Construction of Bridged Tricycles by Lewis Acid-Catalyzed Formal Cross-Dimerization of Bicyclobutanes with Dithioazabicycles.

While formal cross-dimerization of strained rings and (3+X) cycloadditions of bicyclobutanes (BCBs) have enabled diverse cyclic frameworks and bicyclo[n.1.1]alkanes, respectively, their extension to bridged tricycles remains elusive. Herein, we report a Lewis acid-catalyzed formal (6 + 3) cycloaddition of BCBs with dithioazabicycles, enabling efficient access to structurally complex 9-membered thia-aza-bridged tricycles under mild conditions (up to 99% yield). This transformation proceeds via a highly selective C-C/C-S bond cross-exchange process and features broad substrate scope, gram-scale synthesis, and versatile downstream derivatization.