Fig. 1: Previous strategies of azetidine forming 2 + 2 photocycloadditions and this approach.

a Use of substrate π-stacking pre-organization¹³. b Activation via cyclic glyoxylate imine sensitization¹⁶. c Xanthone sensitization¹⁷. d Activation via Ir-mediated sensitization¹⁸. e Alkene activation via Cu-coordination and excitation. Tp = hydrotris(pyrazolyl)borate.