Palladium‐Catalyzed Cyclopropanation of Unsaturated Endoperoxides. A New Peroxide‐Preserving Reaction

Unsaturated bicyclic endoperoxides are efficiently cyclopropanated with excess diazomethane in the presence of catalytic palladium(II) acetate [Pd(OAc)2] in a stereoselective manner. This method represents a new peroxide‐preserving transformation. Whereas the unsaturated endoperoxides in the [2.2.1] series are attacked by the carbene from the exo face, the analogs with larger bridges are preferentially attacked from the face syn to the peroxo bridge. Only in the case of the benzannelated [2.2.2] system does the attack occur exclusively from the face proximal to the benzene ring. Certain strained cyclopropanated endoperoxides are reduced by diazomethane to give cis‐diols. 2‐Methylfuran endoperoxide gives rise to cis‐1‐formyl‐2‐acetylcyclopropane in excellent yield.