Biological properties of two enantiomorphic forms of N-(2,6-dimethylphenyl)-4-hydroxy-2,2-dioxo-1H-2λ6,1-benzothiazine-3-carboxamide, a structural analogue of piroxicam

1-Allyl-4-hydroxy-2,2-dioxo-N-(4-methoxyphenyl)-1H-2λ6,1-benzothiazine-3-carboxamide: polymorphic transition due to grinding with the loss of the biological activity

A study of two polymorphic forms of 1-allyl-4-hydroxy-2,2-dioxo-N-(4-methoxyphenyl)-1-2λ⁶,1-benzothiazine-3-carboxamide (a structural analogue of piroxicam) has revealed some regularities in the crystal structure formation due to different evaporation rates from the tested solvents. The monoclinic polymorph crystallized from ethyl acetate is formed due to a large number of very weak C-H...O and C-H...π interactions as well as one strong stacking interaction. The triclinic polymorph crystallized from N,N-dimethylformamide is formed due to a small number of weak specific interactions and a maximal number of strong stacking interactions. The stacked dimer is a complex building unit in both polymorphic structures. Further analysis showed that the monoclinic structure is layered while the triclinic one is columnar. The two polymorphic structures also differ in their biological activity (antidiuretic and analgesic). The monoclinic polymorph possesses very high biological activity while the triclinic polymorph is almost inactive. The polymorphic transition of the biologically active metastable monoclinic structure into the inactive stable triclinic one within four weeks of grinding is caused by orientational factors rather than conformational ones and is accompanied by a change in the redistribution of interaction energies in the crystal from anisotropic to more isotropic. Thus, a slow polymorphic transition after grinding results in a loss of the biological activity.

Methyl 4-Hydroxy-2,2-Dioxo-1H-2λ6,1-Benzothiazine-3-Carboxylate and Its Analogs Modified in the Benzene Moiety of the Molecule as New Analgesics

In order to identify new regularities of the “structure–analgesic activity” relationship in the series of 2,1-benzothiazine derivatives, the synthesis of methyl 4-hydroxy-2,2-dioxo-1H-2λ6,1-benzothiazine-3-carboxylate and a group of its analogs substituted in the benzene moiety of the molecule, as well as their mono-and diammonium salts, was performed with tris(hydroxymethyl)aminomethane. The algorithm was proposed; it allows for uniquely solving the question of the nature of the substituent and its true position in the benzothiazine core based on the complex use of NMR (1H and 13C) and mass spectrometry data. Using single-crystal X-ray diffraction analysis it was proven that salt formation first passes through the cyclic sulfamide group and only then through the 4-hydroxyl group, and is always accompanied by a significant conformational rearrangement of the molecule. Based on the results of pharmacological tests it was found that modification of the benzene moiety of the molecule can be used as a method for enhancing the analgesic properties of the class of compounds studied. The presence of a substitute in position 7 is particularly effective, regardless of its nature. A comparative analysis of the analgesic activity of the initial esters and their mono- and diammonium salts convincingly showed that the common belief about a direct relationship between the solubility of a substance and the level of its biological effect is not always true. As it turned out, increasing the solubility in water can lead to a variety of consequences: From a significant increase in analgesia to its complete elimination. It was suggested that the analgesic activity of the compounds studied is determined not by solubility, but by the molecular conformations formed during their obtainment.