Photoaddition of 2-(3,3-dimethylbut-1-ynyl)-cyclopent-2-enone to 2,3-dimethylbut-2-ene

Irradiation (350 nm) of the newly synthesized cyclopent-2-enone 6 in the pr esence of 2,3-dimethylbut-2-ene under standard conditions (10(-1) M 6 and 1 0-fold molar excess alkene in benzene) affords a 1 : 1 : 5 mixture of 7, 8 and 9. The main product 9 is the head-to-tail (HT) cis-transoid-cis cyclodi mer of 6 as established by X-ray analysis, its formation becoming negligibl e only at much higher dilution (10(-3) M 6 and 50-fold molar excess alkene) . Compounds 7 and 8 are [enone + alkene] adducts; the latter one isomerizes to 10 on attempted gas chromatographic separation. (C) 1999 Elsevier Scien ce S.A. All rights reserved.