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.