Synthesis of base-modified fluorescent furo[3,2-c]coumarin nucleosides and their photophysical studies

A small library of fluorescent furo[3,2-c]coumarin nucleoside analogues has been successfully synthesized via a one-pot condensation reaction of 3′,5′-di-O-acetyl-5-formyl-2′-deoxyuridine, alkyl isocyanides, and differently substituted 4-hydroxycoumarins in acetonitrile at 80 °C. Further, the synthesized compounds have been characterized by IR, ¹H NMR, ¹³C NMR, ¹H–¹H COSY, ¹H–¹³C HETCOR, 2D NOESY NMR experiments, HRMS measurements and single crystal X-ray structure analysis of compound 5-(2′'-N-(cyclohexyl)-amino-4′'H-furo[3,2-c]chromen-4-one-3′'-yl)-2′-deoxyuridine. The electronic structure of these modified nucleosides has been examined by Density Functional Theory (DFT). The synthesized nucleoside analogues exhibited a prominent emission spectrum, featuring a band around 500 nm (with excitation at 380 nm). Photophysical investigations revealed a noteworthy fluorescence intensity, along with excellent Stokes shift values (54–194 nm) and higher quantum yields (0.010–0.515) in comparison to other pyrimidine-based fluorescent nucleosides. The solvatochromic studies of 5-(2′'-N-(cyclohexyl)-amino-8′'‑chloro-4′'H-furo[3,2-c]chromen-4-one-3′'-yl)-2′-deoxyuridine determined the quantum yield (Φ_F) to be 0.515 in DMSO (λ_abs = 388 nm). This emphasizes the potential utility of these modified nucleosides in probing the local structure and dynamics of nucleic acids. Moreover, the scalability of the synthesis protocol was effectively demonstrated by producing one of the compounds on a gram scale, exhibiting its practical viability for large-scale production.