothiazine ring adopts a distorted sofa conformation: the puckering parameters (Zefirov et al., 1990) ISSN 2056-9890 Figure 1 The synthesis of the title compound, 3. are: S = 0.67, Â = 57.1 , É = 19.0 . Atom S1 deviates from the mean plane of the remaining atoms (N1/C1/C6-C8) by 0.767 (1) Å . The allyl substituent (C-C C) is inclined to this mean plane by 78.5 (7) and the acetate group (O C-O-C) by 66.5 (3) . Atom N1 has a planar configuration, the sum of the bond angles being 359.1 .

Supramolecular features
In the crystal, molecules are linked by C-HÁ Á Á interactions, forming chains propagating along the a-axis direction. (Table 1 and Fig. 3). There are no other significant intermolecular interactions in the crystal structure, despite the presence of a number of potential donor and acceptor atoms.

Figure 3
A view along the c axis of the crystal packing of compound 3. The C-HÁ Á Á interactions are represented by dashed lines (see Table 1) and, for clarity, only H atom H12A (grey ball) is included. Table 1 Hydrogen-bond geometry (Å , ).
Cg is the centroid of the C1-C6 ring.
of the dihydrothiazine ring in both molecules resembles that in the title compound, which has a distorted sofa conformation. A view of the structural overlap of the three molecules is shown in Fig. 4.

Synthesis and crystallization
The synthesis of the title compound, 3, is illustrated in Fig. 1. To a solution of 2.53 g (0.01 mol) of methyl 4-methyl-2,2dioxo-1H-2 6 ,1-benzothiazine-3-carboxylate, 1, in 20 ml DMSO were added 2.07 g (0.015 mol) of K 2 CO 3 and the mixture was stirred for 30 min. Allyl bromide (1.81 g, 0.015 mol) was then added and the mixture was stirred for a further 30 min at 298 K. It was then diluted with cold water and acidified with dilute HCl to pH 4. It was extracted with CH 2 Cl 2 (3 Â 10 ml). The organic extracts were combined and the solvent removed by distillation (at reduced pressure at the end). The residue was dissolved in 20 ml of hot methanol and filtered over charcoal. The resulting solution was then placed in a freezer (253 K) for 24 h, after which crystals of the title compound were harvested (yield 2.55 g, 87%; m.p. 360-362 K).

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. H atoms could all be located in difference Fourier maps. During refinement they were included in calculated positions and treated as riding: C-H = 0.93-0.97 Å with U iso = 1.5U eq (C-methyl) and 1.2U eq (C) for other H atoms. The structural overlap of the two independent molecules of the 4-hydroxy analogue (MINJAW; Shishkina et al., 2013), and the title compound 3, shown in blue.   (Sheldrick, 2015); molecular graphics: SHELXTL (Sheldrick, 2008) and Mercury (Macrae et al., 2008); software used to prepare material for publication: SHELXL2014 (Sheldrick, 2015), PLATON (Spek, 2009) and publCIF (Westrip, 2010).

Special details
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.