Alkynes to (Free) Carbenes\nto Polycyclic Cyclopropanes

Carbenes and carbenoids are commonly employed for the\nsynthesis\nof cyclopropane-containing compounds. Here we report the metal-free,\nintramolecular cyclopropanation of tethered alkenes by free carbenes\nderived from alkynes to construct structurally unique multicyclic\ncyclopropanes with perfect atom economy. The nature of the tether\ninfluences both the rate of carbene formation and subsequent competing\nreaction events. Some of the substrates lead to metastable cyclopropane\nintermediates that further fragment to furnish interesting isomeric\nproducts by mechanistically novel processes. A removable siloxane\ntether can be utilized to achieve formal <i>inter</i>molecular\ncyclopropanations and to access cyclopropanol derivatives.