3,3′-(Phenylmethylene)bis(1-ethyl-3,4-dihydro-1H-2,1-benzothiazine-2,2,4-trione): single-crystal X-ray diffraction study, quantum-chemical calculations and Hirshfeld surface analysis

The tautomeric form of 3,3′-(phenylmethylene)bis(1-ethyl-3,4-dihydro-1H-2,1-benzothiazine-2,2,4-trione) with potential antimicrobial, analgesic, and anti-inflammatory activity was studied.

3,3 0 0 0 -(Phenylmethylene)bis(1-ethyl-3,4-dihydro-1H-2,1-benzothiazine-2,2,4-trione): single-crystal X-ray diffraction study, quantum-chemical calculations and Hirshfeld surface analysis The title compound, C 27 H 26 N 2 O 6 S 2 , possesses potential antimicrobial, analgesic, and anti-inflammatory activity. This compound has three tautomeric forms, which relative energies were estimated with quantum-chemical calculations. All these tautomers (dienol form 7A, keto-enol form 7B, and diketo form 7C) were optimized by the M06-2X/cc-pVTZ method in a vacuum, using the PCM model with chloroform and DMSO as solvent. The diketo form of the title compound proved to be the most energetically favourable as compared to the keto-enol or dienol forms. The diketo form can exist as three possible stereoisomers with the same configuration of one stereogenic center and different configurations of the stereogenic centers at two other atoms: (R, R, R), (S, R, S) and (R, R, S). The (R, R, S) stereoisomer was found in the crystal phase. It was revealed that the thiazine rings of equivalent benzothiazine fragments have different conformations, (a sofa or a half-chair). The two bicyclic fragments connected through the phenylmethylene group are oriented almost orthogonal to each other, subtending a dihedral angle of 82.16(7) .

Chemical context
Nowadays a countless number of heterocyclic scaffolds are being used in nearly every branch of industry, providing necessary properties to the final product. 2,1-Benzothiazine 2,2-dioxide belongs to an important class of heterocyclic cores. In particular, its structural features have led to its wide application in medicinal chemistry research, which is evidenced by the works related to the field as well as recent reviews (Vo & Ngo, 2022;Ukrainets et al., 2019;Chattopadhyay, 2018;Ahmad Saddique et al., 2021;Dobrydnev & Marco-Contelles, 2021).
It incorporates a -keto sultam fragment that allows a molecule to be a versatile synthetic intermediate used for the preparation of diverse molecular platforms (Ahmad et al., 2018;Grombein et al., 2015;D'Amico et al., 2007;Pieroni et al., 2012;Popov et al., 2010;Popov et al., 2009). Moreover, such a fragment is probably responsible for non-trivial reactivity as we have established previously (Lega et al., 2016c;Kolodyazhna et al., 2021). One of such unexpected outcomes was the formation of stable ammonium enolates as a result of interaction between 2,1-benzothiazin-4(3H)-one 2,2-dioxides and aldehydes in the presence of secondary or tertiary amines (Lega et al., 2016c(Lega et al., , 2017. This fact is quite interesting as similar bis-derivatives have previously been isolated in an acid form and not as a salt (Zanwar et al., 2012;Ye et al., 1999). The possibility of such salt formation is most probably caused by the raised CH-acidic properties of the methyne group (as the result of the electron-withdrawing influence of the SO 2 group), which leads to ease of enolization. Moreover, the intramolecular O-HÁ Á ÁO À. hydrogen bond increases the stability of such enolates.
Considering the uniqueness of salts 2 (Fig. 1), we have investigated their antimicrobial, analgesic, and anti-inflammatory properties (Lega et al., 2016a,b). Biological experiments have revealed that compounds 2 are promising platforms to search for a novel NSAID among them. For the reason of further modification of the compounds' structures, we decided to convert salts 2 into an acidic form with the prospect of a comparative study of their NSAID activity to that of the salts. The motive behind the modification method was the removal of the possible toxic amine fragment. Moreover, there was the assumption that the acidic environment of the stomach breaks down the salt and produces the acidic form of 2, which is the true bioactive part of the salt.
Reflux of 1-ethyl-2,1-benzothiazin-4(3H)-one 2,2-dioxide (3) with benzaldehyde (4) and triethylamine (5) (molar ratio 1:2:1) for 1 h in ethyl alcohol results in the triethylammonium salt 6 used in the study (Lega et al., 2016c) (Fig. 2). In order to achieve the planned acidic form, we exposed the salt to a solution of TsOH (1.5 equiv) in ethanol. Short heating of this mixture gave a fine crystalline substance, which was recrystallized from acetic acid and further analyzed. It is worth noting that the reaction can be accomplished by simple reflux of salt 6 in water for 15 h.
To our great surprise, we recorded an unexpected 1 H NMR spectrum (DMSO-d 6 , 200 MHz) with a complicated set of numerous signals (Fig. 3). Such a spectroscopic picture could be the result of a tautomeric conversion cycle of compound 7 initiated by proton movement in the dihydroxy tautomer 7A (Fig. 2). The prototropic transformations are apparently facilitated by the slight basic properties of the solvent (MacGregor, 1967). From the proton spectrum, one can conclude that the mixture contains the monohydroxy tautomer 7B (a singlet at 11.13 ppm) and diketo form 7C Preparation of triethylammonium salt 6, its hydrolysis and possible tautomeric interconversions of product 7 formed (chiral carbon atoms are starred).

