Unsymmetrical Bis‐Coumarin Based on o‐Hydroxy Aldehydes and 4‐Hydroxy Coumarin: Expedient Synthesis, Single Crystal X‐rays and DFT Studies

Abstract

Bis‐coumarins are distinguished derivatives of coumarin which bear numerous pharmaceutical and other applications. In continuation of our ongoing research towards the development of benign protocols for various heterocyclic systems, herein we report an Amberlyst 15 mediated synthesis of unsymmetrical bis‐coumarins via in situ generation and cyclization of symmetrical bis‐coumarinyl methanes derived from o‐hydroxy aromatic aldehydes and 4‐hydroxy coumarin. Aldehydes of both electron‐donating and electron‐withdrawing units underwent smooth conversion to their respective unsymmetrical bis‐coumarins (76–98 % yield) without leaving any side products or intermediates. The catalyst used in the present protocol is cost‐effective, eco‐friendly, and readily available which could be recovered and reused several times. This methodology offers operational simplicity, low catalyst loading, and avoids the use of toxic solvents/reagents and the product can be isolated without chromatography. DFT study indicated that the driving force for the cyclisation of symmetrical bis‐coumarin is the minimization of the tetrahedral bond angle deviations along with other small non‐covalent interactions. Some DFT data was also verified with the data obtained from the X‐rays crystallographic study.