Calcium-Catalyzed Formal [5 + 2] Cycloadditions of Alkylidene β-Ketoesters with Olefins: Chemodivergent Synthesis of Highly Functionalized Cyclohepta[b]indole Derivatives

The calcium-catalyzed, formal [5 + 2] cycloaddition of indolyl alkylidene β-ketoesters with mono- and disubstituted aryl olefins to form cyclohepta[b]indole derivatives has been established. Unanticipated chemodivergence with phenyl vinyl sulfide/ether revealed a double [5 + 2] cycloaddition cascade providing ethano-bridged cyclohepta[b]indoles. Overall, the method’s highlights include: (1) use of a green, calcium-based catalyst (2.5 mol % loading); (2) reaction times under 1 h; (3) mild reaction conditions; (4) substrate-derived chemodivergence; (5) functional group tolerance; and (6) examples of derivatization.