Solid-state and solution photocycloadditions of 4-acyloxy-2-pyrones with maleimide

Solid-state photosensitized reactions of 4-acyloxy-2-pyrones () with maleimide () afforded endo–endo double-[4+2] cycloadducts () with high stereoselectivity. Sensitized photoreactions of with in solution gave exo–endo double-[4+2] cycloadducts (). 2-Pyrones were photolyzed to give carboxylic acids () via their valence isomerization in the solid state and in solution. Such kinds of photoreaction of the 4-acyloxy-2-pyrones were dramatically different from regio- and stereoselective [2+2] cycloadditions of 4-alkyloxy-2-pyrones. The photoreaction mechanisms of with and itself were analyzed by powder X-ray diffraction analysis and MO calculations.