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

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, H-1 NMR, C-13 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. (C) 2021 Elsevier B.V. All rights reserved.

Automated summary

The unexpected spiro cyclic products were obtained from the alkali-catalyzed Claisen-Schmidt reaction of ketones with salicylaldehyde, in addition to the bis-chalcones. Under acid catalysis, the spiro products transformed into benzopyrylium salts. The study characterized the products obtained from cyclopentanone, cylcohexanone, and cylcoheptanone using various analytical techniques, and investigated the thermal properties of the spiro molecule synthesized from cylcoheptanone reactant. The impact of solvents, Lewis acid catalyst, and thermodynamic parameters like entropy were explored to rationalize the phenomenon, as well as the relationship between the bis-chalcone, spiro molecule, and benzopyrylium salt, with the investigation of ring expansion from cyclopentanone to cycloheptanone. The antibacterial potential of the synthesized spirobibenzopyrans and benzopyrylium salts was also presented.