Figure 3
1 (triplet at 5.75 ppm). We would like to emphasize that tautomers 7B and 7C contain several asymmetric carbon atoms and can exist as various optical isomers. The situation becomes more complicated with the SO 2 fragment that can be located in the crystals up or down the thiazine ring, creating an additional pseudo-chiral center as was reported previously (Ukrainets et al., 2016b). At the same time, the 1 H NMR spectrum (400 MHz) of 7 recorded in CDCl 3 solution indicates the presence of only one tautomer, 7C (Fig. 4). With uncertainty about the absolute structure of the isolated product, we decided to perform X-ray experiments to unambiguously establish the structure of compound 7. Moreover, we also aimed to calculate the stability of the tautomers and stereoisomers. The latter has a big value as the binding energy of different isomeric forms to biomolecules depends greatly on the structure and the absolute configuration. Moreover, as was stated in previous works, understanding keto-enol tautomerism is significant in order to substantiate critical biological applications of the tautomeric molecules and to comprehend their biochemical reactions (Tighadouini et al., 2022;Temperini et al., 2009).

Quantum-chemical study
To estimate the relative energies of tautomeric forms of the product 7, quantum-chemical calculations were performed. Dienol form 7A, keto-enol form 7B and diketo form 7C were optimized by the M06-2X/cc-pVTZ method (Zhao & Truhlar, 2007;Kendall et al., 1992) using GAUSSIAN09 software (Frisch et al., 2010). The vacuum approximation and PCM model (Mennucci, 2012) with chloroform or DMSO as a solvent for considering a polarizing environment were used. In addition, vibration frequencies were calculated for all of these optimized molecules, indicating a minimum on the potential energy surface. The results of the optimization showed that the diketo form 7C has the lowest energy (Table 1). Moreover, the diketo form can exist as three possible stereoisomers: 7C(R, R, R), 7C(S, R, S) and 7C(R, R, S). The results of the quantum-chemical calculations revealed that these stereoisomers have close energies, but the most energetically preferable stereoisomer is 7C(R, R, S). It should be noted that the use of the PCM model results in an increase of the energy difference between the dienol A and diketo C forms. In contrary to calculations in a vacuum approximation, the calculations considering a polarizing environment result in almost same energy for the stereoisomers.  Table 1 Relative energies (kcal mol À1 ) of tautomeric and stereoisomeric forms, calculated by the M06-2X/cc-pVTZ method. À2401

Figure 5
Molecular structure of the compound 7C(R,R,S). Only the major component of disorder is shown. Displacement ellipsoids are shown at the 50% probability level.

Supramolecular features
Analysis of the shortest distances between atoms of neighboring molecules of 7 does not reveal any strong intermolecular interactions in the crystal phase. Only two very weak C-HÁ Á ÁO interactions are found: C13-H13Á Á ÁO6(x, 1 2 À y, 1 2 + z) where the HÁ Á ÁO distance is 2.57 Å and the C-HÁ Á ÁO bond angle is 165 , and C23-H23Á Á ÁO2(1 + x, 1 2 À y,  Rowland & Taylor (1996), and 2.46 Å as calculated by Zefirov (1997). As can be seen, the interactions discussed above do not unambiguously indicate the existence of weak hydrogen bonds and thus a further study of the supramolecular features is needed.

Hirshfeld surface analysis
To identify and visualize different types of intermolecular interactions in the crystal structure, a Hirshfeld surface analysis (Turner et al., 2017) as implemented in program CrystalExplorer17 (Spackman et al., 2021) was used. This method allows the crystal space to be split into molecular domains and the detection of short distances between atoms of neighboring molecules. A standard (high) surface resolution with three-dimensional d norm surfaces in the range À0.129 to 1.589 a.u. was used to construct the molecular Hirshfeld surface of the title compound (Fig. 6). Red spots on the d norm surface were found only near to atoms H13 and O6 atoms participating in the   C13-HÁ Á ÁO6 hydrogen bond. No red spots on the Hirshfeld surface indicated the formation of an C23-HÁ Á ÁO2 interaction. Thus, only one C-HÁ Á ÁO intermolecular hydrogen bond can be discussed in the title structure. Molecules bound by this hydrogen bond form a chain in the [001] direction.
Taking into account the potential biological activity of the title compound, which presumes its interaction with a receptor, an analysis of the relative contributions of interactions of different types seems to be useful. All of the intermolecular interactions of the title compound are evident on the two-dimensional fingerprint plot presented in Fig. 7a. The presence of X-HÁ Á ÁO hydrogen bonds in the crystal structure could be indicated by high contribution of OÁ Á ÁH/ HÁ Á ÁO (33.7%) contacts and the sharp spikes in Fig. 7c. The contribution of CÁ Á ÁH/HÁ Á ÁC (16.3%) contacts, which are associated with X-HÁ Á Á interactions, are much lower (Fig. 7d), whereas the contribution of HÁ Á ÁH contacts is the highest at 47.4% (Fig. 7b).  (Shafiq et al., 2009c)]. Both of these two forms are presented in structure NAKZAD (Lega et al., 2016c). As can be seen, the most of the related compounds exist in the crystal phase in the enol tautomeric form. However, the keto tautomeric form of similar compounds also has been found in the crystal phase. This suggests that the difference in the relative energies of the keto and enol tautomeric forms is small enough and tautomeric equilibrium can be distorted during the crystallization process because of the influence of solvation effects.

Synthesis and crystallization
Triethylammonium salt 6 (Lega et al., 2016c) (0.639 g, 0.001 mol) was added to a solution of TsOH (0.258 g, 0.0015 mol) in EtOH (10 mL). The obtained mixture was heated at 343 K for 15 min and cooled to room temperature. The precipitate that formed was collected by filtration, washed with EtOH and dried in air, affording 0.53 g (98% yield) of the target product.

3,3′-(Phenylmethylene)bis(1-ethyl-3,4-dihydro-1H-2,1-benzothiazine-2,2,4-trione)
Crystal data 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.