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.
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