Transition-Metal-Free, Brønsted Acid-Mediated Cascade Sequence in the Reaction of Propargyl Alcohols with Sulfonamido-indoles/-indolines: Highly Substituted δ- and α‑Carbolines

Brønsted acid-mediated,
transition-metal-free reaction of propargyl alcohols with sulfonamido-indoles/-indolines
under mild conditions affords highly substituted δ- or α-carbolines
in good to excellent yields. This protocol involves cascade reaction
sequences of Friedel–Crafts alkylation/ [1,5]-hydrogen shift/electrocyclization/elimination/
[1,2]-aryl migration followed by aromatization. An unexpected regioselective
tosyl group migration from indole 2- to 6-position and arene elimination
leading to α-carbolines has also been discovered.