Isomorphous diethyl 1-(4-chlorobenzyl)-4-(4-chlorophenyl)-2,2-dioxo-3,4,6,7,8,8a-hexahydro-1H-pyrrolo[2,1-c][1,4]thiazine-1,3-dicarboxylate and its 1-(4-methylbenzyl)-4-(4-methylphenyl)-substituted analogue obeying the chloro–methyl exchange rule

The influence of the substituents in the crystals of the title compounds has not made any significant effect on the crystal packing and intermolecular hydrogen bonds. The validity of chlorine–methyl exchange rule is confirmed.

Both (I) and (II) are thiazine derivatives that may potentially exhibit pharmacological activities in view of the presence of nitrogen and sulfur atoms as constituents of the fused pyrrolothiazine ring (Moriyama et al., 2004;Koketsu et al., 2002;Rai et al., 2013). Derivatives of thiazine have been shown to exhibit calcium antagonist activities (Erker, 1998) and various inhibitory activities on central nervous system (Grandolini et al., 1997;Malinka et al., 2002). Pyrrolothiazine derivatives have been employed as anti-inflammatory, antifungal and anti-microbial agents (Armenise et al., 1991;Armenise et al., 1998). The present work reports the detailed description of the crystal structures of (I) and (II) along with Hirshfeld surface analysis of their respective intermolecular interactions.

Supramolecular features
The crystal packing of both compounds (Figs. 3 and 4) features C-HÁ Á ÁO hydrogen bonding (Tables 1 and 2) andinteractions. The C-HÁ Á ÁO interactions, which are similar in strength and geometry, involve only one of the two dioxo oxygen atoms, viz. O1. The non participation of the other oxygen atom (O2) cannot be explained from the viewpoint of intermolecular interactions whereas the absence of such    interactions involving O3 and O5 may be attributed to steric factors arising from an unfavourable packing geometry. In both crystals, molecules are connected into inversion dimers via pairs of weak C-HÁ Á ÁO hydrogen bonds, forming R 2 2 (14) graph-set motifs. These dimers are further connected via weak C-HÁ Á ÁO interactions into chains running along [011]. A parallel-displacedstacking interaction is observed in both compounds between the C21-C26 benzyl rings. In (I), CgÁ Á ÁCg(1 À x, Ày, 2 À z) = 4.0485 (13) Å , with a slippage of 1.749 Å [for (II), CgÁ Á ÁCg(1 À x, 2 À y, 2 À z) = 4.0554 (14) Å , slippage of 1.711 Å ] where Cg is the ring centroid.

Hirshfeld Surface Analysis
Hirshfeld surface analysis is a graphical tool to investigate the packing modes and nature of prominent intermolecular interactions in crystal structures. The Hirshfeld surfaces (Spackman & Jayatilaka, 2009) and the associated two-dimensional fingerprint plots were generated using Crystal-Explorer 3.0 software (Wolff et al., 2012). In the present work, the nature of the intermolecular interactions in the two structures is similar because of their isomorphism. The Hirshfeld surfaces mapped with shape-index together with decomposed fingerprint plots for (I) and (II) are shown in Figs. 5 and 6, respectively. In both the structures, the molecules participate in weak C-HÁ Á ÁO hydrogen bonds, which are indicated by red spots on the surface plots. The OÁ Á ÁH/HÁ Á ÁO intermolecular interactions appear as distinct sharp spikes in the fingerprint plots. The area between the spikes corresponds to the HÁ Á ÁH contacts, which account for nearly 46.7% of the surface in (I) and 70.6% in (II). The ClÁ Á ÁH/HÁ Á ÁCl inter-research communications   Symmetry codes: (i) Àx þ 1; Ày þ 1; Àz þ 2; (ii) x À 1 2 ; Ày þ 1 2 ; z À 1 2 .

Figure 3
Part of the crystal structure of compound (I), showing the formation of an R 2 2 (14) ring. Dashed lines indicate hydrogen bonds. H atoms not involved in the hydrogen bonding have been omitted for the sake of clarity.

Figure 6
Hirshfeld surface of compound (II) mapped over shape-index and decomposed finger print plots of dominant interactions showing (a) all,

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. In both compounds, the carbonbound H atoms were placed in calculated positions (C-H = 0.93-0.97 Å ) and were included in the refinement in the riding-model approximation, with U iso (H) set at 1.2-1.5U eq (C).

Diethyl 1-(4-chlorobenzyl)-4-(4-chlorophenyl)-2,2-dioxo
where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max = 0.002 Δρ max = 0.37 e Å −3 Δρ min = −0.24 e Å −3 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.