Exploring the unexpected formation of spirobibenzopyrans and benzopyrylium salts and effect of Lewis acids on the Claisen-Schmidt reaction

Abstract

Abstract Unexpected spiro cyclic products were obtained from the popular alkali catalyzed Claisen-Schmidt reaction of ketones with salicylaldehyde apart from the bis-chalcones. However, in the acid catalyzed reaction, we observe their transformation to a benzopyrylium salt. In the present study, we demonstrate this phenomenon for three ketones namely cyclopentanone, cylcohexanone and cylcoheptanone. Each of the corresponding products obained were characterized using UV-Vis, FT-IR, 1H-NMR, 13C-NMR, and mass spectrometry. In addition, the structure of the spiro molecule synthesized from cylcoheptanone reactant has been determined by X-ray crystallography and further investigated for its thermal properties using TGA/DTA technique. To rationalize this phenomenon, we first studied the effect of solvents on the reaction, and further explored the effect of Lewis acid catalyst and various solvents for the same reaction conditions. Further, we have evaluated in silico, the impact of thermodynamic parameter like entropy. In this regard, we explored the relationship between the bis-chalcone, spiro molecule and the benzopyrylium salt. Consequently, the effect of the ring expansion from cyclopentanone to cycloheptanone was investigated. In conclusion, we have presented the anti-bacterial potential of the synthsized spirobibenzopyrans and the benzopyrylium salts.