Photochemical behavior of 4-(ω-enyl)- and 4-(ω-dienyl)-2-pyrones

4-(ω-Enyl)-2-pyrone 3 and two types of 4-(ω-dienyl)-2-pyrones 4–6, 9 and 10 were prepared and the photochemical behavior was investigated. Photosensitized reaction of 3 afforded [2+2]-cycloadduct 11 site-specifically. On the other hand, 4 and 5 gave geometrical isomers, respectively. 2-Pyrones 6, 9 and 10 possessing pendant furans gave no adduct. The reasons for the differences are described.