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

Bronsted acid-mediated, transition-metal-free reaction of propargyl alcohols with sulfonamido-indoles/indolines under mild conditions affords highly substituted delta- or alpha-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 alpha-carbolines has also been discovered.