Alkynes to (Free) Carbenes to Polycyclic Cyclopropanes

Carbenes and carbenoids are commonly employed for the synthesis of cyclopropane-containing compounds. Here we report the metal-free, intramolecular cyclopropanation of tethered alkenes by free carbenes derived from alkynes to construct structurally unique multicyclic cyclopropanes with perfect atom economy. The nature of the tether influences both the rate of carbene formation and subsequent competing reaction events. Some of the substrates lead to metastable cyclopropane intermediates that further fragment to furnish interesting isomeric products by mechanistically novel processes. A removable siloxane tether can be utilized to achieve formal intermolecular cyclopropanations and to access cyclopropanol derivatives.