The different modes of chiral [1,2,3]triazolo[5,1-b][1,3,4]thiadiazines: crystal packing, conformation investigation and cellular activity

Abstract

The crystal structures of four new chiral [1,2,3]triazolo[5,1-b][1,3,4]thiadiazines are described, namely, ethyl 5′-benzoyl-5′H,7′H-spiro[cyclohexane-1,6′-[1,2,3]triazolo[5,1-b][1,3,4]thiadiazine]-3′-carboxylate, C19H22N4O3S, ethyl 5′-(4-methoxybenzoyl)-5′H,7′H-spiro[cyclohexane-1,6′-[1,2,3]triazolo[5,1-b][1,3,4]thiadiazine]-3′-carboxylate, C20H24N4O4S, ethyl 6,6-dimethyl-5-(4-methylbenzoyl)-6,7-dihydro-5H-[1,2,3]triazolo[5,1-b][1,3,4]thiadiazine-3-carboxylate, C17H20N4O3S, and ethyl 5-benzoyl-6-(4-methoxyphenyl)-6,7-dihydro-5H-[1,2,3]triazolo[5,1-b][1,3,4]thiadiazine-3-carboxylate, C21H20N4O4S. The crystallographic data and cell activities of these four compounds and of the structures of three previously reported similar compounds, namely, ethyl 5′-(4-methylbenzoyl)-5′H,7′H-spiro[cyclopentane-1,6′-[1,2,3]triazolo[5,1-b][1,3,4]thiadiazine]-3′-carboxylate, C19H22N4O3S, ethyl 5′-(4-methoxybenzoyl)-5′H,7′H-spiro[cyclopentane-1,6′-[1,2,3]triazolo[5,1-b][1,3,4]thiadiazine]-3′-carboxylate, C19H22N4O4S, and ethyl 6-methyl-5-(4-methylbenzoyl)-6-phenyl-6,7-dihydro-5H-[1,2,3]triazolo[5,1-b][1,3,4]thiadiazine-3-carboxylate, C22H22N4O3S, are contrasted and compared. For both crystallization and an MTT assay, racemic mixtures of the corresponding [1,2,3]triazolo[5,1-b][1,3,4]thiadiazines were used. The main manner of molecular packing in these compounds is the organization of either enantiomeric pairs or dimers. In both cases, the formation of two three-centre hydrogen bonds can be detected resulting from intramolecular N—H...O and intermolecular N—H...O or N—H...N interactions. Molecules of different enantiomeric forms can also form chains through N—H...O hydrogen bonds or form layers between which only weak hydrophobic contacts exist. Unlike other [1,2,3]triazolo[5,1-b][1,3,4]thiadiazines, ethyl 5′-benzoyl-5′H,7′H-spiro[cyclohexane-1,6′-[1,2,3]triazolo[5,1-b][1,3,4]thiadiazine]-3′-carboxylate contains molecules of only the (R)-enantiomer; moreover, the N—H group does not participate in any significant intermolecular interactions. Molecular mechanics methods (force field OPLS3e) and the DFT B3LYP/6-31G+(d,p) method show that the compound forming enantiomeric pairs via weak N—H...N hydrogen bonds is subject to greater distortion of the geometry under the influence of the intermolecular interactions in the crystal. For intramolecular N—H...O and S...O interactions, an analysis of the noncovalent interactions (NCIs) was carried out. The cellular activities of the compounds were tested by evaluating their antiproliferative effect against two normal human cell lines and two cancer cell lines in terms of half-maximum inhibitory concentration (IC50). Some derivatives have been found to be very effective in inhibiting the growth of Hela cells at nanomolar and submicromolar concentrations with minimal cytotoxicity in relation to normal cells.