Visible Light-Mediated Thia-Paternò-Büchi Reaction of Indoles through Energy Transfer: Synthesis of Thietane-Fused Polycyclic Indolines.

The thia-Paternò-Büchi reactions represent a valuable yet underexploited transformation compared to analogous [2+2] photocycloadditions involving olefins, carbonyls, and imines. Herein, we present visible light thia-Paternò-Büchi reactions of xanthates derived from tryptophol and tryptamine, enabling the synthesis of various tetracyclic indolines contiguously fused with thietane and additional O/N heterocycles. Mechanistic studies indicate that energy transfer activation of the thiocarbonyl chromophore is instrumental in promoting reactivity, which differs from the established cycloadditions utilizing triplet indoles.