With these experimental results, a possible mechanism was proposed and illustrated in Scheme 3. The reaction presumably underwent a stepwise pathway, commencing with the nucleophilic addition of silyl enol ether with Lewis acidactivated BCB to form the zwitterionic intermediate Int 4. This was succeeded by an intramolecular Aldol-type reaction, resulting in the formation of the bicycle 3. Otherwise, silyl migration might occur after the in situ generation of Int 4, which leads to the cyclobutyl silyl enol ether byproduct 4. Additionally, when BCB reacts with silyl dienol ether to generate Int 4', the reaction could proceed through 1,4-addition, delivering bicyclo[4.1.1]octane 5. Considering the proposed reaction process, the origin of distinction in diastereoselectivity possibly arose from the steric bulk of the attached substituent geminal to silyloxyl group on silyl enol ether in the Aldol-type cyclization step. Scheme 3. Proposed mechanism CONCLUSIONS In summary, we developed a Lewis acid catalyzed formal [2+3] cycloaddition of silyl enol ethers and BCBs. This reaction exhibits high efficiency along with mild condition, operative simplicity, and broad substrate scope with respect to silyl enol ethers, which are readily available from ketone precursors. Notably, ketone is one of the most widely presented functional groups in organic molecules including natural products. Moreover, this formal [2+3] cycloaddition can be further extended to a [4+3] variant with silyl dienol ether. This discovery enables the divergent syntheses of different structurally intriguing polycyclic frameworks and highlights the great potential of silyl enol ethers in polycyclic synthesis. Importantly, this method should provide a broadly useful entry into bicyclo[2.1.1]hexanes from easily accessible starting materials covering a wide range of structural complexity. Further studies for the development of new reactions involving other widely available starting materials and BCBs are ongoing in our laboratory.