X-Ray and MO analysis of highly stereoselective solid-state photocycloadditions of 2-pyrones with maleimide

Photoirradiations of grinding mixtures of 4-(ω-arylalkyloxy)-6-methyl-2-pyrones and maleimide in the solid state quantitatively gave only a [2+2] cycloadduct with high stereoselectivity. The 1:1 complex crystals ·, and ·, ·, ·, ·, ·, · from another 2-pyrones with , were characterized by powder X-ray diffraction technique. The crystal formation was remarkably affected by polar and bulky nature of the substituents at the aryl groups. Four kinds of hydrogen bondings by two ground state species for the crystals were quantitatively estimated, and the photoreaction mechanism was analyzed to proceed via some interactions of the singlet excited state of with ground state of by MO transition state calculation.