Elemental Chalcogen (Se, S) in PEG-400 to the Synthesis of Seleno- and Thioflavones from 2-Chlorophenyl Ethynyl Ketone and Nucleophilic Species of Chalcogen

An alternative green method was developed for the synthesis of thio-and selenoflavones by the ring closure of 2-chlorophenyl ethynyl ketone with NaHY (Y = S, Se). These nucleophilic chalcogen species were generated in situ using NaBH4 to reduce the elemental chalcogen in the presence of polyethylene glycol-400 (PEG-400). The efficiency of this reaction is strongly dependent on the PEG-400 solvent, acting like a crown ether, complexing with the sodium atom of NaHY species, making the chalcogen nucleophile more active. The synthetic protocol proceeded efficiently at 100 °C under argon, using a range of 2-chlorophenyl ethynyl ketone containing alkyl, aryl, or vinyl groups and the sulfur and selenium chalcogen. By this efficient and simple approach, 18 chalcogenoflavones were obtained in good to excellent yields after 2 h.