reactions of 4-acyloxy-2-pyrones (1b,c) with maleimide (2) afforded endo–endo double-[4+2] cycloadducts (3b,c) with high stereoselectivity. Sensitized photoreactions of 1a–d with 2 in solution gave exo–endo double-[4+2] cycloadducts (4a–d). 2-Pyrones 1a–d were photolyzed to give carboxylic acids (5a–d) via their valence isomerization in the solidstate and in solution. Such kinds of photoreaction of the 4-acyloxy-2-pyrones