A metal-free, mild and efficient method for the synthesis of thiazolo[3,2-a]indoles has been developed starting from indoline-2-thiones. The reaction methodology involves first the formation of thermally labile 2-(prop-2-ynylthio)-1H-indole intermediates, which undergo base-mediated intramolecular hydroamination to produce the title compounds in excellent yields.

Synthesis of a variety of 1,2-fused tricyclic S-containing indoles is reported starting from indole sulfides tethered with terminal and internal alkynes via a key hydroamination step. Cu-catalyzed hydroamination reactions resulted in the exclusive formation of tricycles possessing an exocyclic methylene moiety, whereas base-mediated conditions led to thiazolo­[3,2-a]­indoles. Indole sulfides with internal alkyne functionality produced 2H-[1,3]­thiazino­[3,2-a]­indoles under Cu-catalysis.