Irradiation of 5,5‐dimethyl‐6‐oxocyclohex‐ l‐ene‐ l‐carbonitrile (1) in the presence of 2,3‐dimethylbut‐2‐ene afforded 3,3,4,4,7,7‐hexamethyl‐3,4,4a,5,6,7‐hexahydroindeno[1,7‐c,d]‐],2‐oxazole (3) in nearly quantitative yield. In contrast, 4,4‐dimethyl‐5‐oxo‐cyclopent‐l‐ene‐l‐carbonitrile (2) under the same conditions reacted not to a tricyclic isoxazole but to a 2:1 mixture of 3,3,6,6,7,7‐hexamethyl‐2‐oxo‐bicyclo[3.2.0]heptane‐l‐carbonitrile (4) and trans‐3,3‐dimethyl‐2‐oxo‐5‐(2,3‐dimethylbut‐3‐en‐2‐yl)cyclopentane‐l‐carbonitrile (5), respectively.