Intramolecular photosensitization of the pinene- ocimene rearrangement

Bonding of nopol to the para position of acetophenone produces 5,5-dimethyl-2-(2-(p-acetylphenoxy)ethyl)bi- cyclo(3.1.1)hept-2-ene 1, which contains two chromophores: a para-alkoxyacetophenone and an α -pinene, connected by a single methylene group. UV irradiation of 1 in both benzene and methanol produces none of the intramolecular (2 + 2) cycloaddition that most para-(3-buten-1-oxy)acetophenones undergo. Instead, the pinene unit rearranges to a triene skeleton identical to that of ocimene, a known photoproduct of pinene. At modest conversion the diene portion of the triene is cis but gradually is converted to a 52:48 trans:cis ratio. It is concluded that intramolecular triplet energy transfer from the excited ketone chromophore forms the 1,2-biradical triplet state of the pinene moiety, which then un - dergoes cyclobutylcarbinyl ring opening to a 1,4-biradical that cleaves to the 1,3,6-triene structure of ocimene. This mechanism is suggested to be responsible for the earlier reported intermolecularly sensitized rearrangement of α -pinene to the ocimene isomers.