Direct Ketone Cyclopropanation Enabled by Tebbe's Reagent.

A method for the direct transformation of ketones and aldehydes into cyclopropanes is described. This chemistry exploits the ability of titanocene methylidene unveiled from Tebbe's reagent to mediate carbonyl methylenation and subsequently generate a titanacyclobutane from the incipient alkene. This labile titanium heterocycle is then reacted in situ with iodine to trigger cyclopropane ring formation. The synthetic utility of this method was demonstrated by preparing the azaspiro[2.5]heptane fragment of ledipasvir in a single step.