Two pseudo-enantiomeric forms of N-benzyl-4-hydroxy-1-methyl-2,2-dioxo-1H-2λ(6),1-benzothiazine-3-carboxamide and their analgesic properties

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 5-chloro-4-hy-droxy-2,2-dioxo-1H-2λ6,1-benzo-thia-zine-3-carboxyl-ate: structure and Hirshfeld surface analysis

The title compound, C10H8ClNO5S, which has potential analgesic activity, crystallizes in space group P21/n. The benzo-thia-zine ring system adopts an inter-mediate form between sofa and twist-boat conformations. The coplanarity of the ester substituent to the bicyclic fragment is stabilized by an O-H⋯O intra-molecular hydrogen bond. In the crystal, hydrogen bonds of type N-H⋯O(SO2) link the mol-ecules into zigzag chains extending along the b-axis direction. Neighbouring chains are linked by both O-H⋯Cl and C-H⋯Cl inter-actions. A Hirshfeld surface analysis was used to compare different types of inter-molecular inter-actions, giving contributions of O⋯H/H⋯O = 42.0%, C⋯H/H⋯C = 17.3%, Cl⋯H/H⋯Cl = 14.2%, H⋯H = 11.1%.

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.

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

The title benzothiazine-3-carboxamide, C₁₇H₁₆N₂O₄S, crystallized in two enantiomorphic crystal forms with the space groups P3₂ and P3₁ despite the absence of a classic stereogenic atom. The molecular structures are mirror images of each other. Only one sulfonyl O atom takes part in intramolecular hydrogen bonding as a proton acceptor and this atom is different in the two enantiomorphic structures. As a result, the S atom becomes a pseudo-stereogenic centre. This fact is worth taking into account due to the different biological activities of the enantiomorphic forms. One form possesses a high analgesic activity, while the other form revealed a high anti-inflammatory activity.

Synthesis and Regularities of the Structure–Activity Relationship in a Series of N-Pyridyl-4-methyl-2,2-dioxo-1H-2λ6,1-benzothiazine-3-carboxamides

According to our quantum and chemical calculations 4-methyl-2,2-dioxo-1H-2λ6,1-benzothiazine-3-carboxylic acid imidazolide is theoretically almost as reactive as its 2-carbonyl analog, and it forms the corresponding N-pyridyl-4-methyl-2,2-dioxo-1H-2λ6,1-benzothiazine-3-carboxamides with many aminopyridines. However, in practice, the sulfo group introduces significant changes at times and prevents the acylation of sterically hindered amines. One of these products was 2-amino-6-methylpyridine. Thus, it has been concluded that aminopyridines interact with imidazolide in aromatic form where the target for the initial electrophilic attack is the ring nitrogen. To confirm the structure of all substances synthesized, 1H-NMR spectroscopy and X-ray diffraction analysis were used. From X-ray diffraction data it follows that in the crystalline phase the carbonyl and sulfo group may occupy different positions with respect to the plane of the benzothiazine bicycle: this position may be unilateral, typical for 4-methyl-2,2-dioxo-1H-2λ6,1-benzothiazine-3-carboxamides, versatile, and not yet encountered in compounds of this type. A comparison of these data with the results of the pharmacological screening conducted on the standard model of carrageenan inflammation showed that the N-pyridylamides of the first group demonstrated a direct dependence of their analgesic and anti-inflammatory activity on the mutual arrangement of the planes of the benzothiazine and pyridine fragments. The new molecular conformation of the benzothiazine nucleus provides a sufficiently high level of analgesic (but not anti-inflammatory) properties in all N-pyridylamides of the second group with an extremely weak dependence on the spatial arrangement of the pyridine cycle. All substances presented this article proved themselves in varying degrees as analgesics and antiphlogistics. Moreover, two of them—N-(5-methylpyridin-2-yl)- and N-(pyridin-3-yl)-4-methyl-2,2-dioxo-1H-2λ6,1-benzothiazine-3-carboxamides—exceeded the most effective drug of oxicam type Lornoxicam by these indicators.

