Photocycloisomerization of Boc‐Protected 5‐Alkenyl‐2,5‐dihydro‐1H‐ pyrrol‐2‐ones

On irradiation (254 nm), the newly synthesized Boc‐protected 5‐alkenyl‐2,5‐dihydro‐1H‐pyrrol‐2‐ones 13 undergo regioselective intramolecular 2+2] photocycloadditions. While the allyl derivatives 13a – 13c afford mainly azatricyclo3.3.0.0^2,7]octanones, i.e., crossed cycloadducts, the butenyl‐ and pentenyl‐substituted compounds 13d and 13e isomerize preferentially to straight cycloadducts.