Stereoselective Synthesis of Dihydrofuranoindoles via the Friedel–Crafts Alkylation/Annulation Cascade Process

A stereoselective annulation protocol was developed to construct dihydrofuranoindoles from readily available starting materials. In the presence of a bifunctional squaramide, the Friedel–Crafts alkylation/annulation cascade process occurred smoothly to provide dihydrofuranoindoles in 26–95% isolated yields exclusively as trans-diastereomers (38–99% ee). This catalytic protocol was compatible with a range of structurally distinct hydroxyindoles bearing the hydroxyl group at different positions, providing four kinds of dihydrofuranoindoles. Moreover, gram-scale synthesis and further synthetic manipulation of the product were also demonstrated.