Domino-Annulation-Based Approach to Synthesize Bridged Bis-thiopyrano[2,3-b]indoles and Unbridged Thiopyrano[2,3-b]indoles

A novel domino strategy for the construction of intricate bridged bis-thiopyrano[2,3-b]indoles was disclosed, which involved one molecule of 3-formylchromones and two molecules of indoline-2-thiones, enabling the formation of four new bonds in a single operation. The transformation occurred under mild reaction conditions, facilitated by the synergistic action of the base NaHCO3 and the catalytic acid ZnCl2. Notably, when these bridged bis-thiopyrano[2,3-b]indole skeletons were treated with alkyl halides under basic conditions, they underwent deconstruction to yield alkylthio-substituted indole-decorated thiopyrano[2,3-b]indoles.