Molecular Conformations and Biological Activity of N-Hetaryl(aryl)alkyl-4-methyl-2,2-dioxo-1H-2λ6,1-benzothiazine-3-carboxamides

The analysis of our previous studies on the search for synthetic analgesics among N-R-amides of bicyclic hetaryl-3-carboxylic acids has been performed; on its basis N-hetaryl(aryl)-alkyl-4-methyl-2,2-dioxo-1H-2λ6,1-benzothiazine-3-carboxamides have been selected as new study objects. The “one pot synthesis” of these compounds, which is simple to perform and at the same time highly effective, has been offered. The method consists in the initial reaction of 4-methyl-2,2-dioxo-1H-2λ6,1-benzothiazine-3-carboxylic acid and N,N′-carbonyldiimidazole in anhydrous N,N-dimethylformamide with the subsequent amidation of imidazolide formed with hetarylalkyl- or benzylamines in the same solvent. The peculiarities of 1H- and 13C-NMR spectra of the substances obtained, as well as their electrospray ionization liquid chromato-mass spectra are discussed. According to the results of the pharmacological tests carried out on the model of carrageenan inflammation it has been found that all without exception N-hetaryl(aryl)alkyl-4-methyl-2,2-dioxo-1H-2λ6,1-benzothiazine-3-carboxamides demonstrate the statistically significant analgesic and anti-inflammatory properties. Among the substances presented in this article analgesics and antiphlogistics, which increase the pain threshold and suppress the inflammatory response more effectively than Lornoxicam and Diclofenac in the same doses, have been identified. The molecular and crystal structures of a large group of the substances synthesized have been studied by X-ray diffraction analysis. Comparison of these data with the results of biological tests has revealed the fact of excellent correlation between the molecular conformations of N-hetaryl(aryl)alkyl-4-methyl-2,2-dioxo-1H-2λ6,1-benzothiazine-3-carboxamides recorded in the crystal and the potency of their analgesic effect. N-Thiophen-2-ylmethyl- and N-4-methoxybenzyl-amides of 4-methyl-2,2-dioxo-1H-2λ6,1-benzothiazine-3-carboxylic acid has shown a high analgesic and anti-inflammatory effect, therefore, they deserve more careful research.

4-Methyl-2,2-dioxo-1H-2λ⁶,1-benzothiazine-3-carboxylic Acid. Peculiarities of Preparation, Structure, and Biological Properties

In order to determine the regularities of the structure–analgesic activity relationship, the peculiarities of obtaining, the spatial structure, and biological properties of 4-methyl-2,2-dioxo-1H-2λ⁶,1-benzothiazine-3-carboxylic acid and some of its derivatives have been studied. Using nuclear magnetic resonance (NMR) spectroscopy and X-ray diffraction analysis, it has been proven that varying the reaction conditions using alkaline hydrolysis of methyl 4-methyl-2,2-dioxo-1H-2λ⁶,1-benzothiazine-3-carboxylate makes it possible to successfully synthesize a monohydrate of the target acid, its sodium salt, or 4-methyl-2,2-dioxo-1H-2λ⁶,1-benzothiazine. The derivatographic study of the thermal stability of 4-methyl-2,2-dioxo-1H-2λ⁶,1-benzothiazine-3-carboxylic acid monohydrate has been carried out; based on this study, the optimal conditions completely eliminating the possibility of unwanted decomposition have been proposed for obtaining its anhydrous form. It has been shown that 4-methyl-2,2-dioxo-1H-2λ⁶,1-benzothiazine is easily formed during the decarboxylation of not only 4-methyl-2,2-dioxo-1H-2λ⁶,1-benzothiazine-3-carboxylic acid, but also its sodium salt, which is capable of losing СО₂ both in rather soft conditions of boiling in an aqueous solution, and in more rigid conditions of dry heating. The NMR spectra of the compounds synthesized are given; their spatial structure is discussed. To study the biological properties of 4-methyl-2,2-dioxo-1H-2λ⁶,1-benzothiazine-3-carboxylic acid and its sodium salt, the experimental model of inflammation caused by subplantar introduction of the carrageenan solution in one of the hind limbs of white rats was used. The anti-inflammatory activity and analgesic effect were assessed by the degree of edema reduction and the ability to affect the pain response compared to the animals of control groups. According to the results of the tests performed, it has been found that after intraperitoneal injection, the substances synthesized demonstrate a moderate anti-inflammatory action and simultaneously increase the pain threshold of the experimental animals very effectively, exceeding Lornoxicam and Diclofenac in a similar dose by their analgesic activity.

