Synthesis, study of antileishmanial and antitrypanosomal activity of imidazo pyridine fused triazole analogues

Abstract

Four groups, thirty-five compounds in total, of novel 1,2,3-triazole analogues of imidazo-[1,2-a]-pyridine-3-carboxamides were designed and synthesized using substituted pyridine, propargyl bromide, 2-azidoethyl 4-methyl benzenesulfonate and substituted acetylenes. These compounds were characterized using ¹H NMR, ¹³C NMR, LCMS and elemental analyses and a crystal structure was obtained for one of the significantly active compounds, 8f. All the synthesized and characterized compounds were screened in vitro for antileishmanial and antitrypanosomal activity against Leishmania major and Trypanosoma brucei parasites, respectively. Among the tested analogues, five compounds (8d, 8f, 8j, 10b and 10d) exhibited significant antileishmanial activity while three compounds (10b, 11a and 11b) showed substantial activity against T. brucei parasite. In silico ADME prediction studies depicted that the essential compounds obeyed Lipinski's rule of five. The predicted in silico toxicity profile suggested that the tested compounds would be non-toxic, which was confirmed experimentally by the lack of cytotoxicity against HeLa cells. Finally, a molecular docking study was also performed, for 10d the most active antileishmanial compound, to study its putative binding pattern at the active site of the selected leishmanial trypanothione reductase target.

Fig. 1. Imidazo-[1,2-a]-pyridine based drugs.

Fig. 2. Biologically active imidazo-[1,2-a]-pyridine derivatives.

Fig. 3. Imidazo-[1,2-a]-pyridine based derivatives as antileishmanial agents.

Fig. 4. Triazole containing drugs.

Fig. 5. Triazole based antileishmanial analogues.

Fig. 6. Piperazine linked antileishmanial derivatives.

Fig. 7. Design of the titled compounds 7a–j, 8a–p, 10a–d, and 11a–e.

Scheme 1. Preparation of intermediate compounds 5a and 5b. Reagents and conditions: (i) 1a/1b (1.0 eq.), ethyl 2-chloro-3-oxobutanoate (1.5 eq.), 1,4-dioxane, reflux, 36 h; (ii) LiOH (2.0 eq.), EtOH, water, 75 °C, 22 h; (iii) 1-Boc-piperazine (1.1 eq.), EDC·HCl (1.5 eq.), hydroxybenzotriazole (1.5 eq.), DIPEA (3 eq.) DMF, rt, 16 h; (iv) 4 M dioxane–HCl (2V) DCM, 0 °C to rt, 4 h.

Scheme 2. Preparation of title compounds 7a–j and 8a–p. Reagents and conditions: (v) 5a/5b (1.0 eq.), propargyl bromide (80% in toluene) (1.5 eq.), K₂CO₃ (4 eq.), DMF, 100 °C, 12 h; (vi) substituted azides (1.5 eq.), CuSO₄^·5H₂O, (10 mol%), sodium ascorbate (10 mol%), DMF : water (8 : 2), rt, 7–12 h.

Scheme 3. Preparation of title compounds 10a–d and 11a–e. Reagents and conditions: (vii) 5a/5b (1.0 eq.), 2-azidoethyl 4-methylbenzenesulfonate (1.5 eq.), K₂CO₃ (4 eq.), DMF, 100 °C, 12 h; (viii) substituted terminal alkynes (1.5 eq.), CuSO₄^·5H₂O, (10 mol%), sodium ascorbate (10 mol%), DMF : water (8 : 2), rt, 7–12 h.

Fig. 8. Superimposed view of the FAD molecule in the active site of the target PDB-2JK6, and its redocked pose in the same target (RMSD – 0.20 Å).

Fig. 9. Exposed amino-acid residue interactions of the FAD molecule in the actives site of the target PDB-2JK6 (black lines – hydrogen bonds, blue – pi interaction in 3D, magenta – hydrogen bond, green – pi interaction in 2D).

Fig. 10. Exposed amino-acid residue interactions of the significantly active compound 10d in the actives site of the target PDB-2JK6 (yellow lines – hydrogen bonds, blue – pi interaction in 3D, magenta – hydrogen bond in 2D).

Fig. 11. ORTEP diagram of compound 8f.

Fig. 12. Packing diagram of 8f showing pi–pi stacking arrangement as observed along a (left) and c (right) axis.