Synthesis and antibacterial activity of new 1,2,3-triazolylmethyl-2H-1,4-benzothiazin-3(4H)-one derivatives

Background A novel series of 1,2,3-triazole derivatives containing 1,4-benzothiazin-3-one ring (7a–9a, 7b–9b), (10a–12a, 10b–12b) and (13–15) were synthesized by 1,3-dipolar cycloaddition reactions of azides α-d-galactopyranoside azide F, 2,3,4,6-tetra-O-acetyl-(d)-glucopyranosyl azide G and methyl-N-benzoyl-α-azidoglycinate H with compounds 4–6. Findings Initially, the reactions were conducted under thermal conditions in ethanol. The reaction leads, each time, to the formation of two regioisomers: (Schemes 2, 3) with yields of 17 to 21% for 1,5-disubstituted 1,2,3-triazole-regioisomers (7b–12b) and yields ranging from 61 to 65% for the 1,4-disubstituted regioisomers (7a–12a). In order to report an unequivocal synthesis of the 1,4-regioisomers and confirm the structures of the two regioisomers obtained in thermal conditions (Huisgen reactions), the method click chemistry (Copper-Catalyzed Azide-Alkyne Cycloaddition) has been used. Conclusions The newly synthesized compounds using cycloaddition reactions were evaluated in vitro for their antibacterial activities against some Gram positive and Gram negative microbial strains. Among the compounds tested, the compound 8a showed excellent antibacterial activities against PA ATCC and Acin ESBL (MIC = 31.2 μg/ml).

The structures of compounds isolated have been identified on the basis of 1 H NMR and 13 C NMR spectral data. The 1 H NMR spectrum of the compounds 4-6 in DMSO d 6 shows signals for the propargyl group as a doublet at 4.74, 4.90 and 4.86 ppm, respectively and a triplet centered at 2.20 (2.21) and 3.31 ppm corresponding to methylene groups bonded to the nitrogen atom and acetylenic HC≡C-proton, respectively. The 13 C NMR spectrum showed the signal of hydrogenated acetylenic carbon at 75.0, 75.5 and 75.47 ppm, respectively. The structures of compounds 4 and 5 were confirmed by a crystallographic studies [22,23] (Fig. 2).
The crystallographic study confirms that compounds 5, 6 have Z configuration about the exocyclic double bond. This result will allow to assign the Z configuration to all compounds coming from the products 5, 6 in future ulterior cycloaddition reactions the dipolarophiles 4-6 are then involved in cycloaddition reactions with the dipoles given above leading to new benzothiazine derivatives containing various 1,2,3-triazole moieties able to modulate their biological activities [24,25].
It should be noted that when compounds 4-6 reacted with azide H it has allowed us to isolate in each case only one isomer 13-15 (Scheme 3) with yields between 77 and 83%. For compounds 13-15 the 1 H NMR in DMSO d 6 exhibit in particular signals as singlets at 5.16 (13) The crystallographic analysis of compound 13 indicates that the triazole nitrogen atom is unsubstituted and confirms the structures of compounds 13-15 (Figs. 3 and 4). It is interesting to note that compound 13 crystallizes in monoclinic system (P2 1 /c). The crystallographic data have been assigned to the deposition number. CCDC 1564624.
The formation of compounds 13-15 suggests that the reaction operates via a traditional mechanism of 1,3-dipolar cycloaddition of azide H with alkynes 4-6, followed by a transesterification. The nucleophilic substitution of triazole unit by ethanol leads to compounds 13-15 next to the glycine derivative 16, Scheme 4.