New Synthesis, Structure and Analgesic Properties of Methyl 1-R-4-Methyl-2,2-Dioxo-1H-2λ⁶,1-Benzothiazine-3-Carboxylates

According to the principles of the methodology of bioisosteric replacements a series of methyl 1-R-4-methyl-2,2-dioxo-1H-2λ⁶,1-benzothiazine-3-carboxylates has been obtained as potential analgesics. In addition, a fundamentally new strategy for the synthesis of compounds of this chemical class involving the introduction of N-alkyl substituent at the final stage in 2,1-benzothiazine nucleus already formed has been proposed. Using nuclear magnetic resonance (NMR) spectroscopy, mass spectrometry and X-ray diffraction analysis it has been proven that in the DMSO/K₂CO₃ system the reaction of methyl 4-methyl-2,2-dioxo-1H-2λ⁶,1-benzothiazine-3-carboxylate and alkyl halides leads to formation of N-substituted derivatives with good yields regardless of the structure of the alkylating agent. The peculiarities of NMR (¹Н and ¹³С) spectra of the compounds synthesized, their mass spectrometric behavior and the spatial structure are discussed. In N-benzyl derivative the ability to form a monosolvate with methanol has been found. According to the results of the pharmacological testing conducted on the model of the thermal tail-flick it has been determined that replacement of 4-ОН-group in methyl 1-R-4-hydroxy-2,2-dioxo-1H-2λ⁶,1-benzothiazine-3-carboxylates for the methyl group is actually bioisosteric since all methyl 1-R-4-methyl-2,2-dioxo-1H-2λ⁶,1-benzothiazine-3-carboxylates synthesized demonstrated a statistically significant analgesic effect. The majority of the substances can inhibit the thermal pain response much more effective than piroxicam in the same dose. Under the same conditions as an analgesic the N-methyl-substituted analog exceeds not only piroxicam, but more active meloxicam as well. Therefore, it deserves in-depth biological studies on other experimental models.

Synthesis, Spatial Structure and Analgesic Activity of Sodium 3-Benzylaminocarbonyl-1-methyl-2,2-dioxo-1H-2λ⁶,1-benzothiazin-4-olate Solvates

In order to obtain and then test pharmocologically any possible conformers of the new feasible analgesic N-benzyl-4-hydroxy-1-methyl-2,2-dioxo-1H-2λ⁶,1-benzothiazine-3-carboxamide, its 4-O-sodium salt was synthesized using two methods. X-ray diffraction study made possible to determine that, depending on the chosen synthesis conditions, the above-mentioned compound forms either monosolvate with methanol or monohydrate, where organic anion exists in the form of three different conformers. Pharmacological testing of the two known pseudo-enantiomeric forms of the original N-benzylamide and of the two solvates of its sodium salt was performed simultaneously under the same conditions and in equimolar doses. Comparison of the results obtained while studying the peculiarities of the synthesized compounds spatial structure and biological properties revealed an important structure-action relationship. In particular, it was shown that the intensity of analgesic effect of different conformational isomers of N-benzyl-4-hydroxy-1-methyl-2,2-dioxo-1H-2λ⁶,1-benzothiazine-3-carboxamide may change considerably: while low active conformers are comparable with piroxicam, highly active conformers are more than twice as effective as meloxicam.