Mechanochemical synthesis of coumarins via Pechmann condensation under solvent-free conditions: an easy access to coumarins and annulated pyrano[2,3-f] and [3,2-f]indoles

A green protocol has been developed for the synthesis of simple coumarins and linear pyrano[2,3-f] and [3,2-f]indoles by the reaction of phenol derivatives with beta-ketoesters under ball milling at ambient temperature in the presence of methanesulfonic acid as a mild acid catalyst. The significant advantages of this procedure are high yields, scalability, no use of hazardous acids or solvents, shorter reaction time, ambient temperature, low cost, and straightforward purification without column chromatography. This procedure is associated with high EcoScale metrics and a low E-factor. In contrast to traditional Pechmann condensation procedures, the mechanochemical protocol leads to the synthesis of pyranoindoles with excellent regioselectivity and high yields.