Biological evaluation in vitro antibacterial evaluation
The compounds tested showed an average antibacterial activity and the results of the assessments are shown in Fig. 5 and Table 1.
Finally the compound 10b obtained by cycloaddition with azide G the results of the antibacterial activity of the products tested showed the absence of growth inhibition for compound 10b towards all tested bacteria. In general, the molecular specifications of the 1,2,3-triazoles can also be used as a linker and show bioisosteric effects on peptide linkage, aromatic ring, double bonds. Some unique features like hydrogen bond formation, dipoledipole and π stacking interactions of triazole compounds have increased their importance in the field of medicinal chemistry as they bind with the biological target with high affinity due to their improved solubility. This study is expected to take anti-inflammatory tests, antifungal, antiparasitic and anti-cancer, because the literature gives a lot of interesting results on these topics. Also, other bacteria should be selected to expand the investigation [36][37][38]. The 1,2,3-triazole based heterocycles have been well exploited for the generation of many medicinal scaffolds exhibiting anti-HIV, anticancer, antibacterial activities.

Conclusion
In conclusion, in the development of this work, the synthesis of the new heterocyclic systems derived from 1,2,3-triazolyl-1,4-benzothiazin-3-one was carried out in satisfactory yields by cycloaddition reactions under thermal and catalytic conditions (Cu I). The results showed a periselectivity and regioselectivity as a function of the dipole (azides F, G and H) 1, 2, 4, 5, 7a, 7b, 8a, 8b, 10a, 10b, 11a and 11b  obtained results highlight an original synthesis reaction of 1,2,3-triazoles monosubstituted by the action of azideglycine (H) on dipolarophiles 4-6 under thermal conditions. The heterocyclic systems obtained were identified by 1 H NMR, 13 C NMR, and confirmed for product 13 by X-ray diffraction. The synthesized products were subjected to the evaluation of antibacterial activity. Several compounds tested showed significant activity.

Experimental part
General: Column chromatography was performed on silica gel 60 (Merck 230-400 mesh). Nuclear magnetic resonance spectra were recorded on a Varian Unity Plus spectrometer 1 H NMR at 300 MHz; the chemical shifts (d) are expressed in parts per million (ppm) and the coupling constants (J) in Hertz (Hz). DMSO was used as the solvent and SiMe4 as the reference.

General procedure of synthesis compounds 4, 5 and 6
To a solution of (6.05 mmol) of 2-substituted)-1,4-benzothiazin-3-one 1 (2 or 3) in 15 ml of DMF, were added 11.3 mmol of potassium carbonate. The reaction mixture was stirred magnetically for 5 min then added 0.6 mmol of bromide tetra-nbutylammonium (BTBA) and 7.26 mmol of propargyl bromide, then the mixture was stirred magnetically for 24 h. After removal of salts by filtration, the solution was evaporated under reduced pressure, and the residue obtained is dissolved in dichloromethane. The remaining salts are extracted with distilled water, and the mixture obtained was chromatographed on silica gel column [eluent: ethyl acetate/hexane (1/9)].

General procedure for the synthesis of compounds 7a-12a, 7b-12b and 13-15 via Huisgen 1,3-dipolar cycloaddition reactions
To a solution of dipolarophile 4 (5 or 6) (8 mmol) in absolute ethanol (20 ml) was added azide F (G or H) (16 mmol). The reaction mixture was stirred at reflux and the reaction monitored by thin layer chromatography (TLC). After concentration under reduced pressure, the residue was purified by column chromatography on silica gel using a mixture [ethyl acetate/hexane (1/9)] as eluent.

General procedure for the synthesis of compounds 7a-12a by click chemistry: [Copper-Catalyzed Azide-Alkyne Cycloaddition (CuAAC)]
To a solution of 1 mmol of compound 4 (5 or 6) and 2 mmol of azide F (G) in 15 ml of ethanol were added 0.5 mmol of CuSO 4 and 1 mmol of sodium ascorbate dissolved in 7 ml of distilled water. The reaction mixture was stirred for 24 h at room temperature. The reaction was monitored by TLC. After filtration and concentration of the solution under reduced pressure the residue obtained was chromatographed on silica gel column using as eluent ethyl acetate/hexane (1/9). The compounds have been obtained with yields ranging from 86 to 90%.