Intramolecular photocycloadditions of 6,6′‐dimethyl‐4,4′‐polymethylenedioxy‐di‐2‐pyrones

Photosensitized reactions of 4,4′‐polymethylene‐di‐2‐pyrones 2b‐e afforded [2+2]‐cycloadducts 3‐5 , site‐selectively. The selectivity depended upon the methylene chain length. Namely, the three carbon chain gave a syn head‐to‐head adduct 3b at the 3,4‐position of the 2‐pyrone ring, the four to six carbon chains gave syn head‐to‐head adducts 4c‐e at the 5,6‐position and/or anti head‐to‐head adducts 5d,e at the 5,6‐position, respectively. The intramolecular cycloaddition mechanism was also explained from results calculated by means of PM3‐CI method.