Enantioselective [2π\n+ 2σ] Photocycloaddition\nEnabled by Brønsted Acid Catalyzed Chromophore Activation

Bicyclo[2.1.1]hexanes have emerged as valuable scaffolds\nfor the\ndesign of new pharmaceutical and agrochemical active ingredients.\nThese structures can be efficiently synthesized via [2π + 2σ]\nphotocycloadditions; however, control over the absolute stereochemistry\nof these strain-releasing reactions has remained challenging. Herein,\nwe demonstrate that Brønsted acid catalyzed chromophore activation\nof <i>C</i>-acyl imidazoles enables highly enantioselective\n[2π + 2σ] photocycloadditions. Because this approach is\nagnostic to the identity of the coupling partner, the same strategy\ncan be used to synthesize several other medicinally relevant strained\nsmall-ring structures.