Catalyst-free domino-Knoevenagel-hetero-Diels-Alder reaction of terminal alkynes in water: an efficient one-step synthesis of indole-annulated thiopyranobenzopyran derivatives

The domino-Knoevenagel-hetero-Diels–Alder reaction of O-propargylated salicylaldehyde and 1-methylindoline-2-thione in aqueous medium in the absence of Lewis acid has been described for the synthesis of hitherto unreported indole-annulated pentacyclic heterocycles containing oxygen, nitrogen and sulfur. This methodology involves only one step and easy work-up procedure to give the products in 72–80% yields.