Microwave-assisted synthesis of base-modified fluorescent 1,4-dihydropyridine nucleosides: photophysical characterization and insights

Abstract

A synthesis of a small library of fluorescent 1,4-dihydropyridine nucleoside analogues has been successfully carried out under solvent-free conditions via a one-pot three-component Hantzsch condensation reaction. The process involved a Ba(NO₃)₂ catalyzed solvent-free reaction between 3′,5′-di-O-acetyl-5-formyl-2′-deoxyuridine, differently substituted β-keto ester and ammonium acetate under microwave irradiation. This facile methodology yielded the desired products with very high yields (86–96%) under solvent-free reaction conditions in a short reaction time, which was followed by a simple workup. Yields obtained under microwave and conventional heating were compared, with the microwave irradiation condition displaying superior results. The synthesized compounds were characterized by IR, ¹H-NMR, ¹³C-NMR, ¹H–¹H COSY, ¹H–¹³C HETCOR, 2D NOESY NMR and HRMS analysis. These nucleoside analogues exhibited significant fluorescence, with a prominent emission band around 460 nm (excitation at 235 nm). Photophysical studies revealed strong fluorescence intensity, excellent Stokes shifts (70–162 nm), and high quantum yields (0.022–0.579), outperforming other pyrimidine-based fluorescent nucleosides. Notably, 5-(diethyl 2′′,6′′-propyl-1′′,4′′-dihydropyridine-3′′,5′′-dicarboxylate)-4′′-yl-2′-deoxyuridine demonstrated a quantum yield as high as 0.579 in DMSO during solvatochromic studies, highlighting their potential for probing local nucleic acid structure and dynamics. Additionally, we demonstrated the scalability of the synthesis protocol by producing one of the compounds on a gram scale, confirming its practical viability for large-scale production. This study underscores the potential of these fluorescent nucleoside analogues for various biochemical applications.