Facile Synthesis of Fully Substituted Dihydro-β-carbolines via Brønsted Acid Promoted Cascade Reactions of α‑Indolyl Propargylic Alcohols with Nitrones

A straightforward synthesis of fully substituted β-carbolines via Brønsted acid promoted cyclizations of α-indolyl propargylic alcohols with nitrones is described. The use of nitrones bearing alkenyl or electron-rich aryl groups as the R4 substituent dramatically switches the reaction pathway to afford tetrasubstituted alkenes and amines, which is assumed to proceed through a rearrangement reaction involving N–O bond cleavage and 1,2-migration of the R4 group to an adjacent nitrogen atom.