Multicomponent catalyst-free regioselective synthesis and binding studies of 3-aroyl-2-methylimidazo[1,2- a]pyrimidines with BSA using biophysical and computational techniques

Abstract

A facile and environmentally benign protocol for regioselective synthesis of diversely substituted imidazo[1,2-a]pyrimidines 5a-h has been described via multicomponent reaction of unsymmetrical β-diketones 1, N-bromosuccinimide 2 and 2-aminopyrimidine 4 in DCM. The reaction proceeds through in situ formation of α-bromo-β-diketones 3 and their ensuing condensation with 2-aminopyrimidine without the need of any organic or inorganic catalyst. The structure of the regioisomeric product was characterized by ¹H, ¹³C NMR, heteronuclear 2D NMR and HRMS studies. The present protocol offers several advantages such as avoidance of metal-based and toxic catalysts, broad substrate scope with respect to substitutions on β-diketones, operational simplicity, easy work-up and high yields. Computational molecular docking studies were carried out to examine the interaction of imidazo[1,2-a]pyrimidines with bovine serum albumin (BSA). Moreover, different spectroscopic approaches viz. UV-visible, steady-state fluorescence and competitive displacement assays were carried out to investigate the binding mechanisms of imidazo[1,2-a]pyrimidines (5c, 5e and 5h) with BSA. The results thus obtained revealed that imidazo[1,2-a]pyrimidines showed moderate binding with BSA through a static quenching mechanism and compound 5e had more affinity to bind in site I of BSA.

Fig. 1. Representative molecules containing imidazo[1,2-a]pyrimidine scaffold.

Scheme 1. Retrosynthetic approach for the construction of imidazo[1,2-a]pyrimidine.

Scheme 2. Sequential and multicomponent regioselective synthesis of 3-aroylimidazo[1,2-a]pyrimidines.

Fig. 2. ¹H (in red), ¹³C (in blue) and ¹⁵N (in green) NMR chemical shift values and 2D NMR correlation illustration for compound 5a and 5c.

Scheme 3. Plausible mechanism for regioselective synthesis of imidazo[1,2-a]pyrimidine.

Fig. 3. (a and b) 2D and 3D poses of the interaction of ligand 5e with BSA protein.

Fig. 4. (a–j) 2D diagram showing of the interaction of ligand 5a–h, phenylbutazone (PBZ) and ibuprofen (IBP) with BSA protein.

Fig. 5. UV-visible spectra of BSA-ligand complex system at increasing concentrations of ligand at constant BSA concentration of 15 μM in physiological pH 7.2 of Tris–HCl buffer at room temperature. (a) BSA spectra at variable concentration of 5e (0–40 μM); (b) BSA spectra at variable concentration of 5h (0–28 μM); (c) BSA spectra at variable concentration of 5c (0–36 μM). (d) Job's plot for BSA-5e complex system.

Fig. 6. Emission spectra of BSA (15 μM) and in presence of increasing concentration of compounds (a) 5e (0–60 μM), (b) 5h (0–56 μM), (c) 5c (0–56 μM) and (d) emission spectra of ligand 5e (0.1 mM) and in presence of increasing concentration of BSA (0–40 μM).

Fig. 7. (a) Stern–Volmer plot of BSA quenching by compounds 5e, 5h and 5c. (b) Double logarithmic plot employed to determine binding parameters for compounds 5e, 5h and 5c.

Fig. 8. Fluorescence spectra of (a) BSA-PBZ and (b) BSA-IBP complex in the absence and presence of increasing concentration of 5e (0–62 μM) at 298 K. (c) Double logarithmic plot employed to determine binding constants for compound 5e the absence (DMSO) and presence of site markers, ibuprofen and phenylbutazone.