One-Pot Synthesis of Spirocyclopenta[<i>a</i>]indene Derivatives via a Cascade Ring Expansion and Intramolecular Friedel–Crafts-Type Cyclization

A one-pot efficient synthetic approach for the rapid construction of spirocyclopenta­[a]­indene derivatives has been developed via an iodine-initiated cascade ring expansion and intramolecular Friedel–Crafts-type cyclization from propargyl alcohol-tethered alkylidenecyclobutanes under mild conditions with broad substrate scope. This cascade process can be elegantly conducted on a gram scale. A plausible reaction mechanism has been proposed on the basis of a series of deuterium labeling and control experiments.