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

Abstract

Solid-state photosensitized 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 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 1 with 2 and 1 itself were analyzed by powder X-ray diffraction analysis and MO calculations.