A general approach is described for the formation of tetrahydrothiepines using donor-acceptor cyclopropanes. Thiochalcones, functioning as sulfur-containing four-atom building blocks, were reacted in a Lewis-acid-catalyzed formal (4+3)-cycloaddition reaction with donor-acceptor cyclopropanes as three-atom building blocks. Under mild conditions various tetrahydrothiepines were synthesized in good yields in a stereosepecific reaction with high functional group tolerance.