Duality of Donor–Acceptor Cyclopropane Reactivity as a Three‐Carbon Component in Five‐Membered Ring Construction: [3+2] Annulation Versus [3+2] Cycloaddition

Quo vadis? The Lewis acid catalyzed reaction of (hetero)aryl-derived donor-acceptor cyclopropanes with alkenes can be selectively directed along a [3+2] annulation pathway (see scheme). This new process provides convenient and efficient access to indanes and other cyclopentannulated (hetero)arenes, among which polyoxygenated 1-arylindanes exhibit significant cytotoxicity against several cancer cell lines with an IC50 of 10(-6)-10(-5) M.