3,3'-(Phenyl-methyl-ene)bis-(1-ethyl-3,4-di-hydro-1 H -2,1-benzo-thia-zine-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 anti-microbial, 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 chloro-form 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 thia-zine rings of equivalent benzo-thia-zine fragments have different conformations, (a sofa or a half-chair). The two bicyclic fragments connected through the phenyl-methyl-ene group are oriented almost orthogonal to each other, subtending a dihedral angle of 82.16(7 ) °.