6-(omega-Alkenyl)-2-pyrones 3a-c and 8a,b were prepared and the photoc
hemical reactions were investigated. Photosensitized reactions of 3b,c
gave intramolecular [2 + 2]-cycloadducts 11b,c as tricyclic lactones,
site- and regio-specifically. They are not frontier-orbital-controlle
d adducts. On the other hand, 3a, 8a,b afforded cyclobutenecarboxylic
acids, 10a, 14a,b, respectively. The end-ester group at the side-chain
is thought not to be effective for the intramolecular photoaddition.