The Chemistry of Short-Lived α-Fluorocarbocations.

The present study of the chemistry of short-lived α-fluorocarbocations reveals that even inactive methyl carbons can serve as nucleophiles, attacking a cationic center. This, in turn, facilitates the synthesis of a cyclopropane ring in certain triterpene backbones. We report the synthesis of compounds similar to 2, containing a bridgehead cyclopropane, and compounds of type 3 with an 11 membered bicyclic ring consisting of two bridgehead double bonds (anti-Bredt) within a triterpene skeleton. The synthesis involves three unconventional chemical processes: (a) a methyl group serving as a nucleophile; (b) the unexpected and unprecedented synthesis of a strained system in the absence of an external neighboring trigger; and (c) the formation of an 11-membered bicyclic diene ring within a triterpenoid skeleton. An α-fluorocarbocation mechanism is proposed and supported by density functional theory calculations.