Photocycloaddition of 2‐Cyanocycloalk‐2‐Enones to 2, 3‐Dimethylbut‐2‐Ene

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.