Indium/TFA-Catalyzed Synthesis of Tetracyclic [6,5,5,6] Indole Ring, via a Tandem Cycloannulation of β‑Oxodithioester with Tryptamine

The cycloannulation of β-oxodithioesters and tryptamine in dichloromethane in the presence of a catalytic amount of InCl₃ and TFA gave the novel 5-aryl/heteroaryl 2a¹,9b-dihydro-1H-2a,5a-diaza-cyclopenta[jk]fluorene-3(2H)-thiones in moderate to good yields. The reaction was proposed to involve a tandem transformation of thioamide, protonation, and dehydrative cyclization. KMnO₄-oxidation of these newly prepared compounds yielded oxidative desulfurization products in good yields.