Indole, a π-nucleophile, reacts with Brønsted acid activated imide carbonyl group in an intramolecular fashion via a 6-exo-trig cyclization, to deliver a condensed tetrahydro-β-carboline unit. This methodology is effectively applied to assemble the tetrahydro-β-carboline skeleton containing alkaloids such as harmicine and 10-desbromoarborescidine-A.
