Intermolecular Electrophilic Addition of Epoxides to Alkenes: [3+2] Cycloadditions Catalyzed by Lewis Acids

Described are the first examples of intermolecular electrophilic additions of epoxides to alkenes, which proceed through a classic cationic mechanism initiated by epoxide C–O bond cleavage. Treatment of styrene oxides and either styrenes or dienes with a variety of Lewis‐acidic triflate salts generates tetrahydrofurans as products of [3+2] cycloaddition in moderate to good yields (up to 71 %). Careful choice of catalyst and reaction conditions favors the desired intermolecular reaction over epoxide degradation without requiring additional reagents or additives. The reaction proceeds diastereoselectively and provides only one regioisomer of the product. Additional highlights include inexpensive precursors, mild conditions, short reaction times, low catalyst loading, and scalability.