Direct photochemical intramolecular [4 + 2] cycloadditions of dehydrosecodine-type substrates for the synthesis of the iboga-type scaffold and divergent [2 + 2] cycloadditions employing micro-flow system

Abstract

Photocyclisation reactions offer a convenient and versatile method for constructing complex polycyclic scaffolds, particularly in the synthesis of natural products. While the [2 + 2] photocycloaddition reaction is well-established and extensively reported, the [4 + 2] counterpart via direct photochemical means remains challenging and relatively unexplored. In this work, we devised the rapid assembly of the iboga-type scaffold through photochemical intramolecular Diels–Alder reaction using a common biomimetic dehydrosecodine-type intermediate having vinyl indole and dihydropyridine (DHP) sub-units. Exploiting a micro-flow system, the medicinally important iboga-type scaffold was obtained up to 77% yield under mild, neutral conditions at room temperature. This study demonstrated the site-selective activation of the DHP moiety by direct UV-LED irradiation, eliminating the need for external photocatalysts or photosensitisers and showing good tolerance to a wide range of stabilised dehydrosecodine-type substrates. By adjusting the spatial arrangement of the DHP ring and the vinyl indole group, this versatile photochemical approach efficiently facilitates both [4 + 2] and [2 + 2] cyclisations, assembling architecturally complex multicyclic scaffolds. Precise photoactivation of the DHP subunit, generating short-lived biradical species, enabled the new way of harnessing the hidden but innately pre-encoded reactivity of the polyunsaturated dehydrosecodine-type intermediate. These photo-mediated [4 + 2] cyclisation and divergent [2 + 2] cycloadditions are distinct from biosynthetic processes, which are mainly mediated through concerted thermal cycloadditions.