Synthesis of Tricyclo [6.2.1.01,5]undecadiones via Intramolecular Photoaddition of 5-(1-Cyclopentenylmethyl)-3-alkoxy-2-cyclopentenones

Irradiation of the dienone methyl ether 3a furnished the unstable regioisomeric photoadducts 4a and 5a which on Lewis acid-catalyzed fragmentation afforded the tricyclic diketones 6,7 and 8 . The same mixture was obtained from the isobutyl ether 3b (Scheme 2).