Synthesis and characterization of coumarin-derived sulfur analogues using Lawesson's reagent

The synthesis and characterization of six novel coumarin derivatives containing O and S atoms are described here, namely, ethyl 2-oxo-2H-chromene-3-carboxylate, C₁₂H₁₀O₄ (S1a), ethyl 2-sulfanylidene-2H-chromene-3-carboxylate, C₁₂H₁₀O₃S (S2a), ethyl 2-sulfanylidene-2H-chromene-3-carbothioate, C₁₂H₁₀O₂S₂ (S3a), ethyl 8-methoxy-2-oxo-2H-chromene-3-carboxylate, C₁₃H₁₂O₅ (S1b), ethyl 8-methoxy-2-sulfanylidene-2H-chromene-3-carboxylate, C₁₃H₁₂O₄S (S2b), and ethyl 8-methoxy-2-sulfanylidene-2H-chromene-3-carbothioate, C₁₃H₁₂O₃S₂ (S3b). Compounds S1a/b were synthesized in good yields following the Knoevenagel condensation method. The thiocarbonyl analogues of these compounds, S2a/b and S3a/b, were obtained using Lawesson's reagent as a thionating compound. The structures of S2a/b and S3a/b were confirmed using FT-IR, ¹H and ¹³C NMR, and UV-Vis spectroscopy, and single-crystal X-ray diffraction. Hirshfeld surface and energy framework analyses show that stacked π-π ring interactions occur for all the structures obtained here, and various hydrogen-bond interactions link the stacks to form three-dimensional energy frameworks.