Fig. 1: Intermolecular [2 + 2] photocycloadditions of indoles.

Intermolecular [2 + 2] photocycloadditions of indoles. a Escape from flat indoles into diverse 3D polycyclic indolines. b Challenges for intermolecular [2 + 2] photocycloaddition of indoles to build a fused saturated four-membered ring. c Naturally occurring ladderanes as biologically important membrane lipids. d The present work: intermolecular aza- Paternò-Büchi reactions with indoles that afford 3D ladder-shape azetidine-fused indoline pentacycles with divergent H–H and T–T selectivity and exclusive exo stereoselectivity. H–T, head-to-head; H–T, head-to-tail. For nomenclature of head and tail, see Supplementary Table 1.