A metal-free tandem C-C/C-O bond formation approach to densely functionalized indolyl 4H-chromenes catalyzed by polystyrene-supported p-toluenesulfonic acid under solvent-free conditions.

A new environmentally benign and highly convergent protocol for the synthesis of indolyl 4H-chromene derivatives has been developed. This one-pot three-component condensation reaction of salicylaldehyde, cyclic 1,3-diketones, and indole is promoted by PS-PTSA as a reusable heterogeneous acid catalyst under solvent-free conditions. This protocol demonstrates several notable advantages such as that the catalyst is readily available and can be recovered and reused for at least five runs without any significant impact on product yields, high atom economy, excellent yields, and efficiency of producing three new bonds (two C-C and one C-O) and one stereo center in a single operation.