Intramolecular Photochemical Cycloaddition Reactions of 3-(1,5-Dimethylhex-4-enyl)cyclohex-2-enone: Regio- and Stereochemical Aspects

The intramolecular photocycloaddition reaction of the title enone (1) and its 1-demethyl analogue (2) gives head-to-head, [2 + 2] adducts containing cyclopenta[1,4]cyclobuta[1,2]benzen-5(6H)-one skeletons and none of the head-to-tail isomers of the tricyclo[5.3.1.0^2,7]undecan-3-one variety. In addition, formal photoene adducts of a spiro[5,4]decan-3-one nature which are related to the acorane sesquiterpene carbon skeleton are also observed. The stereochemical consequences of the acyclic methyl-bearing chiral center in 1 are probed, and the ratio of epimeric products is found to be temperature dependent. Attempts to thermally convert the gem-dimethylcyclobutane moiety in the [2 + 2] adducts to spirocyclic precursors of the acoranes results instead in reversion to starting enone.